Vibrational circular dichroism of gramicidin D in organic solvents

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.

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.

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.

Vibrational circular dichroism of gramicidin D in vesicles and micelles

The vibrational circular dichroism (VCD) and absorption spectra of gramicidin D in three model membranes (dioctadecyldimethylammonium chloride vesicles, dimyristoyl-phosphatidylcholine vesicles, and sodium dodecyl sulfate micelles) are presented. The absorption and VCD spectra suggest that the stable gramicidin D conformation in the model membranes is different from those in organic solvents. The presence of cations does not change the membrane-bound conformation of gramicidin D.

Noncovalent interactions of peptides with porphyrins in aqueous solution: conformational study using vibrational CD spectroscopy

Noncovalent interactions of poly(L-lysine) (PL), oligopeptides L-lysyl-L-alanyl-L-alanine and (L-lysyl-L-alanyl-L-alanine)(2) with meso-tetrakis(4-sulfonatophenyl)porphine (TPPS), and poly(L-glutamic acid) (PLGA) with meso-tetrakis(1-methyl-4-pyridyl)porphine tetra-p-tosylate (TMPyP) in aqueous solutions have been studied using combination of spectroscopic methods: Vibrational circular dichroism (VCD) spectroscopy in the mid-infrared region provides a direct information on conformational changes of the polypeptides and oligopeptides caused by interactions with porphyrins; ultraviolet-visible absorption, fluorescence, and electronic circular dichroism (ECD) reveal the aggregation characterization of the porphyrin part of the complexes. Interactions of TPPS with tripeptide, hexapeptide, and PL containing about ten amino acid residues in the molecular chain are accompanied with the changes of VCD patterns in the amide I' region. In these cases, the conformation of the oligopeptide part of complexes is obviously influenced by interactions with TPPS and partial changes of random coil structure are observed in VCD. When PL was composed of the hundreds of lysine residues, just a weak intensity decrease was detected and the shape of VCD spectrum typical for the random coil structure was preserved. As follows from the uv-vis absorption and fluorescence spectra, porphyrin molecules are attached to peptides by electrostatic interaction as a monomer or dimer and interaction between porphyrin and peptide depends on the polypeptide chain length. For the PLGA-TMPyP system with PLGA containing from tens to hundreds of glutamic acid residues in the chain, the VCD spectra were unchanged when TMPyP was presented in the aqueous solution of PLGA and random coil conformation of PLGA-TMPyP aggregates was preserved.