Type II beta-turn conformation of pivaloyl-L-prolyl-alpha-aminoisobutyryl-N-methylamide: Theoretical, spectroscopic, and X-ray studies

Pivaloyl-L-Pro-Aib-N-methylamide has been shown to possess one intramolecular hydrogen bond in (CD3)2SO solution, by 1H-nmr methods, suggesting the existence of beta -turns, with Pro-Aib as the corner residues. Theoretical conformational analysis suggests that Type II beta-turn conformations are about 2 kcal mol-1 more stable than Type III structures. A crystallographic study has established the Type II beta-turn in the solid state. The molecule crystallizes in the space group P21 with a = 5.865 Å, b = 11.421 Å, c = 12.966 Å, beta = 97.55°, and Z = 2. The structure has been refined to a final R value of 0.061. The Type II -turn conformation is stabilized by an intramolecular 4 1 hydrogen bond between the methylamide NH and the pivaloyl CO group. The conformational angles are Pro = -57.8°, Pro = 139.3°, Aib = 61.4°, and Aib = 25.1°. The Type II beta-turn conformation for Pro-Aib in this peptide is compared with the Type III structures observed for the same segment in larger peptides.

EXAFS studies of arsenic chalcogenide glasses

An EXAFS study at the AsK edge of the ternary glasses As2(S, Se)3 and As2(Se, Te)3 and the binary As2S3, As2Se3 and As2Te3 glasses has been carried out. Radial structure functions show that the environment of As in glasses of intermediate compositions is quite different from that in the binary glasses. In the As2(S, Se)3 system, this might arise from chemical disorder in the network while in the As2(Se, Te)3 system increased ionicity could be the cause of this behaviour. Glasses where the constituent atoms are of similar size seem to exhibit fewer peaks in the radial structure function.

XPES studies of oxides of second- and third-row transition metals including rare earths

X-ray photoelectron spectroscopy has been employed to investigate oxides of second- and third-row transition metals, including those of rare earths. Systematics in the spin—orbit splittings and binding energies of core levels of the metals are described. In most of the cases studied, the dependence of the spin—orbit splittings on the atomic number Z is given by the relation ΔE = a(Z - Z0)4, where a is the quantum defect parameter and Z0 is the effective screening. Core-level binding energies are found to increase with the oxidation state of the metal. Most of the core-level binding energies are related to the atomic number Z by the expression E = x(Z - Z0)2, giving rise to linear plots of ln E versus ln Z. Specific features of individual oxides, with respect to satellites, multiplet structure, configuration mixing, and other properties are also discussed. The spectra of PrO2, Pr6O11, TbO2 and Tb4O7 are reported for the first time.

Studies on the reactions of thionyl fluoride

Thionyl fluoride undergoes quantitative oxidation with chloramine-T and reduction with lithium aluminium hydride and sodium borohydride. At elevated temperatures, (>150°C) it reacts with metals such as copper, silver, zinc and lead forming the corresponding metal sulphides, fluorides and sulphur dioxide. With the respective metal oxides, the metal fluorides and sulphur dioxide are formed.

Preparation and catalytic studies of palladium nanoparticles stabilized by dendritic phosphine ligand-functionalized silica

Silica is a prominently utilized heterogeneous metal catalyst support. Functionalization of the silica with poly(ether imine) based dendritic phosphine ligand was conducted, in order to assess the efficacy of the dendritic phosphine in reactions facilitated by a silica supported metal catalyst. The phosphinated poly(ether imine) (PETIM) dendritic ligand was bound covalently to the functionalized silica. For this purpose, the phosphinated dendritic ligand containing an amine at the focal point was synthesized initially. Complexation of the dendritic phosphine functionalized silica with Pd(COD)Cl-2 yielded Pd(II) complex, which was reduced subsequently to Pd(0), by conditioning with EtOH. The Pd metal nanoparticle thus formed was characterized by physical methods, and the spherical nanoparticles were found to have >85% size distribution between 2 nm and 4 nm. The metal nanoparticle was tested as a hydrogenation catalyst of olefins. The catalyst could be recovered and recycled more than 10 times, without a loss in the catalytic efficiency.

Studies on characteristics of lightning generated currents in an interconnected lightning protection system

An in-depth knowledge about the characteristics of lightning generated currents will facilitate evaluation of the interception efficacy of lightning protection systems. In addition, it would aid in extraction of valuable statistics (from measured current data) on local lightning parameters. Incidentally, present day knowledge on characteristics of lightning induced current in typical lightning protection systems is rather limited. This is particularly true with closely interconnected protection systems, like the one employed in Indian Satellite Launch Pad-II. This system is taken as a specific example in the present study. Various aspects suggest that theoretical modelling would be the best possible approach for the intended work. From the survey of pertinent literature, it is concluded that electromagnetic modelling of lightning return-stroke with current source at the channel base is best suited for this study. Numerical electromagnetic code was used for the required electromagnetic field solution and Fourier transform techniques were employed for computing time-domain results. A validation for the numerical modelling is provided by laborator experiments on a reduced scale model of the system. Apart from ascertaining the influence of various parameters, salient characteristics of tower base currents for different kinds of events are deduced. This knowledge can be used in identifying the type of event, as well as its approximate location. A method for estimation of injected stroke current has also been proposed.

