Conformational study of sequential Lys and Leu based polymers and oligomers using vibrational and electronic CD spectra.
Vibrational CD (VCD) and electronic CD (ECD) spectra of some sequential Lys and Leu based oligo- and polypeptides were studied as a function of added salt and (for ECD) as a function of concentration in aqueous solution. For these samples, the VCD spectra can only be measured at relatively high concentrations under which the well-known salt-induced transition to a beta-sheet form can be observed for the KL based species, but only the end-state alpha-helical conformation is obvious for the LKKL based samples. ECD concentration dependence demonstrates that, at high concentration with no added or with added salt, LKKL based oligomers and polymers give alpha-helical spectra. These data provide evidence of aggregation induced secondary structure formation in an exceptionally simple peptide system. Similarly, the KL based oligomers and polymers give beta-sheet like spectra at high concentration or at high salt. These systems further provide model systems under "normal" aqueous conditions that yield VCD band shapes that correlate to the major secondary structural types of polypeptides. They are in substantial agreement with those spectra obtained on homopolypeptides and on proteins, confirming the relative independence of the VCD technique from side-chain and solvent effects.