Internal quantum efficiency of III-nitride quantum dot superlattices grown by plasma-assisted molecular-beam epitaxy

We present a study of the optical properties of GaN/AlN and InGaN/GaN quantum dot (QD) superlattices grown via plasma-assisted molecular-beam epitaxy, as compared to their quantum well (QW) counterparts. The three-dimensional/two-dimensional nature of the structures has been verified using atomic force microscopy and transmission electron microscopy. The QD superlattices present higher internal quantum efficiency as compared to the respective QWs as a result of the three-dimensional carrier localization in the islands. In the QW samples, photoluminescence (PL) measurements point out a certain degree of carrier localization due to structural defects or thickness fluctuations, which is more pronounced in InGaN/GaN QWs due to alloy inhomogeneity. In the case of the QD stacks, carrier localization on potential fluctuations with a spatial extension smaller than the QD size is observed only for the InGaN QD-sample with the highest In content (peak emission around 2.76 eV). These results confirm the efficiency of the QD three-dimensional confinement in circumventing the potential fluctuations related to structural defects or alloy inhomogeneity. PL excitation measurements demonstrate efficient carrier transfer from the wetting layer to the QDs in the GaN/AlN system, even for low QD densities (~1010 cm-3). In the case of InGaN/GaN QDs, transport losses in the GaN barriers cannot be discarded, but an upper limit to these losses of 15% is deduced from PL measurements as a function of the excitation wavelength.

Inmovilización de TiO2 sobre polímeros transparentes en el UV-A para la eliminación fotocatalítica de tricloroetileno en aire

