Characterization of folded conformations in a tetrapeptide containing two tryptophan residues by vibrational circular dichroism

Abstract

The intramolecularly hydrogen bonded conformations of the tetrapeptide Boc-Trp-Aib-Gly-Trp-OMe (WUGW) are investigated using experimental and quantum chemical predictions of vibrational circular dichroism (VCD) in the 1800-1550 cm^-1 region. The predicted VCD spectrum, for a conformation (conformer A) obtained from optimization of crystal structure, reproduced the dominant negative VCD band observed experimentally in CH₃OH and CHCl₃ solvents. However, the predicted VCD spectrum of Conformation A also has an extra positive band which is not seen in the experimental spectra. This mismatch appears to be due to the lack of solvent influence in the quantum chemical geometry optimizations. However, Conformations I and II, obtained, respectively, from constrained optimization of crystal and NMR structures, mimic the solvent stabilized structures and are predicted to have dominant negative VCD band as found in the experimental spectra. It is noted that, for the peptide investigated here, unconstrained quantum chemical geometry optimizations in vacuum converged to structures that are not the realistic models of conformations found in solution. It is also noted that undertaking quantum chemical vibrational property calculations directly using geometries obtained from crystal data or NMR data resulted in unrealistic vibrational frequencies and descriptions. However, constraining the backbone dihedral angles to those found in condensed medium, and optimizing the remaining geometrical parameters resulted in a better reproduction of the observed VCD in condensed medium. The vibrational origins of bands in all of the predicted VCD spectra for the WUGW-tetrapeptide have also been presented.