Conformational equilibria in a synthetic copolypeptide

A synthetic copolymer of L ‐glutamic acid and L ‐tyrosine (23:1) with molecular weight 17,000 was examined conformationally as a function of pH, using circular dichroism, difference spectrophotometry, fluorescence, potentiometic titration, and high‐resolution nuclear magnetic resonance (220 MHz). A water‐dioxan mixture (2:1) was used to avoid complications due to aggregation (which was shown by infrared spectroscopy to lead to the formation of β‐structures). In the α‐helical form the tyrosine residues generate a sizeable negative Cotton effect in the near ultraviolet; this is a consequence of perturbation of the chromophores by the helix, and not of tyrosine‐tyrosine interactions, which are known to give rise in the right‐handed α‐helical state to positive Cotton effects. The pH profile of this Cotton effect is different from that of the peptide Cotton effects, which reflect the helix‐random coil equilibrium. The data are interpreted in terms of preferential breakdown of the α‐helix in the neighborhood of the tyrosine residues. An ultraviolet difference spectrum in the tyrosine absorption bands is generated at the low pH extreme of the conformational transition, the absorbance change being largely complete at a pH at which the other optical parameters have only begun to change. A possible explanation is the formation of a hydrogen bond between the phenolic hydroxyl and a carboxylate, the pK of which is lowered by the hydrogen bonding. An alternative explanation is the freezing of side‐chain rotations at a pH below the onset of the helix‐random coil transition, when the degree of side chain ionization approaches zero. Some support for the latter scheme comes from the splitting of side‐chain methylene proton resonances, indicating partial immobilization, as well as small changes in chemical shift of tyrosine ring protons in the pH (or pD) region in which the difference spectrum appears.