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Prasad L Polavarapu

Publications and source records attributed to Prasad L Polavarapu.

32 records · Page 2Linked to original sources
PubMed2005

Film techniques for vibrational circular dichroism measurements.

Vibrational circular dichroism (VCD) spectra in the 1800-900 cm(-1) region have been measured recently for protein films and for carbohydrate films prepared from aqueous solutions. In addition to the advantages of avoiding the interference from strong infrared absorption of water at 1650 cm(-1) and requiring a two-orders-of-magnitude-smaller amount of sample for film measurements (than those required for the same in water solutions), the VCD bands in film measurements are found to be the same as those in solution measurements and to be independent of film orientation. This latter observation indicated the possibility for routine VCD measurements in the film state. In this manuscript we report different film techniques for VCD measurements. These techniques include preparation of supported films, free-standing films, film-on-film, and facilitation of film formation for substances that otherwise do not form amorphous films. The experimental data for films prepared by each of these techniques are presented and the relative merits of these different methods are discussed.

Absorption↗
PubMed2004

Vibrational circular dichroism of carbohydrate films formed from aqueous solutions.

Vibrational circular dichroism (VCD) spectra in the entire 2000-900 cm(-1) region have been recorded, for the first time, for films of carbohydrates prepared from aqueous solutions. Eight different carbohydrates, alpha-D-glucopyranosyl-(1-->4)-D-glucose, cyclomaltohexaose, alpha-D-glucopyranosyl alpha-D-glucopyranoside, beta-D-glucopyranosyl-(1-->6)-D-glucose, beta-D-glucopyranosyl-(1-->4)-D-glucose, D-glucose, and both enantiomers of 6-deoxygalactose and of allose, were investigated. The VCD spectra obtained for films are found to be identical to the corresponding spectra obtained for aqueous solutions of carbohydrates. These measurements demonstrate several advantages of significant importance. The strong infrared absorption of water has prevented, in the past, the pursuit for routine applications of VCD in determining the structures of carbohydrates in aqueous solutions. This limitation is not present for film studies because water solvent is removed in the process of preparing the films. Also, strong infrared absorption of water at 1650 cm(-1) requires the use of very short-pathlength (6 microm) cells for measurements on aqueous solutions. This requirement and concomitant inconveniences (such as laborious assembling of a demountable liquid cell or purchasing an expensive variable pathlength liquid cell) have been eliminated for film measurements. The removal of interfering water absorption in film studies resulted in higher light throughput and better signal-to-noise ratios for VCD measurements. Another point of significance is that the amount of carbohydrate sample required for VCD measurements on films is approximately one to two orders of magnitude smaller than that required for corresponding VCD measurements on aqueous solutions. Since carbohydrate samples can now be studied as films, VCD spectroscopy becomes much more broadly applicable for carbohydrates than previously believed. The present work, in combination with other film measurements in our laboratory, indicate that VCD studies on films can be used more generally, providing a convenient and powerful approach for probing structural information for biologically important compounds.

Carbohydrate Conformation↗
PubMed2004

Vibrational circular dichroism spectra of protein films: thermal denaturation of bovine serum albumin.

Vibrational circular dichroism (VCD) spectroscopy has been used for the first time to investigate the thermal denaturation of proteins in H(2)O solutions. Films prepared from heated aqueous solutions were used for these investigations. A well-known alpha-helical protein, bovine serum albumin (BSA), is used for this first study. Both VCD and infrared absorption results obtained for BSA films indicate that the heat treatment of BSA induces significant amounts of beta-sheet structure and that the denaturation process is irreversible. To verify the irreversible nature of thermal denaturation, optical rotation was also measured as a function of temperature in both heating and cooling cycles. These results also indicate that thermal denaturation of BSA in solution is irreversible. This study establishes the usefulness of films for VCD investigations and offers new avenues for VCD studies on biologically important systems.

Animals↗
PubMed2004

Vibrational circular dichroism of protein films.