El gran desarrollo industrial y demográfico de las últimas décadas ha dado lugar a un consumo crecientemente insostenible de energía y materias primas, que influye negativamente en el ambiente por la gran cantidad de contaminantes generados. Entre las emisiones tienen gran importancia los compuestos orgánicos volátiles (COV), y entre ellos los compuestos halogenados como el tricloroetileno, debido a su elevada toxicidad y resistencia a la degradación. Las tecnologías generalmente empleadas para la degradación de estos compuestos presentan inconvenientes derivados de la generación de productos tóxicos intermedios o su elevado coste. Dentro de los procesos avanzados de oxidación (Advanced Oxidation Processes AOP), la fotocatálisis resulta una técnica atractiva e innovadora de interés creciente en su aplicación para la eliminación de multitud de compuestos orgánicos e inorgánicos, y se ha revelado como una tecnología efectiva en la eliminación de compuestos orgánicos volátiles clorados como el tricloroetileno. Además, al poder aprovechar la luz solar como fuente de radiación UV permite una reducción significativa de costes energéticos y de operación. Los semiconductores más adecuados para su empleo como fotocatalizadores con aprovechamiento de la luz solar son aquellos que tienen una banda de energía comparable a la de los fotones de luz visible o, en su defecto, de luz ultravioleta A (Eg < 3,5 eV), siendo el más empleado el dióxido de titanio (TiO2). El objetivo principal de este trabajo es el estudio de polímeros orgánicos comerciales como soporte para el TiO2 en fotocatálisis heterogénea y su ensayo para la eliminación de tricloroetileno en aire. Para ello, se han evaluado sus propiedades ópticas y su resistencia a la fotodegradación, y se ha optimizado la fijación del fotocatalizador para conseguir un recubrimiento homogéneo, duradero y con elevada actividad fotocatalítica en diversas condiciones de operación. Los materiales plásticos ensayados fueron el polietileno (PE), copolímero de etil vinil acetato con distintos aditivos (EVA, EVA-H y EVA-SH), polipropileno (PP), polimetil (metacrilato) fabricado en colada y extrusión (PMMA-C y PMMA-E), policarbonato compacto y celular (PC-C y PC-Ce), polivinilo rígido y flexible (PVC-R y PVC-F), poliestireno (PS) y poliésteres (PET y PETG). En base a sus propiedades ópticas se seleccionaron el PP, PS, PMMA-C, EVA-SH y PVC-R, los cuales mostraron un valor de transmitancia superior al 80% en el entorno de la región estudiada (λ=365nm). Para la síntesis del fotocatalizador se empleó la tecnología sol-gel y la impregnación multicapa de los polímeros seleccionados por el método de dip-coating con secado intermedio a temperaturas moderadas. Con el fin de evaluar el envejecimiento de los polímeros bajo la radiación UV, y el efecto sobre éste del recubrimiento fotoactivo, se realizó un estudio en una cámara de exposición a la luz solar durante 150 días, evaluándose la resistencia química y la resistencia mecánica. Los resultados de espectroscopía infrarroja y del test de tracción tras el envejecimiento revelaron una mayor resistencia del PMMA y una degradación mayor en el PS, PVC-R y EVA SH, con una apreciable pérdida del recubrimiento en todos los polímeros. Los fotocatalizadores preparados sobre soportes sin tratamiento y con tres capas de óxido de titanio mostraron mejores resultados de actividad con PMMA-C, PET y PS, con buenos resultados de mineralización. Para conseguir una mayor y mejor fijación de la película al soporte se realizaron tratamientos químicos abrasivos con H2SO4 y NaOH y tratamientos de funcionalización superficial por tecnología de plasma a presión atmosférica (APP) y a baja presión (LPP). Con los tratamientos de plasma se consiguió una excelente mojabilidad de los soportes, que dio lugar a una distribución uniforme y más abundante del fotocatalizador, mientras que con los tratamientos químicos no se obtuvo una mejora significativa. Asimismo, se prepararon fotocatalizadores con una capa previa de dióxido de silicio con la intervención de surfactantes (PDDA-SiO2-3TiO2 y SiO2FC-3TiO2), consiguiéndose buenas propiedades de la película en todos los casos. Los mejores resultados de actividad con tratamiento LPP y tres capas de TiO2 se lograron con PMMA-C (91% de conversión a 30 ppm de TCE y caudal 200 ml·min-1) mejorando significativamente también la actividad fotocatalítica en PVC-R y PS. Sin embargo, el material más activo de todos los ensayados fue el PMMA-C con el recubrimiento SiO2FC-3TiO2, logrando el mejor grado de mineralización, del 45%, y una velocidad de 1,89 x 10-6 mol· m-2 · s-1, que dio lugar a la eliminación del 100 % del tricloroetileno en las condiciones anteriormente descritas. A modo comparativo se realizaron ensayos de actividad con otro contaminante orgánico tipo, el formaldehído, cuya degradación fotocatalítica fue también excelente (100% de conversión y 80% de mineralización con 24 ppm de HCHO en un caudal de aire seco de 200 ml·min-1). Los buenos resultados de actividad obtenidos confirman las