Lipid-amphotericin B complex structure in solution: A possible first step in the aggregation process in cell membranes

The interactions between the polyene antibiotic amphotericin B with dipalmitoylphosphatidylcholine were investigated in vesicles (using circular dichroism) and in chloroform solution (using circular dichroism and IH, I3C, and 31P nuclear magnetic resonance). The results show that amphotericin B readily aggregates in vesicles and that the extent of aggregation depends on the 1ipid:drug concentration ratio. Introduction of sterol molecules into the membrane hastens the process of aggregation of amphotericin B. In chloroform solutions amphotericin B strongly interacts with phospholipid molecules to form a stoichiometric complex. The results suggest that there are interactions between the conjugated heptene stretch of amphotericin B and the methylene groups of lipid acyl chains, while the sugar moiety interacts with the phosphate head group by the formation of a hydrogen bond. A model is proposed for the lipid-amphotericin B complex, in which amphotericin B interacts equally well with the two lipid acyl chains, forming a 1:l complex.

Nucleotide sequence of the first cosmid from the Mycobacterium leprae genome project: structure and function of the Rif-Str regions

The nucleotide sequence of cosmid B1790, carrying the Rif-Str regions of the Mycobacterium leprae chromosome, has been determined. Twelve open reading frames were identified in the 36716bp sequence, representing 40% of the coding capacity. Five ribosomal proteins, two elongation factors and the β and β'subunits of RNA polymerase have been characterized and two novel genes were found. One of these encodes a member of the so-called ABC family of ATP-binding proteins while the other appears to encode an enzyme involved in repairing genomic lesions caused by free radicals. This finding may well be significant as M. leprae, an intracellular pathogen, lives within macrophages.

Role of oestradiol-17β in the regulation of synthesis and secretion of human chorionic gonadotrophin by first trimester human placenta

Inhibition of aromatase, a key enzyme in the biosynthesis of oestradiol-17 beta, by the addition of 1,4,6-androstatrien-3,17-dione resulted in a significant increase in the levels of immunoreactive human chorionic gonadotrophin (hCG) in the medium and tissue. This increase was partially reversed by the simultaneous addition of oestradiol-17 beta. These effects on the levels of immunoreactive hCG were also reflected by the increased levels of mRNA specific for the alpha and beta subunits of hCG following the addition of the aromatase inhibitor. However, addition of tamoxifen resulted in a drastic decrease in the levels of both the messages. Based on these results, it is suggested that the synthesis of hCG is negatively modulated by oestradiol-17 beta in the human placenta.

Size dependence of the bulk modulus of semiconductor nanocrystals from first-principles calculations

The variation in the bulk modulus of semiconductor nanoparticles has been studied within first-principles electronic-structure calculations using the local density approximation (LDA) for the exchange correlation. Quantum Monte Carlo calculations carried out for a silicon nanocrystal Si87H76 provided reasonable agreement with the LDA results. An enhancement was observed in the bulk modulus as the size of the nanoparticle was decreased, with modest enhancements being predicted for the largest nanoparticles studied here, a size just accessible in experiments. To access larger sizes, we fit our calculated bulk moduli to the same empirical law for all materials, the asymptote of which is the bulk value of the modulus. This was found to be within 2-10% of the independently calculated value. The origin of the enhancement has been discussed in terms of Cohen's empirical law M.L. Cohen, Phys. Rev. B 32, 7988 (1985)] as well as other possible scenarios.

Edge Stabilities of Hexagonal Boron Nitride Nanoribbons: A First-Principles Study

We investigate the comparative stability of sp(2) bonded planar hexagonal boron nitride (h-BN) nanoribbon (BNNR) edges, using first principles calculations. We find that the pristine armchair edges have the highest degree of stability. Pristine zigzag edges are metastable, favoring planar reconstructions in the form of 5-7 rings] that minimizes the energy. Our investigation further reveals that the pristine zigzag edges can be stabilized against 5-7 reconstructions by passivating the dangling bonds at the edges by other elements, such as hydrogen (H) atoms. Electronic and magnetic properties of nanoribbons depend on the edge shapes and are strongly affected by edge reconstructions.

First total synthesis of (±)-myltayl-4(12)-ene and single-crystal X-ray structure of exo-12-normyltaylan-4-yl 4-nitrobenzoate

The total synthesis of the unusual sesquiterpene (+/-)-myltayl-4(12)-ene 3 starting from the readily available cyclogeraniol 5 and the single-crystal X-ray structure of the 4-nitrobenzoate 12 of the noralcohol 11 are described.

