Ultracold collisions between two light indistinguishable diatomic molecules: Elastic and rotational energy transfer in HD plus HD

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Factors controlling the competition among rotational and vibrational energy transfer channels in glyoxal

The state-to-state transfer of rotational and vibrational energy has been studied for S1 glyoxal (CHOCHO) in collisions with D2, N2, CO and C2H4 using crossed molecular beams. A laser is used to pump glyoxal seeded in He to its S1 zero point level with zero angular momentum about its top axis (K′ = 0). The inelastic scattering to each of at least 26 S1 glyoxal rotational and rovibrational levels is monitored by dispersed S1–S0 fluorescence. Various collision partners are chosen to investigate the relative influences of reduced mass and the collision pair interaction potential on the competition among the energy transfer channels. When the data are combined with that obtained previously from other collision partners whose masses range from 2 to 84 amu, it is seen that the channel competition is controlled primarily by the kinematics of the collisional interaction. Variations in the intermolecular potential play strictly a secondary role.

A quasiclassical trajectory study of the OH plus SO reaction: The role of rotational energy

A full dimensional quasiclassical trajectory study of the OH+SO reaction is presented with the aim of investigating the role of the reactants rotational energy in the reactivity. Different energetic combinations with one and both reactants rotationally excited are studied. A passive method is used to correct zero-point-energy leakage in the classical calculations. The reactive cross sections, for each combination, are calculated and fitted to a capturelike model combined with a factor accounting for recrossing effects. Reactivity decreases as rotational energy is increased in any of both reactants. This fact provides a theoretical support for the experimental dependence of the rate constant on temperature.

Intermolecular energy transfer and photostability of luminescence-tuneable multicolour PMMA films doped with lanthanide-beta-diketonate complexes

It is reported in this work the preparation, characterisation and photoluminescence study of poly(methylmethacrylate) (PMMA) thin films co-doped with [Eu(tta)(3)(H(2)O)(2)] and [Tb(acac)(3)(H(2)O)(3)] complexes. Both the composition and excitation wavelength may be tailored to fine-tune the emission properties of these Ln(3+)-beta-diketonate doped polymer films, exhibiting green and red primary colours, as well as intermediate colours. In addition to the ligand-Ln(3+) intramolecular energy transfer, it is observed an unprecedented intermolecular energy transfer process from the (5)D(4) emitting level of the Tb(3+) ion to the excited triplet state T(1) of the tta ligand coordinated to the Eu(3+) ion. The PMMA polymer matrix acts as a co-sensitizer and enhances the overall luminescence intensity of the polymer films. Furthermore, it provides considerable UV protection for the luminescent species and improves the photostability of the doped system.

Energy transfer upconversion determination by thermal-lens and Z-scan techniques in Nd(3+)-doped laser materials

The Z-scan and thermal-lens techniques have been used to obtain the energy transfer upconversion parameter in Nd(3+)-doped materials. A comparison between these methods is done, showing that they are independent and provide similar results. Moreover, the advantages and applicability of each one are also discussed. The results point to these approaches as valuable alternative methods because of their sensitivity, which allows measurements to be performed in a pump-power regime without causing damage to the investigated material. (C) 2009 Optical Society of America

Photoinduced electron transfer and electronic energy transfer in naphthyl-appended cyclams

A series of novel macrocyclic tetraaza ligands that incorporate a naphthalene moiety as a photoactive chromophore have been prepared and structurally characterized as their Cu(II) complexes. Variable-temperature photophysical studies have concluded that the luminescence quenching evident in the Cu(H) complexes is due to intramolecular electronic energy transfer (EET). In their free-base forms, these ligands undergo reductive luminescence quenching via photoinduced electron transfer (PET) reactions, with proximate amine lone pairs acting as electron donors. Consequently, the emission behavior can be modulated by variations in pH and/or the presence of other Lewis acids such as Zn(H).

Intramolecular electronic energy transfer in bichromophoric macrocyclic complexes

Efficient intramolecular electronic energy transfer (EET) has been demonstrated for three novel bichromophoric compounds utilizing a macrocyclic spacer as the bridge between the electronic energy donor and acceptor fragments. As their free base forms, emission from the electronically excited donor is absent and the acceptor emission is reductively quenched via photoinduced oxidation of proximate amine lone pairs. As their Zn(II) complexes, excitation of the donor results in sensitization of the electronic acceptor emission.