enormes posibilidades que ofrecen los polímeros transparentes en el UV-A como soportes del dióxido de titanio para la eliminación fotocatalítica de contaminantes en aire. ABSTRACT The great industrial and demographic development of recent decades has led to an unsustainable increase of energy and raw materials consumption that negatively affects the environment due to the large amount of waste and pollutants generated. Between emissions generated organic compounds (VOCs), specially the halogenated ones such as trichloroethylene, are particularly important due to its high toxicity and resistance to degradation. The technologies generally used for the degradation of these compounds have serious inconveniences due to the generation of toxic intermediates turn creating the problem of disposal besides the high cost. Among the advanced oxidation processes (AOP), photocatalysis is an attractive and innovative technique with growing interest in its application for the removal of many organic and inorganic compounds, and has emerged as an effective technology in eliminating chlorinated organic compounds such as trichloroethylene. In addition, as it allows the use of sunlight as a source of UV radiation there is a significant reduction of energy costs and operation. Semiconductors suitable to be used as photocatalyst activated by sunlight are those having an energy band comparable to that of the visible or UV-A light (Eg <3,5 eV), being titanium dioxide (TiO2), the most widely used. The main objective of this study is the test of commercial organic polymers as supports for TiO2 to be applied in heterogeneous photocatalysis and its assay for removing trichloroethylene in air. To accomplish that, its optical properties and resistance to photooxidation have been evaluated, and different operating conditions have been tested in order to optimize the fixation of the photocatalyst to obtain a homogeneous coating, with durable and high photocatalytic activity. The plastic materials tested were: polyethylene (PE), ethyl vinyl acetace copolymers with different additives (EVA, EVA-H and EVA -SH), polypropylene (PP), poly methyl (methacrylate) manufactured by sheet moulding and extrusion (PMMA-C and PMMA-E), compact and cellular polycarbonates (PC-C PC-Ce), rigid and flexible polyvinyl chloride (PVC-R and PVC-F), polystyrene (PS) and polyesters (PET and PETG). On the basis of their optical properties PP, PS, PMMA-C, EVA-SH and PVC-R were selected, as they showed a transmittance value greater than 80% in the range of the studied region (λ = 365nm). For the synthesis of the photocatalyst sol-gel technology was employed with multilayers impregnation of the polymers selected by dip-coating, with intermediate TiO2 drying at moderate temperatures. To evaluate the polymers aging due to UV radiation, and the effect of photoactive coating thereon, a study in an sunlight exposure chamber for 150 days was performed, evaluating the chemical resistance and the mechanical strength. The results of infrared spectroscopy and tensile stress test after aging showed the PMMA is the most resistant sample, but a greater degradation in PS, PVC-R and EVA SH, with a visible loss of the coating in all the polymers tested. The photocatalysts prepared on the untreated substrates with three layers of TiO2 showed better activity results when PMMA-C, PET and PS where used. To achieve greater and better fixation of the film to the support, chemical abrasive treatments, with H2SO4 and NaOH, as well as surface functionalization treatments with atmospheric pressure plasma (APP) and low pressure plasma (LPP) technologies were performed. The plasma treatment showed the best results, with an excellent wettability of the substrates that lead to a better and uniform distribution of the photocatalyst compared to the chemical treatments tested, in which no significant improvement was obtained. Also photocatalysts were prepared with the a silicon dioxide previous layer with the help of surfactants (SiO2- 3TiO2 PDDA-and-3TiO2 SiO2FC), obtaining good properties of the film in all cases. The best activity results for LPP-treated samples with three layers of TiO2 were achieved with PMMA-C (91% conversion, in conditions of 30 ppm of TCE and 200 ml·min-1 air flow rate), with a significant improvement of the photocatalytic activity in PVC-R and PS samples too. However, among all the materials assayed, PMMA-C with SiO2FC-3TiO2 coating was the most active one, achieving the highest mineralization grade (45%) and a reaction rate of 1,89 x 10-6 mol· m-2 · s-1, with total trichloroethylene elimination in the same conditions. As a comparative assay, an activity test was also performed with another typical organic contaminant, formaldehyde, also with good results (100% conversion with 24 ppm of HCHO and 200 ml·min-1 gas flow rate). The good activity results obtained in this study confirm the great potential of organic polymers which are transparent in the UV-A as supports for titanium dioxide for photocatalytic removal of air organic pollutants.

