Circular Dichroism, Optical Rotatory Dispersion and Helix Coil Transition of Polytyrosine and Tyrosine Peptides in Non-Aqueous Solvents

A randomly coiled conformation is found for polytyrosine in dimethylsulfoxide and for tyrosine peptides with chain length of 1 to 12 in dimethyl-sulfoxide, 1,2-propanediol and N,N-dimethylformamide. In the latter two solvents, polytyrosine assumes a helical conformation. The circular dichroism is well represented assuming additivity of the spectrum characteristic of a non-aromatic right-handed α-helix and aromatic side chain contributions. A cooperative intramolecular helix ⇌ coil transition of polytyrosine is induced by addition of dimethylsulfoxide to 1,2-propanediol or N,N-dimethylformamide. For the estimation of protein structures from optical rotatory dispersion or circular dichroism it is important to note that in unfavorbale cases one tyrosine side chain may compensate for the contribution of about one peptide bond in an α-helical or β-conformation.