Rates of electronic energy transfer in conformationally flexible bichromophoric macrocyclic complexes: a combined experimental and molecular modeling study

Electronic energy transfer (EET) rate constants between a naphthalene donor and anthracene acceptor in [ZnL4a](ClO4)(2) and [ZnL4b](ClO4)(2) were determined by time-resolved fluorescence where L-4a and L-4b are the trans and cis isomers of 6-((anthracen-9-yl-methyl)amino)-6,13-dimethyl-13-((naphthalen-1-yl-methyl)amino)-1,4,8,11-tetraazacyclotetradecane, respectively. These isomers differ in the relative disposition of the appended chromophores with respect to the macrocyclic plane. The trans isomer has an energy transfer rate constant (k(EET)) of 8.7 x 10(8) s(-1), whereas that of the cis isomer is significantly faster (2.3 x 10(9) s(-1)). Molecular modeling was used to determine the likely distribution of conformations in CH3CN solution for these complexes in an attempt to identify any distance or orientation dependency that may account for the differing rate constants observed. The calculated conformational distributions together with analysis by H-1 NMR for the [ZnL4a](2+) trans complex in the common trans-III N-based isomer gave a calculated Forster rate constant close to that observed experimentally. For the [ZnL4b](2+) cis complex, the experimentally determined rate constant may be attributed to a combination of trans-Ill and trans-I N-based isomeric forms of the complex in solution.

Efficient energy transfer from Si clusters to Er3+ in complex silicate glasses

We present an extensive study of the structural and optical emission properties in aluminum silicates and soda-lime silicates codoped with Si nanoclusters (Si-nc) and Er. Si excess of 5 and 15¿at.¿% and Er concentrations ranging from 2×1019 up to 6×1020¿cm¿3 were introduced by ion implantation. Thermal treatments at different temperatures were carried out before and after Er implantation. Structural characterization of the resulting structures was performed to obtain the layer composition and the size distribution of Si clusters. A comprehensive study has been carried out of the light emission as a function of the matrix characteristics, Si and Er contents, excitation wavelength, and power. Er emission at 1540¿nm has been detected in all coimplanted glasses, with similar intensities. We estimated lifetimes ranging from 2.5¿to¿12¿ms (depending on the Er dose and Si excess) and an effective excitation cross section of about 1×10¿17¿cm2 at low fluxes that decreases at high pump power. By quantifying the amount of Er ions excited through Si-nc we find a fraction of 10% of the total Er concentration. Upconversion coefficients of about 3×10¿18¿cm¿3¿s¿1 have been found for soda-lime glasses and one order of magnitude lower in aluminum silicates.

Energy transfer mechanism and Auger effect in Er3+ coupled silicon nanoparticle samples

We report a spectroscopic study about the energy transfer mechanism among silicon nanoparticles (Si-np), both amorphous and crystalline, and Er ions in a silicon dioxide matrix. From infrared spectroscopic analysis, we have determined that the physics of the transfer mechanism does not depend on the Si-np nature, finding a fast (< 200 ns) energy transfer in both cases, while the amorphous nanoclusters reveal a larger transfer efficiency than the nanocrystals. Moreover, the detailed spectroscopic results in the visible range here reported are essential to understand the physics behind the sensitization effect, whose knowledge assumes a crucial role to enhance the transfer rate and possibly employing the material in optical amplifier devices. Joining the experimental data, performed with pulsed and continuous-wave excitation, we develop a model in which the internal intraband recombination within Si-np is competitive with the transfer process via an Auger electron"recycling" effect. Posing a different light on some detrimental mechanism such as Auger processes, our findings clearly recast the role of Si-np in the sensitization scheme, where they are able to excite very efficiently ions in close proximity to their surface. (C) 2010 American Institute of Physics.

Use of phycoerythrin and allophycocyanin for fluorescence resonance energy transfer analyzed by flow cytometry: advantages and limitations.

BACKGROUND: This study validates the use of phycoerythrin (PE) and allophycocyanin (APC) for fluorescence energy transfer (FRET) analyzed by flow cytometry. METHODS: FRET was detected when a pair of antibody conjugates directed against two noncompetitive epitopes on the same CD8alpha chain was used. FRET was also detected between antibody conjugate pairs specific for the two chains of the heterodimeric alpha (4)beta(1) integrin. Similarly, the association of T-cell receptor (TCR) with a soluble antigen ligand was detected by FRET when anti-TCR antibody and MHC class I/peptide complexes (&lt;&lt;tetramers&gt;&gt;) were used. RESULTS: FRET efficiency was always less than 10%, probably because of steric effects associated with the size and structure of PE and APC. Some suggestions are given to take into account this and other effects (e.g., donor and acceptor concentrations) for a better interpretation of FRET results obtained with this pair of fluorochromes. CONCLUSIONS: We conclude that FRET assays can be carried out easily with commercially available antibodies and flow cytometers to study arrays of multimolecular complexes.