MOVPE growth of III-V semiconductors on silicon for third generation solar cells

Esta Tesis trata sobre el desarrollo y crecimiento -mediante tecnología MOVPE (del inglés: MetalOrganic Vapor Phase Epitaxy)- de células solares híbridas de semiconductores III-V sobre substratos de silicio. Esta integración pretende ofrecer una alternativa a las células actuales de III-V, que, si bien ostentan el récord de eficiencia en dispositivos fotovoltaicos, su coste es, a día de hoy, demasiado elevado para ser económicamente competitivo frente a las células convencionales de silicio. De este modo, este proyecto trata de conjugar el potencial de alta eficiencia ya demostrado por los semiconductores III-V en arquitecturas de células fotovoltaicas multiunión con el bajo coste, la disponibilidad y la abundancia del silicio. La integración de semiconductores III-V sobre substratos de silicio puede afrontarse a través de diferentes aproximaciones. En esta Tesis se ha optado por el desarrollo de células solares metamórficas de doble unión de GaAsP/Si. Mediante esta técnica, la transición entre los parámetros de red de ambos materiales se consigue por medio de la formación de defectos cristalográficos (mayoritariamente dislocaciones). La idea es confinar estos defectos durante el crecimiento de sucesivas capas graduales en composición para que la superficie final tenga, por un lado, una buena calidad estructural, y por otro, un parámetro de red adecuado. Numerosos grupos de investigación han dirigido sus esfuerzos en los últimos años en desarrollar una estructura similar a la que aquí proponemos. La mayoría de éstos se han centrado en entender los retos asociados al crecimiento de materiales III-V, con el fin de conseguir un material de alta calidad cristalográfica. Sin embargo, prácticamente ninguno de estos grupos ha prestado especial atención al desarrollo y optimización de la célula inferior de silicio, cuyo papel va a ser de gran relevancia en el funcionamiento de la célula completa. De esta forma, y con el fin de completar el trabajo hecho hasta el momento en el desarrollo de células de III-V sobre silicio, la presente Tesis se centra, fundamentalmente, en el diseño y optimización de la célula inferior de silicio, para extraer su máximo potencial. Este trabajo se ha estructurado en seis capítulos, ordenados de acuerdo al desarrollo natural de la célula inferior. Tras un capítulo de introducción al crecimiento de semiconductores III-V sobre Si, en el que se describen las diferentes alternativas para su integración; nos ocupamos de la parte experimental, comenzando con una extensa descripción y caracterización de los substratos de silicio. De este modo, en el Capítulo 2 se analizan con exhaustividad los diferentes tratamientos (tanto químicos como térmicos) que deben seguir éstos para garantizar una superficie óptima sobre la que crecer epitaxialmente el resto de la estructura. Ya centrados en el diseño de la célula inferior, el Capítulo 3 aborda la formación de la unión p-n. En primer lugar se analiza qué configuración de emisor (en términos de dopaje y espesor) es la más adecuada para sacar el máximo rendimiento de la célula inferior. En este primer estudio se compara entre las diferentes alternativas existentes para la creación del emisor, evaluando las ventajas e inconvenientes que cada aproximación ofrece frente al resto. Tras ello, se presenta un modelo teórico capaz de simular el proceso de difusión de fosforo en silicio en un entorno MOVPE por medio del software Silvaco. Mediante este modelo teórico podemos determinar qué condiciones experimentales son necesarias para conseguir un emisor con el diseño seleccionado. Finalmente, estos modelos serán validados y constatados experimentalmente mediante la caracterización por técnicas analíticas (i.e. ECV o SIMS) de uniones p-n con emisores difundidos. Uno de los principales problemas asociados a la formación del emisor por difusión de fósforo, es la degradación superficial del substrato como consecuencia de su exposición a grandes concentraciones de fosfina (fuente de fósforo). En efecto, la rugosidad del silicio debe ser minuciosamente controlada, puesto que éste servirá de base para el posterior crecimiento epitaxial y por tanto debe presentar una superficie prístina para evitar una degradación morfológica y cristalográfica de las capas superiores. En este sentido, el Capítulo 4 incluye un análisis exhaustivo sobre la degradación morfológica de los substratos de silicio durante la formación del emisor. Además, se proponen diferentes alternativas para la recuperación de la superficie con el fin de conseguir rugosidades sub-nanométricas, que no comprometan la calidad del crecimiento epitaxial. Finalmente, a través de desarrollos teóricos, se establecerá una correlación entre la degradación morfológica (observada experimentalmente) con el perfil de difusión del fósforo en el silicio y por tanto, con las características del emisor. Una vez concluida la formación de la unión p-n propiamente dicha, se abordan los problemas relacionados con el crecimiento de la capa de nucleación de GaP. Por un lado, esta capa será la encargada de pasivar la subcélula de silicio, por lo que su crecimiento debe ser regular y homogéneo para que la superficie de silicio quede totalmente pasivada, de tal forma que la velocidad de recombinación superficial en la interfaz GaP/Si sea mínima. Por otro lado, su crecimiento debe ser tal que minimice la aparición de los defectos típicos de una heteroepitaxia de una capa polar sobre un substrato no polar -denominados dominios de antifase-. En el Capítulo 5 se exploran diferentes rutinas de nucleación, dentro del gran abanico de posibilidades existentes, para conseguir una capa de GaP con una buena calidad morfológica y estructural, que será analizada mediante diversas técnicas de caracterización microscópicas. La última parte de esta Tesis está dedicada al estudio de las propiedades