Conformational Structure of an L-Asparagine/L-Aspartate Random Copolypeptide as Revealed by Vibrational and CD Spectroscopy

The infrared and Raman spectra of an L-asparagine/L-aspartate random copolypeptide with an L-asparagine residue content of 69% mol have been measured in aqueous solution and in the solid state and analyzed for the first time. The infrared amide I, I', and II bands have been related to the conformational structure of the polypeptide backbone with the aid of CO spectroscopy, whereby it has been concluded that this biopolymer adopts a random coiled secondary structure. The backbone conformation remains unordered even when side-chain carboxyl groups are in the undissociated form (-COOH). The fact that this polypeptide takes this secondary structure in preference to α-helices and β-sheets can be explained by weak hydrophobic bonding between side chains and/or by hydrogen bonding between peptide CONH groups and side-chain polar groups. Another interesting site is the amide III Raman region, which in spite of the above results shows a band in the range usually attributed to α-helical structure. This finding is due to contributions from methine bonds and CH₂ groups and indicates that the side chains can influence the position of the bands in the amide III region. Consequently, this result reveals that associating polypeptide backbone conformation with characteristic frequency ranges in the amide III region may be incorrect.