Vibrational circular dichroism (VCD) spectra in the 1800-1400 cm(-)(1) region have been measured for the first time for protein films prepared from aqueous buffer solutions. These measurements demonstrate several advantages of significant importance. First, the interference from infrared absorption of water in the amide I region, which is a serious limitation for measurements in water solutions, is eliminated. Second, the amounts of protein samples required for VCD measurements on films are approximately 2 orders of magnitude smaller than those required for the same in water solutions. In addition, the amide I absorption and VCD bands of protein films are found to be independent of film orientation. Furthermore, characteristic VCD patterns have been observed for protein films whose secondary structure is dominated by alpha-helix, beta-sheet, and alpha + beta combinations. These results demonstrate that VCD can be used to study the structure of proteins in the film state.

Animals↗
PubMed2004

A study of the conformations of valinomycin in solution phase.

Vibrational absorption and vibrational circular dichroism (VCD) spectra of valinomycin are measured, in different solvents, in the ester and amide carbonyl stretching regions. The influence of cations, namely Li(+), Na(+), K(+), and Cs(+), in methanol-d(4) solvent is also investigated. Ab initio quantum mechanical calculations using density functional theory and 6-31G* basis set are used to predict the absorption and VCD spectra. A bracelet-type structure for valinomycin that reproduces the experimental absorption and VCD spectra in inert solvents is identified. For the structure of valinomycin in polar solvents, a propeller-type structure was optimized, but further investigations are required to confirm this structure. A symmetric octahedral environment for the ester carbonyl groups in the valinomycin-K(+) complex is supported by the experimental VCD spectra. The results obtained in the present study demonstrate that even for large macrocyclic peptides, such as valinomycin, VCD can be used as an independent structural tool for the study of conformations in solution.

Amino Acid Sequence↗
PubMed2004

Structure of A beta(25-35) peptide in different environments.

The fragment A beta(25-35) of the Alzheimer's amyloid beta-peptide, like its full-length peptide A beta(1-42), has shown neurotoxic activities in cultured cells. The conformational preference of this important peptide is examined here in solution, gel, and film states (obtained with organic and aqueous solvents) by vibrational circular dichroism spectroscopy for the first time. For comparative studies, vibrational absorption and electronic circular dichroism measurements were also carried out under identical conditions. The peptide was found to adopt beta-sheet and beta-turn structures, with their relative proportions changing in different environments.

Amyloid beta-Peptides↗
PubMed2003

Conformational analysis of melittin in solution phase: vibrational circular dichroism study.

The vibrational absorption and vibrational circular dichroism (VCD) spectra of melittin in D(2)O solutions at different pH values, different salt concentrations, or different 2,2,2-trifluoroethanol (TFE) concentrations are recorded in the amide I' (1850-1600 cm(-1)) region. Two models are used to simulate this peptide in different conditions, and a coupled oscillator program is used to obtain the calculated absorption and VCD spectra. This study indicates that melittin adopts a mixed structure in D(2)O solution at low pH, low salt concentration, or low TFE concentration. With an increase in pH, salt concentration, or TFE concentration, the structure changes to alpha-helix and further increases lead to aggregation. These results demonstrate the versatility of VCD in probing the conformations of peptides under different environmental perturbations.

Circular Dichroism↗
PubMed2003

Intrinsic rotation and molecular structure.

The intrinsic rotation, limiting value of specific rotation at zero concentration, of (R)-(-)-epichlorohydrin was measured in four different solvents, CH(3)OH, CH(2)Cl(2), CHCl(3) and CCl(4). It was found that the sign of rotation in CH(3)OH, and CH(2)Cl(2) solvents is opposite to that in CCl(4). The intrinsic rotation in CHCl(3) is close to zero. This observed pattern was explained using density functional calculations of specific rotation using very large basis sets. It was found that the g-I and g-II conformations of epichlorohydrin have nearly the same magnitude of specific rotation but with opposite sign. When these two conformations have equal populations, as in CHCl(3) solvent, the net rotation is close to zero. When g-II conformation dominates, as in CCl(4) solvent, the observed sign of rotation will be opposite to that when g-I conformation dominates, as in CH(2)Cl(2) and CH(3)OH solvents. A combination of intrinsic rotation measurement with density functional prediction of specific rotation is demonstrated to be a practical method for determining the structures of molecules.