fotovoltaicas de la célula inferior. En ella se analiza la evolución de los tiempos de vida de portadores minoritarios de la base durante dos etapas claves en el desarrollo de la estructura Ill-V/Si: la formación de la célula inferior y el crecimiento de las capas III-V. Este estudio se ha llevado a cabo en colaboración con la Universidad de Ohio, que cuentan con una gran experiencia en el crecimiento de materiales III-V sobre silicio. Esta tesis concluye destacando las conclusiones globales del trabajo realizado y proponiendo diversas líneas de trabajo a emprender en el futuro. ABSTRACT This thesis pursues the development and growth of hybrid solar cells -through Metal Organic Vapor Phase Epitaxy (MOVPE)- formed by III-V semiconductors on silicon substrates. This integration aims to provide an alternative to current III-V cells, which, despite hold the efficiency record for photovoltaic devices, their cost is, today, too high to be economically competitive to conventional silicon cells. Accordingly, the target of this project is to link the already demonstrated efficiency potential of III-V semiconductor multijunction solar cell architectures with the low cost and unconstrained availability of silicon substrates. Within the existing alternatives for the integration of III-V semiconductors on silicon substrates, this thesis is based on the metamorphic approach for the development of GaAsP/Si dual-junction solar cells. In this approach, the accommodation of the lattice mismatch is handle through the appearance of crystallographic defects (namely dislocations), which will be confined through the incorporation of a graded buffer layer. The resulting surface will have, on the one hand a good structural quality; and on the other hand the desired lattice parameter. Different research groups have been working in the last years in a structure similar to the one here described, being most of their efforts directed towards the optimization of the heteroepitaxial growth of III-V compounds on Si, with the primary goal of minimizing the appearance of crystal defects. However, none of these groups has paid much attention to the development and optimization of the bottom silicon cell, which, indeed, will play an important role on the overall solar cell performance. In this respect, the idea of this thesis is to complete the work done so far in this field by focusing on the design and optimization of the bottom silicon cell, to harness its efficiency. This work is divided into six chapters, organized according to the natural progress of the bottom cell development. After a brief introduction to the growth of III-V semiconductors on Si substrates, pointing out the different alternatives for their integration; we move to the experimental part, which is initiated by an extensive description and characterization of silicon substrates -the base of the III-V structure-. In this chapter, a comprehensive analysis of the different treatments (chemical and thermal) required for preparing silicon surfaces for subsequent epitaxial growth is presented. Next step on the development of the bottom cell is the formation of the p-n junction itself, which is faced in Chapter 3. Firstly, the optimization of the emitter configuration (in terms of doping and thickness) is handling by analytic models. This study includes a comparison between the different alternatives for the emitter formation, evaluating the advantages and disadvantages of each approach. After the theoretical design of the emitter, it is defined (through the modeling of the P-in-Si diffusion process) a practical parameter space for the experimental implementation of this emitter configuration. The characterization of these emitters through different analytical tools (i.e. ECV or SIMS) will validate and provide experimental support for the theoretical models. A side effect of the formation of the emitter by P diffusion is the roughening of the Si surface. Accordingly, once the p-n junction is formed, it is necessary to ensure that the Si surface is smooth enough and clean for subsequent phases. Indeed, the roughness of the Si must be carefully controlled since it will be the basis for the epitaxial growth. Accordingly, after quantifying (experimentally and by theoretical models) the impact of the phosphorus on the silicon surface morphology, different alternatives for the recovery of the surface are proposed in order to achieve a sub-nanometer roughness which does not endanger the quality of the incoming III-V layers. Moving a step further in the development of the Ill-V/Si structure implies to address the challenges associated to the GaP on Si nucleation. On the one hand, this layer will provide surface passivation to the emitter. In this sense, the growth of the III-V layer must be homogeneous and continuous so the Si emitter gets fully passivated, providing a minimal surface recombination velocity at the interface. On the other hand, the growth should be such that the appearance of typical defects related to the growth of a polar layer on a non-polar substrate is minimized. Chapter 5 includes an exhaustive study of the GaP on Si nucleation process, exploring different nucleation routines for achieving a high morphological and structural quality, which will be characterized by means of different microscopy techniques. Finally, an extensive study of the photovoltaic properties of the bottom cell and its evolution during key phases in the fabrication of a MOCVD-grown III-V-on-Si epitaxial structure (i.e. the formation of the bottom cell; and the growth of III-V layers) will be presented in the last part of this thesis. This study was conducted in collaboration with The Ohio State University, who has extensive experience in the growth of III-V materials on silicon. This thesis concludes by highlighting the overall conclusions of the presented work and proposing different lines of work to be undertaken in the future.