Lanthanide Chelates as Donors in Fluorescence Resonance Energy Transfer: Exciting Prospects for Bioaffinity Assay Detection

Fluorescence resonance energy transfer (FRET) is a non-radiative energy transfer from a fluorescent donor molecule to an appropriate acceptor molecule and a commonly used technique to develop homogeneous assays. If the emission spectrum of the donor overlaps with the excitation spectrum of the acceptor, FRET might occur. As a consequence, the emission of the donor is decreased and the emission of the acceptor (if fluorescent) increased. Furthermore, the distance between the donor and the acceptor needs to be short enough, commonly 10-100 Å. Typically, the close proximity between the donor and the acceptor is achieved via bioaffinity interactions e.g. antibody binding antigen. Large variety of donors and acceptors exist. The selection of the donor/acceptor pair should be done not only based on the requirements of FRET but also the performance expectancies and the objectives of the application should be considered. In this study, the exceptional fluorescence properties of the lanthanide chelates were employed to develop two novel homogeneous immunoassays: a non-competitive hapten (estradiol) assay based on a single binder and a dual-parametric total and free PSA assay. In addition, the quenching efficiencies and energy transfer properties of various donor/acceptor pairs were studied. The applied donors were either europium(III) or terbium(III) chelates; whereas several organic dyes (both fluorescent and quenchers) acted as acceptors. First, it was shown that if the interaction between the donor/acceptor complexes is of high quality (e.g. biotin-streptavidin) the fluorescence of the europium(III) chelate could be quenched rather efficiently. Furthermore, the quenching based homogeneous non-competitive assay for estradiol had significantly better sensitivity (~67 times) than a corresponding homogeneous competitive assay using the same assay components. Second, if the acceptors were chosen to emit at the emission minima of the terbium(III) chelate, several acceptor emissions could be measured simultaneously without significant cross-talk from other acceptors. Based on these results, the appropriate acceptors were chosen for the dual-parameter assay. The developed homogeneous dual-parameter assay was able to measure both total and free PSA simultaneously using a simple mix and measure protocol. Correlation of this assay to a heterogeneous single parameter assay was excellent (above 0.99 for both) when spiked human plasma samples were used. However, due to the interference of the sample material, the obtained concentrations were slightly lower with the homogeneous than the heterogeneous assay, especially for the free PSA. To conclude, in this work two novel immunoassay principles were developed, which both are adaptable to other analytes. However, the hapten assay requires a rather good antibody with low dissociation rate and high affinity; whereas the dual-parameter assay principle is applicable whenever two immunometric complexes can form simultaneously, provided that the requirements of FRET are fulfilled.

Frequency-Domain and Wide-Pulse Time-Domain Measurements of Lanthanide Luminescence and Lanthanide-Based Resonance Energy Transfer

