Co-oligopeptides of aromatic amino acids and glycine with a variable distance between the aromatic residues. V. pH-dependent conformational changes in peptides containing two adjacent tryptophyl residues as evidenced by circular dichroism studies

Rizzo, V.; Luigiluisi, P.

Biopolymers 16(2): 437-448


ISSN/ISBN: 0006-3525
PMID: 12842
DOI: 10.1002/bip.1977.360160216
Accession: 068517943

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The influence of pH upon circular dichroic [CD] spectra of H-Trp-Trp-OH, H-Trp-Trp-Gly-OH and H-Gly-Trp-Trp-OH is investigated and data are compared with those obtained for peptides containing only 1 tryptophyl residue. A negative Cotton effect at around 225 nm, which in previous work has been related to an increase of the conformational rigidity in compounds having the sequence -CO-Trp-Trp, is also observed in the case of H-Trp-Trp-OH and H-Trp-Trp-Gly-OH upon deprotonation of the terminal .alpha.-amino group. These data, together with observations arising from solvent and temperature effects, give evidence that H-Trp-Trp-OH undergoes a conformational change upon going from acid to alkaline conditions, where the 2 aromatic side chains become conformationally more rigid relative to each other. This rigidity generates an exciton coupling between the Bb transitions of the 2 indoles. Hydrophobic forces, including stacking interactions, do not appear important in stabilizing this conformationally rigid structure. Rather, intramolecular electrostatic interactions (e.g., H-bondings or polar interactions between the aromatic side chain and the peptide backbone) as well as interactions with the OH group(s) of the solvent, are suggested to be the salient forces. Possible structures which obey these requisites are discussed.