Induction of ovulation in rabbit does using purified nerve growth factor and camel seminal plasma

The presence of an ovulation-inducing factor (OIF) in the seminal plasma (SP) of several species with spontaneous and induced ovulation, including the rabbit, has been documented. Recent studies have demonstrated that the OIF in the SP of camels (SPCAM) is a nerve growth factor (β-NGF). The aim of this study was to determine if purified β-NGF from mouse submandibular glands or SPCAM could provoke ovulation induction in the rabbit doe. A total of 35 females were synchronized with 25 IU of equine chorionic gonadotropin (Serigan, Laboratorios Ovejero, Spain) and allocated into 4 groups. Forty-eight hours later (Day 0), does were given a single dose (IM) of 1 mL of saline solution (SS; n = 8); 1 mL of gonadorelin (GnRH; Inducel, Laboratorios Ovejero, Spain; n = 9); 24 µg of β-NGF (2.5S-NGF; Promega, USA; n = 10); or 1 mL of centrifuged raw camel SP (SPCAM; 127 pg mL–1 NGF; n = 8). After treatment, an empty catheter was introduced through the vagina to simulate the nervous/mechanical stimulus of coitus (4 animals per group). Plasma LH concentrations were determined in blood samples taken 30 min before treatment and at 0, 30, 60, 90, and 120 min after injection. Progesterone concentrations were assessed at 0 and 120 min and every 2 days until Day 6 after treatment. Concentrations of β-NGF in camel SP and hormone determinations were made by enzyme immunoassay. Ovulation rate (OR) was determined after euthanasia on Day 7.

Photodynamic therapy results in induction of WAF1/CIP1/P21 leading to cell cycle arrest and apoptosis

Photodynamic therapy (PDT) is a promising new modality that utilizes a combination of a photosensitizing chemical and visible light for the management of a variety of solid malignancies. The mechanism of PDT-mediated cell killing is not well defined. We investigated the involvement of cell cycle regulatory events during silicon phthalocyanine (Pc4)-PDT-mediated apoptosis in human epidermoid carcinoma cells A431. PDT resulted in apoptosis, inhibition of cell growth, and G0-G1 phase arrest of the cell cycle, in a time-dependent fashion. Western blot analysis revealed that PDT results in an induction of the cyclin kinase inhibitor WAF1/CIP1/p21, and a down-regulation of cyclin D1 and cyclin E, and their catalytic subunits cyclin-dependent kinase (cdk) 2 and cdk6. The treatment also resulted in a decrease in kinase activities associated with all the cdks and cyclins examined. PDT also resulted in (i) an increase in the binding of cyclin D1 and cdk6 toward WAF1/CIP1/p21, and (ii) a decrease in the binding of cyclin D1 toward cdk2 and cdk6. The binding of cyclin E and cdk2 toward WAF1/CIP1/p21, and of cyclin E toward cdk2 did not change by the treatment. These data suggest that PDT-mediated induction of WAF1/CIP1/p21 results in an imposition of artificial checkpoint at G1 → S transition thereby resulting in an arrest of cells in G0-G1 phase of the cell cycle through inhibition in the cdk2, cdk6, cyclin D1, and cyclin E. We suggest that this arrest is an irreversible process and the cells, unable to repair the damages, ultimately undergo apoptosis.

Inactivation of DNA proofreading obviates the need for SOS induction in frameshift mutagenesis

Translesion synthesis at replication-blocking lesions requires the induction of proteins that are controlled by the SOS system in Escherichia coli. Of the proteins identified so far, UmuD′, UmuC, and RecA* were shown to facilitate replication across UV-light-induced lesions, yielding both error-free and mutagenic translesion-synthesis products. Similar to UV lesions, N-2-acetylaminofluorene (AAF), a chemical carcinogen that forms covalent adducts at the C8 position of guanine residues, is a strong replication-blocking lesion. Frameshift mutations are induced efficiently by AAF adducts when located within short repetitive sequences in a two-step mechanism; AAF adducts incorporate a cytosine across from the lesion and then form a primer-template misaligned intermediate that, upon elongation, yields frameshift mutations. Recently, we have shown that although elongation from the nonslipped intermediate depends on functional umuDC+ gene products, elongation from the slipped intermediate is umuDC+-independent but requires another, as yet biochemically uncharacterized, SOS function. We now show that in DNA Polymerase III-proofreading mutant strains (dnaQ49 and mutD5 strains), elongation from the slipped intermediate is highly efficient in the absence of SOS induction—in contrast to elongation from the nonslipped intermediate, which still requires UmuDC functions.

Estimating the probability of initiated cell death before tumor induction

The effects of cell toxicity are known to be inherent in carcinogenesis induced by radiation or chemical carcinogens. The event of cell death precludes tumor induction from occurring. A long standing problem is to estimate the proportion of initiated cells that die before tumor induction. No experimental techniques are currently available for directly gauging the rate of cell death over extended periods of time. The obstacle can be surmounted by newly developed theoretical methods of carcinogenesis modeling. In this paper, we apply such methods to published data on multiple lung tumors in mice receiving different schedules of urethane. Bioassays of this type play an important role in testing environmental chemicals for carcinogenic activity. Our estimates for urethane-induced carcinogenesis show that, unexpectedly, many initiated cells die early in the course of tumor promotion. We present numerical estimates for the probability of initiated cell death for different schedules (and doses) of urethane administration.

