Conformational studies of poly(L‐tyrosine). A helix–coil transition in dimethyl sulfoxide–dichloroacetic acid mixtures

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Abstract
Poly(L‐tyrosine) is a random coil in dimethyl sulfoxide. Upon addition of dichloroacetic acid, poly(L‐tyrosine) undergoes a conformational transition centered at about 10% dichloroacetic acid. The transition is nearly complete at 20% dichloroacetic acid. Further addition of dichloroacetic acid leads to precipitation of poly(L‐tyrosine). We have characterized this transition by optical rotation, viscosity, circular dichroism, and infrared. The optical rotation at 350 nm and the intrinsic viscosity increase sharply to values that are consistent with a transition to the α‐helix conformation. The circular dichroism of poly(L‐tyrosine) in dimethyl sulfoxide and in dimethyl sulfoxide/dichloroacetic acid (80:20 v/v) agrees with previous reports for random‐coil and α‐helix conformations, respectively. The infrared spectrum of poly(L‐tyrosine) in dimethyl sulfoxide/dichloroacetic acid (80:20 v/v) shows no evidence of β‐structure. We conclude that the transition on going from dimethyl sulfoxide to 20% dichloroacetic acid in dimethyl sulfoxide is a coil → α‐helix transition. The amide‐I band of poly(L‐tyrosine) in dimethyl sulfoxide/dichloroacetic acid (80:20) is found to be at 1662 cm−1. It has been suggested that this high frequency may be indicative of a left‐handed α‐helix. However, this high amide‐I frequency is consistent with conformational energy calculations of Scheraga and co‐workers. The mechanism of the dichloroacetic acid‐induced transition to an α‐helix is discussed. Dichloroacetic acid and dimethyl sulfoxide interact strongly and the transition presumably involves a marked decrease in the ability of dimethyl sulfoxide to solvate the peptide backbone and aromatic side chains upon complex formation with dichloroacetic acid.