Amide III frequencies for Ala‐X peptides depend on the X amino acid size

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Abstract
Raman spectra of series of aqueous solutions of peptides containing two amino acids, glycine‐X, alanine‐X, and serine‐X, where X is an uncharged amino acid, show that the amide III band shifts systematically to lower frequencies as the side chain of the X amino acid becomes larger. The range of this shift is about 20cm−1, starting at 1275cm−1 for alanine‐glycine and moving to 1251 cm−1 for alanine‐tryptophan, with a correlation coefficient of 0.93 with the mass of the X amino acid side chain for 10 peptides. The amide I frequencies remain constant as the X amino acid is changed. This shift may result from a change in the average conformational preference of the peptide, a change in vibrational coupling of the amide III modes with the X amino acid side chain, a change in molecular force constants, or a combination of these. These results present a test for computational methods.