Journal Article↗
PubMed2002

Resolution of tert-butyl-1-(2-methyinaphthyl)phosphine oxide using selectors identified from a chemical combinatorial library.

Resolution of racemic tert-butyl-1-(2-methylnaphthyl)-phosphine oxide 1, a chiral phosphorus compound, was achieved using selectors developed from a small peptide library. Separation factors as high as 3.2 were observed. The library consists of 81 peptide-based potential chiral selectors on polymeric synthesis resins. The linker needed to immobilize the identified chiral selectors onto silica gel proved important in the chiral separation; a longer linker provided a significantly higher separation factor in this study.

Chromatography, High Pressure Liquid↗
PubMed2002

Absolute configuration of tert-butyl-1-(2-methylnaphthyl)phosphine oxide.

The enantiomers of tert-butyl-1-(2-methylnaphthyl)phosphine oxide 1 have been separated using a homemade HPLC column and an analytical gradient system. Vibrational absorption and circular dichroism spectra for both enantiomers have been measured in CD2Cl2 and CH2Cl2 solutions in the 2000-900 cm(-1) region. The fully relaxed potential energy surface of (S)-tert-butyl-1-(2-methylnaphthyl)phosphine oxide, obtained using the B3LYP functional with a 6-31G basis set, indicated two stable conformers with their populations in a approximately 2:1 ratio. The vibrational absorption and VCD spectra are predicted for these two conformers using the B3LYP functional with a 6-31G basis set. The comparison of predicted and experimental spectra indicated that (+)-tert-butyl-1-(2-methylnaphthyl)phosphine oxide is in the (S)-configuration. This assignment is supported by the ab initio prediction of positive optical rotation for the most stable conformer with an (S)-configuration and the nonequivalence sense of the tert-butyl group chemical shift observed in the 1H NMR spectrum of this enantiomer measured in the presence of (+)-(S)-mandelic acid as a chiral solvating agent.

Chromatography, High Pressure Liquid↗
PubMed2002

Absolute configuration and conformational analysis of a degradation product of inhalation anesthetic Sevoflurane: A vibrational circular dichroism study.

1,1,1,3,3-pentafluoro-2-(fluoromethoxy)-3-methoxypropane, compound B, is a product obtained in the degradation of the anesthetic Sevoflurane. Enantiopure (+)-B was investigated using vibrational circular dichroism (VCD). Experimental absorption and VCD spectra of (+)-B in CDCl(3) solution in the 2,000-900 cm(-1) region are compared with the ab initio predictions of absorption and VCD spectra obtained from density functional theory using B3LYP/6-31G* basis set for different conformers of (S)-1,1,1,3,3-pentafluoro-2-(fluoromethoxy)-3-methoxypropane. This comparison indicates that (+)-B is of the (S)-configuration in CDCl(3) solution, in agreement with previous literature results. Our results also indicate that this compound adopts six predominant conformations in CDCl(3) solution.

Anesthetics, Inhalation↗
PubMed2002

Optical rotation: recent advances in determining the absolute configuration.

The field of optical rotations is currently undergoing a renaissance, which is a direct result of the advances in quantum mechanics and the availability of faster desktop computers. In the last few years, the field of optical rotations has taken an important role for determining the three-dimensional molecular structure (absolute configuration and conformations) with confidence. The purpose of this review is to present the latest advances in this area so that practicing chemists can utilize them in their respective areas of research and development.

Journal Article↗
PubMedDate not supplied

Circular dichroism spectra of regioregular poly[3 [(S)-2-methylbutyl]-thiophene] and of poly[3,4-di[(S)-2-methylbutyl]-thiophene].

Ultraviolet absorption and Circular Dichroism spectra in the region 300-700 nm for highly regioregular samples of Poly[3[(S)-2-methylbutyl]-thiophene] are reported for solutions in good and poor solvents, at variable temperature, and for films in various conditions. In the latter case novel CD data up to the melting temperature provide some hints for the self-assembling process towards supramolecular organization. UV-visible and CD data are also reported on the related polymer Poly[3,4-di[(S)-2-methylbutyl]-thiophene].

Journal Article↗