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Etude par spectroscopie infrarouge de la transformation hélice–chaîne statistique des polypeptides. II. Etude de l'interaction de quelques polypeptides à chaînes latérales polaires avec l'acide trifluoracétique
Author(s) -
GarrigouLagrange P. Combelas et C.,
Lascombe et. J.
Publication year - 1974
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.1974.360130310
Subject(s) - chemistry , hydrogen bond , trifluoroacetic acid , side chain , amide , random coil , chloroform , solvation , crystallography , stereochemistry , polymer , solvent , infrared spectroscopy , helix (gastropod) , circular dichroism , molecule , organic chemistry , ecology , snail , biology
The various polymer–acid solvation possibilities occuring in the helix–coil transition process of polypeptides with polar side chains were systematically analyzed by infrared spectroscopy. The following samples have been considered: poly‐γ‐benzyl‐ L ‐glutamate (PBLG), alternating poly‐γ‐benzyl‐ D , L ‐glutamate (PBD‐LG), and poly‐β‐benzyl‐ L ‐as‐partate (PBLA). The behavior of the amide A, I, II, and νCO ester absorptions of each polymer dissolved in trifluoroacetic acid–chloroform mixtures was studied in depth. The classical assumptions concerning the interaction between a polypeptide and a proton donor solvent are discussed. This interaction was previously proposed in a theoretical model of helix–coil transition. For PBLG, the spectral characteristics of the cooperative transition are evidenced by the amide bands. These bands also show main chain–acid hydrogen bonding (I)Quantitative analysis of phenomenon (I) was performed in order to localize the “binding sites” of the polymer. In agreement with the theory, only the complexation of peptide units belonging to random coil and terminal helical regions were observed. However, in contrast to the theory in which the association constants K CO and K NH of these residues are generally kept equal, the present results have shown that the main binding site is the carbonyl group ( K NH ≃ 0 or « K CO ). The behavior of the polar side chains of these polypeptides were analyzed during the transition. Similarly to the peptide backbone, they bind the acid by hydrogen bonding (II)Furthermore, this association is more important when the side chains are localized in the coiled regions than in the helical ones. This result suggests, by analogy with the main chain behavior, that the helix–coil transition theory should take into account two more association constants for polar side chains, namely k 1 for the helical regions and k 2 > k 1 for the coiled ones.