Vibrational Raman optical activity of alanyl peptide oligomers: A new perspective on aqueous solution conformation

Abstract

SYNOPSISThe vibrational Raman optical activity (ROA) spectra of di-and tri-L-alanine in the range 650-1750 cm-' have been measured in HzO and DzO solution at high, neutral, and low pH and pD. Corresponding ROA spectra for tetra-and penta-L-alanine have also been obtained, but over a more restricted set of pH and pD conditions. There are similarities with the ROA spectrum of L-alanine below -1200 cm-', but the spectra are very different above this wavenumber due to the influence of the vibrational coordinates of the peptide group. The similar overall appearance of the di-, tri-, and tetrapeptide ROA under selected conditions of pH and pD, and of all four peptide ROA spectra in DCI and HCI solutions, in the backbone skeletal stretch region -1050-1200 cm-' and the extended amide I11 region -1250-1350 cm-', suggests that the backbone conformation is approximately the same in all four structures. One difference, however, is a shift of a large positive ROA band in H20 at -1341 cm-' in the dipeptide, assigned to C,-H and in-plane N-H deformations, down to -1331 cm-' in the tripeptide and to -1315 cm-' in the tetrapeptide and pentapeptide (the last in HCI due to insufficient solubility in H 2 0 ) , which indicates increasing delocalization of the corresponding normal mode with increasing chain length. Our results do not support the suggestion that stabilizing interactions of the zwitterionic end groups in tri-L-alanine at neutral pH leads to a different solution structure to that at high pH.

Isotope-edited two-dimensional vibrational spectroscopy of trialanine in aqueous solution

“…21 Vibrational circular dichroism studies [22][23][24] have suggested that trialanine has a preferred conformation in aqueous solution, and our 2D results provided strong evidence confirming this. Since trialanine has only two peptide units, the only structure-related degrees of freedom are the two dihedral angles and ͑see Fig.…”

Visible and UV‐resonance Raman spectroscopy of model peptides

“…This couplet is absent at alkaline pD, which according to Lee et al 65 suggests that a specific structure is stabilized by the electrostatic interaction between the terminal groups in the zwitterionic state. This was more recently contested by Ford et al, 66 who, based on visible Raman and vibrational Raman optical activity (ROA) spectra, found no significant structural difference between the zwitterionic and the fully deprotonated state of trialanine. Krimm and coworkers 67 investigated the Raman spectra of crystallized trialanine on the basis of a normal-coordinate analysis with an empirical force field.…”

INFRARED AND RAMAN VIBRATIONAL OPTICAL ACTIVITY: Theoretical and Experimental Aspects

“…They include ab initio calculations of VCD (92) and ROA (109) of N-acetyl-L-alanine Nmethylamide, empirical ROA studies of simple alanyl peptides (171,172), and VCD studies in the NH-stretching region of a family of blocked alanyl peptides (173). A VCD study of the conformation of alanyl and leucyl tripodal peptides has appeared (174) in which the sense of twist of a threefold symmetric propeller pattern stabilized by inter-chain hydrogen bonding was deduced by using a coupled oscillator formalism (175).…”