Repeated, but not acute, stress suppresses inflammatory plasma extravasation

Clinical findings suggest that inflammatory disease symptoms are aggravated by ongoing, repeated stress, but not by acute stress. We hypothesized that, compared with single acute stressors, chronic repeated stress may engage different physiological mechanisms that exert qualitatively different effects on the inflammatory response. Because inhibition of plasma extravasation, a critical component of the inflammatory response, has been associated with increased disease severity in experimental arthritis, we tested for a potential repeated stress-induced inhibition of plasma extravasation. Repeated, but not single, exposures to restraint stress produced a profound inhibition of bradykinin-induced synovial plasma extravasation in the rat. Experiments examining the mechanism of inhibition showed that the effect of repeated stress was blocked by adrenalectomy, but not by adrenal medullae denervation, suggesting that the adrenal cortex mediates this effect. Consistent with known effects of stress and with mediation by the adrenal cortex, restraint stress evoked repeated transient elevations of plasma corticosterone levels. This elevated corticosterone was necessary and sufficient to produce inhibition of plasma extravasation because the stress-induced inhibition was blocked by preventing corticosterone synthesis and, conversely, induction of repeated transient elevations in plasma corticosterone levels mimicked the effects of repeated stress. These data suggest that repetition of a mild stressor can induce changes in the physiological state of the animal that enable a previously innocuous stressor to inhibit the inflammatory response. These findings provide a potential explanation for the clinical association between repeated stress and aggravation of inflammatory disease symptoms and provide a model for study of the biological mechanisms underlying the stress-induced aggravation of chronic inflammatory diseases.

In situ synthesis of β-glucan microfibrils on tobacco plasma membrane sheets

A major concern in plant morphogenesis is whether cortical microtubules are responsible for the arrangement and action of β-glucan synthases in the plasma membrane. We prepared isolated plasma membrane sheets with cortical microtubules attached and tested whether β-glucan synthases penetrated through the membrane to form microfibrils and whether these synthases moved in the fluid membrane along the cortical microtubules. This technique enabled us to examine synthesis of β-glucan as a fiber with a two-dimensional structure. The synthesis of β-glucan microfibrils was directed in arrays by cortical microtubules at many loci on the membrane sheets. The microfibrils were mainly arranged along the microtubules, but the distribution of microfibrils was not always parallel to that of the microtubules. The rate of β-glucan elongation as determined directly on the exoplasmic surface was 620 nm per min. When the assembly of microtubules was disrupted by treatment with propyzamide, the β-glucans were not deposited in arrays but in masses. This finding shows that the arrayed cortical microtubules are not required for β-glucan synthesis but are required for the formation of arranged microfibrils on the membrane sheet.

Chemical synthesis of the precursor molecule of the Aequorea green fluorescent protein, subsequent folding, and development of fluorescence

The present paper describes the total chemical synthesis of the precursor molecule of the Aequorea green fluorescent protein (GFP). The molecule is made up of 238 amino acid residues in a single polypeptide chain and is nonfluorescent. To carry out the synthesis, a procedure, first described in 1981 for the synthesis of complex peptides, was used. The procedure is based on performing segment condensation reactions in solution while providing maximum protection to the segment. The effectiveness of the procedure has been demonstrated by the synthesis of various biologically active peptides and small proteins, such as human angiogenin, a 123-residue protein analogue of ribonuclease A, human midkine, a 121-residue protein, and pleiotrophin, a 136-residue protein analogue of midkine. The GFP precursor molecule was synthesized from 26 fully protected segments in solution, and the final 238-residue peptide was treated with anhydrous hydrogen fluoride to obtain the precursor molecule of GFP containing two Cys(acetamidomethyl) residues. After removal of the acetamidomethyl groups, the product was dissolved in 0.1 M Tris⋅HCl buffer (pH 8.0) in the presence of DTT. After several hours at room temperature, the solution began to emit a green fluorescence (λmax = 509 nm) under near-UV light. Both fluorescence excitation and fluorescence emission spectra were measured and were found to have the same shape and maxima as those reported for native GFP. The present results demonstrate the utility of the segment condensation procedure in synthesizing large protein molecules such as GFP. The result also provides evidence that the formation of the chromophore in GFP is not dependent on any external cofactor.