An investigation of the first-order spin-state transition in Fe(Phen)(2)(NCS)(2) EXAFS and infrared spectroscopy

The structure of Fe(Phen)(2)(NCS)(2) has been examined across the first-order spin-state transition by EXAFS with full multiple scattering analysis. The EXAFS data at 298 K can be satisfactorily assigned to the high-spin state, but the analysis of the low-temperature data at 90 K is not entirely unequivocal, although consistent with the predominant presence of the low-spin state. That some proportion of the high-spin state remains at low temperatures, well below the first-order transition, is clearly evidenced in the infrared spectra, suggesting possible sublattice ordering.

The unusual formation of methyl alpha-(5,6-dimethoxycarbonyl-2,3-dimethoxyazepin-7-ylidene)-alpha-[5-methoxycarbonyl-2,3-dimethoxypyrid-6-yl)acetate during the pyrolysis of ''azido-meta-hemipinate'' - First example of a reaction involving a concomitant ring expansion and ring extrusion

he ortho methoxycarbonyl substituent constitutes a sole exception in the ring closure reactions of ortho substituted aryl azides, as it provides no rate acceleration to this reaction. Pyrolysis of ''azido-meta-hemipinate'', an aryl azide containing such a substituent, led us to the title compound, a new azepinylidenepyridylacetic ester, whose structure has been established unambiguously by a single crystal X-ray diffraction study. This is the first report of a reaction involving both a ring expansion to an azaheptafulvalene and a ring extrusion to a pyridyl ring residue.

First-order hyperpolarizabilities and pKa of weak organic acids in protic solvents are lineraly related

Second-order nonlinearities (beta) of five weak organic acids in protic solvents have been measured by the double-quantum Rayleigh scattering (DRS) technique. beta is found to bear a linear relationship to the pK(a) of these compounds in those solvents. A direct implication of this observation is that the DRS technique can be used to determine the pK(a) of weak organic acids in any solvent.

First-order hyperpolarizabilities of octupolar aromatic molecules: symmetrically substituted triazines

The first hyperpolarizabilities of some symmetrically substituted triazines have been measured and compared with those of the corresponding symmetrically substituted benzenes. The octupolar triazines have higher quadratic polarizabilities than the corresponding octupolar benzenes. The triazine ring seems to be a better central acceptor than the benzene ring, but if it acts as a donor as in sym-triphenyl triazine, the nonlinearity improves further.

First-Order Hyperpolarizabilities of Sulfophthalein Dyes

In this paper we report the first hyperpolarizabilities (beta) of 12, sulfophthalein dyes. Since these dyes are ionic in nature, their second-order nonlinearities were measured by the hyper-Rayleigh scattering technique in solution. The measured beta values are large and highly solvent dependent. Inclusion of solvent polarity in ab initio estimates of static second-order polarizability does not fully account for the experimental beta values. Contributions from the dissociated forms of the dye in different solvents seem to play an important role in enhancing beta in these systems.

Further studies on Norrish type II reactions including a reaction in the first excited singlet state and cyclization of 1:4 biradicals

The Norrish type II processes of methyl-2,2-dimethyl- cyclopropyl ketone, alpha-alkoxy acetones and alkyl pyruvates have been examined using the AM1 semi-empirical molecular orbital method with complete geometry optimization at the partial configuration interaction level in the restricted Hartree-Fock (RHF) frame. The results reveal that the methyl-substituted cyclopropyl ketone has a constrained geometry favourable for hydrogen abstraction from the gamma-position relative to the carbonyl group in the excited singlet state. The presence of the ether oxygen atom in the beta-position relative to the carbonyl group in alkoxy acetones and alkyl pyruvates leads to increased reactivity relative to alkyl monoketones and diketones respectively. The cyclization of 1:4 biradicals has been studied in the unrestricted Hartree-Fock (UHF) frame, and the results reveal that the 1:4 biradical derived from alkoxy acetones readily cyclizes to form oxetanols. On the other hand, in the 1:4 biradicals derived from methyl-substituted cyclopropyl ketone, the three-membered ring breaks readily to form an enol intermediate. Delocalization of an odd electron in 1:4 biradicals derived from alkyl pyruvates is thought to make cyclization difficult.