Vibrational circular dichroism of polypeptides. VI. Polytyrosine alpha-helical and random-coil results

We have measured the VCD of polytyrosine in the amide I and II regions in dimethyl sulfoxide (DMSO) and in 80:20 mixtures of DMSO with trifluoroethanol, trifluoroacetic acid (TFA), and dichloroacetic acid and in 50:50 mixtures of DMSO and trimethyl phosphate (TMP). Additionally, VCD was obtained for deuterated polytyrosine in DMSO and DMSO:D₂O, DMSO:TFA(d₁), and DMSO:TMP mixtures as before. Amide A VCD was obtained in DMSO and DMSO:TMP mixtures. In the pure solvent, VCD of an opposite sign was seen as compared with that seen in the mixtures. The latter were characteristic in sign pattern and shape of right-handed α-helices for poly(L -tyrosine). The pure polytyrosine:DMSO results are similar to those of polylysine:D₂O at neutral pH and poly(β-benzyl-aspartate) in DMSO and may be characteristic of random-coil VCD.