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

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Abstract
The vibrational Raman optical activity (ROA) spectra of di‐ and tri‐L‐alanine in the range 650–1750 cm−1 have been measured in H2O and D2O 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−1, 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 DCl and HCl solutions, in the backbone skeletal stretch region ∼ 1050–1200 cm−1 and the extended amide III region ∼ 1250–1350 cm−1, 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 H2O at ∼ 1341 cm−1 in the dipeptide, assigned to Cα–H and in‐plane N–H deformations, down to ∼ 1331 cm −1 in the tripeptide and to ∼ 1315 cm−1 in the tetrapeptide and pentapeptide (the last in HCl due to insufficient solubility in H2O), 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. © 1994 John Wiley & Sons, Inc.