FTIR and CD spectroscopic detection of H-bonded folded polypeptide structures

The FTIR spectra of a selection of cyclic and linear peptides were measured in DMSO and TFE, The backbone conformation in DMSO of the cyclic models has been inferred from ROESY-based interproton connectivities and the temperature coefficients of the NH protons. The FTIR measurements give support to the following assignment of low-frequency amide I bands: > 1645 cm(-1), open (weakly H-bonded) beta- and gamma-turns; approximate to 1640 cm(-1), beta-turns (1 <-- 4 H-bonded); approximate to 1625 cm(-1), gamma-turns (1 <-- 3 H-bonded); < 1620 cm(-1), beta- and gamma-turns with bifurcated H-bondings, Solvent-dependence studies on the diamide models Ac-Xxx-NHCH3 (8, Xxx = Pro, Ala and Gly) suggest that it is the inverse gamma-turn (C-7(eq)) structure which is capable of forming a strong 1 <-- 3 intramolecular H-bond (band at approximate to 1625 cm(-1)). DMSO destroys 1 <-- 3 IHBs of gamma-turns and bifurcated turn systems even in cyclic peptides but does not affect 1 <-- 4 IHBs. TFE has a stabilizing effect an both 1 <-- 4 and 1 <-- 3 H-bondings which gives rise to mixtures of beta- and gamma-turn conformers. Contrary to vibrational spectroscopy, circular dichroism can differentiate between type I(III) and type II beta-turns showing class C and class B CD spectra, respectively. Based on the above findings, a class C CD spectrum measured in TFE can reflect the predominance of type I(III) beta-turn conformation, but it may also be a composite of subspectra of H-bonded beta-turn and gamma-turn as well as open conformers. (C) 1997 Elsevier Science B.V.