1H and 19F NMR relaxation time studies in (NH4)2ZrF6 superionic conductor

1H and 19F spin-lattice relaxation times in polycrystalline diammonium hexafluorozirconate have been measured in the temperature range of 10–400 K to elucidate the molecular motion of both cation and anion. Interesting features such as translational diffusion at higher temperatures, molecular reorientational motion of both cation and anion groups at intermediate temperatures and quantum rotational tunneling of the ammonium group at lower temperatures have been observed. Nuclear magnetic resonance (NMR) relaxation time results correlate well with the NMR second moment and conductivity studies reported earlier.

Circular dichroism and nuclear magnetic resonance studies on the complexation of valinomycin with calcium

Complexation of valinomycin (VM) with the divalent cation Ca2+ in a lipophilic solvent, acetonitrile (CH3-CN), has been studied by using circular dichroism and proton and carbon- 13 nuclear magnetic resonance (‘H NMR and I3C NMR). From analyses of the spectral data, it is concluded that VM forms a 2:l (peptideion-peptide) sandwich complex with Ca2+, at low concentration of VM. At moderate conocentrations of the salt, in addition to the sandwich complex, an equimolar (1:l) complex different from those observed for potassium and sodium is also observed. At very large concentrations of the calcium salt, the data suggested a complex with a conformation similar to that of the free VM in polar solvents. Possible conformations for the sandwich and the equimolar VM-calcium complexes are proposed.

Molecularly adsorbed oxygen on metals: electron spectroscopic studies

Oxygen is shown to adsorb molecularly on gold as well as on Ag and Pt. UV and X-ray photoelectron spectroscopy and Auger electron spectroscopy have been employed to investigate electron states of molecularly adsorbed oxygen.

Immunological studies on nucleic acids and their components.8.Antibodies specific to a deoxyribotrinucleotide sequence

Bovine serum albumin conjugates of two trinucleotides, dpTpTpA and dTpTpAp, were prepared by linking the trinucleotides through their end phosphates by the ‘carbodiimide method’. Antibodies were raised in rabbits by injecting the trinucleotide-bovine serum albumin conjugates. Analysis by double diffusion in agar gel, quantitative precipitin reaction and its inhibition by haptens showed clearly the presence of antibodies specific to the whole trinucleotide molecule. The titre of antibodies obtained by the trinucleotide-rabbit serum albumin conjugates with their respective antisera was approximately the same, indicating that linking the trinucleotide through either 5′ or 3′ phosphate does not have an appreciable effect on the titre of antibodies. The results also demonstrate that the nucleotide(s) away from the carrier protein is more immunodominant than the one linked directly to the protein.

Theoretical studies on the conformation of beta-D-N-acetyl neuraminic acid (sialic acid)

The possible conformations of sialic acid were analysed using semi-empirical potential functions. The solid state conformation has approx. 0.2 kcal/mol higher energy than the minimum energy conformation. These studies suggest that in solution sialic acid may exist preponderantly in two different conformations which differ in the orientation of the terminal hydroxymethyl group of glycerol side-chain. The present model is consistent with 1H- and 13C-NMR data, but differs from the earlier models.

beta.-Turn conformation of N-acetyl-L-prolylglycyl-L-phenylalanine. Crystal structure and solution studies

Pro-Gly segments in peptides and proteins are prone to adopt the 0-turn conformation. This paper reports experimental data for the presence of this conformation in a linear tripeptide N-acetyl-L-prolylglycyl-L-phenylalanineb oth in the solid state and in solution. X-ray diffraction data on the tripeptide crystal show that it exists in the type I1 0-turn conformation. CD and proton NMR data show that this conformation persists in trifluoroethanol and methanol solutions in equilibrium with the nonhydrogen-bonded structures. Isomerization around the acetyl-prolyl bond is seen to take place in dimethyl sulfoxide solutions of the tripeptide.

Studies on serum progesterone levels in Zebu × Holstein heifers during pre- and peripubertal periods

The peripheral serum progesterone levels of six normal Zebu × Holstein heifers (75% Holstein inheritance) during the prepubertal period (−150 days) were low (0.23 ± 0.06 ng/ml). They reached maximal values (0.73 ± 0.06 ng/ml) by −45 days (P<0.05 for progesterone values on −90th vs. −45th days) and thereafter decreased to the base level at the time of puberty. The mean (± SEM) age and body weight at puberty of these six heifers were 720 ± 20.70 days and 260.67 ± 8.82 kg, respectively. The serum progesterone levels remained low (0.38 ± 0.17 ng/ml) during early oestrus (up to 28 h) and gradually increased to 2.3 ± 0.84 ng/ml by the 15th day of the cycle.