Dihedral angles of trialanine in D2O determined by combining FTIR and polarized visible Raman spectroscopy

We have measured the polarized visible Raman and FTIR spectra of trialanine and triglycine in D2O at acid, neutral, and alkaline pD. From the Raman sp ectra we obtained the isotropic and the anisotropic scattering. A self-cons istent spectral analysis of the region between 1550 and 1800 cm(-1) was car ried out to obtain the intensities, frequencies, and halfwidths of the resp ective amide I bands. A model was developed by means of which the intensity ratios of the amide I bands in all spectra and the respective frequency di fferences were utilized to determine the orientational angle theta between the peptide groups and the strength of excitonic coupling between the corre sponding amide I modes. By exploiting, results from a recent ab initio stud y on triglycine (Torii, H; Tasumi, M. J. Raman Spectrosc. 1998, 29, 81), we used these parameters to determine the dihedral angles phi and psi between the peptide groups. Our results show that trialanine adopts a 3(1)-helical structure in D2O for all of its three protonation states. The structure is insensitive to the carboxylate protonation and changes only slightly with N-terminal protonation. Triglycine is structurally more heterogeneous in th e zwitterionic and the cationic state. Our spectral analysis suggests that 3(1)-helices coexist with right-handed alpha -helical and/or with beta -tur n conformations. The N-terminal protonation stabilizes the 3(1)-structure. Our study provides compelling evidence that tripeptides adopt stable confor mations in aqueous solution and that they are suitable model systems to inv estigate the initiation of secondary structure formation.