Resonance energy transfer (RET) is a non-radiative transfer of the excitation energy from the initially excited luminescent donor to an acceptor. The requirements for the resonance energy transfer are: i) the spectral overlap between the donor emission spectrum and the acceptor absorption spectrum, ii) the close proximity of the donor and the acceptor, and iii) the suitable relative orientations of the donor emission and the acceptor absorption transition dipoles. As a result of the RET process the donor luminescence intensity and the donor lifetime are decreased. If the acceptor is luminescent, a sensitized acceptor emission appears. The rate of RET depends strongly on the donor–acceptor distance (r) and is inversely proportional to r6. The distance dependence of RET is utilized in binding assays. The proximity requirement and the selective detection of the RET-modified emission signal allow homogeneous separation free assays. The term lanthanide-based RET is used when luminescent lanthanide compounds are used as donors. The long luminescence lifetimes, the large Stokes’ shifts and the intense, sharply-spiked emission spectra of the lanthanide donors offer advantages over the conventional organic donor molecules. Both the organic lanthanide chelates and the inorganic up-converting phosphor (UCP) particles have been used as donor labels in the RET based binding assays. In the present work lanthanide luminescence and lanthanide-based resonance energy transfer phenomena were studied. Luminescence lifetime measurements had an essential role in the research. Modular frequency-domain and time-domain luminometers were assembled and used successfully in the lifetime measurements. The frequency-domain luminometer operated in the low frequency domain ( 100 kHz) and utilized a novel dual-phase lock-in detection of the luminescence. One of the studied phenomena was the recently discovered non-overlapping fluorescence resonance energy transfer (nFRET). The studied properties were the distance and temperature dependences of nFRET. The distance dependence was found to deviate from the Förster theory and a clear temperature dependence was observed whereas conventional RET was completely independent of the temperature. Based on the experimental results two thermally activated mechanisms were proposed for the nFRET process. The work with the UCP particles involved the measurement of the luminescence properties of the UCP particles synthesized in our laboratory. The goal of the UCP particle research is to develop UCP donor labels for binding assays. In the present work the effect of the dopant concentrations and the core–shell structure on the total up-conversion luminescence intensity, the red–green emission ratio, and the luminescence lifetime was studied. Also the non-radiative nature of the energy transfer from the UCP particle donors to organic acceptors was demonstrated for the first time in aqueous environment and with a controlled donor–acceptor distance.

Homogeneous assay technologies in drug screening: Quenching Resonance Energy Transfer (QRET) technique

Information gained from the human genome project and improvements in compound synthesizing have increased the number of both therapeutic targets and potential lead compounds. This has evolved a need for better screening techniques to have a capacity to screen number of compound libraries against increasing amount of targets. Radioactivity based assays have been traditionally used in drug screening but the fluorescence based assays have become more popular in high throughput screening (HTS) as they avoid safety and waste problems confronted with radioactivity. In comparison to conventional fluorescence more sensitive detection is obtained with time-resolved luminescence which has increased the popularity of time-resolved fluorescence resonance energy transfer (TR-FRET) based assays. To simplify the current TR-FRET based assay concept the luminometric homogeneous single-label utilizing assay technique, Quenching Resonance Energy Transfer (QRET), was developed. The technique utilizes soluble quencher to quench non-specifically the signal of unbound fraction of lanthanide labeled ligand. One labeling procedure and fewer manipulation steps in the assay concept are saving resources. The QRET technique is suitable for both biochemical and cell-based assays as indicated in four studies:1) ligand screening study of β2 -adrenergic receptor (cell-based), 2) activation study of Gs-/Gi-protein coupled receptors by measuring intracellular concentration of cyclic adenosine monophosphate (cell-based), 3) activation study of G-protein coupled receptors by observing the binding of guanosine-5’-triphosphate (cell membranes), and 4) activation study of small GTP binding protein Ras (biochemical). Signal-to-background ratios were between 2.4 to 10 and coefficient of variation varied from 0.5 to 17% indicating their suitability to HTS use.

From Unspecific Quenching to Specific Signaling: Functional GTPase Assays Utilizing Quenching Resonance Energy Transfer (QRET) Technology

The aim of the work presented in this study was to demonstrate the wide applicability of a single-label quenching resonance energy transfer (QRET) assay based on time-resolved lanthanide luminescence. QRET technology is proximity dependent method utilizing weak and unspecific interaction between soluble quencher molecule and lanthanide chelate. The interaction between quencher and chelate is lost when the ligand binds to its target molecule. The properties of QRET technology are especially useful in high throughput screening (HTS) assays. At the beginning of this study, only end-point type QRET technology was available. To enable efficient study of enzymatic reactions, the QRET technology was further developed to enable measurement of reaction kinetics. This was performed using proteindeoxyribonuclei acid (DNA) interaction as a first tool to monitor reaction kinetics. Later, the QRET was used to study nucleotide exchange reaction kinetics and mutation induced effects to the small GTPase activity. Small GTPases act as a molecular switch shifting between active GTP bound and inactive GDP bound conformation. The possibility of monitoring reaction kinetics using the QRET technology was evaluated using two homogeneous assays: a direct growth factor detection assay and a nucleotide exchange monitoring assay with small GTPases. To complete the list, a heterogeneous assay for monitoring GTP hydrolysis using small GTPases, was developed. All these small GTPase assays could be performed using nanomolar protein concentrations without GTPase pretreatment. The results from these studies demonstrated that QRET technology can be used to monitor reaction kinetics and further enable the possibility to use the same method for screening.