Experimental and Theoretical Spectroscopic Study of 310-Helical Peptides Using Isotopic Labeling to Evaluate Vibrational Coupling

Coupling between the amide linkages in a peptide or protein is the key physical property that gives vibrational spectra and circular dichroism sensitivity to secondary structures. By use of 13C isotopic labeling on individual and pairs of amide CO groups, the amide I band for selected residues was effectively isolated in designed hexa- and octapeptides having dominant 310-helical conformations. The resultant frequency and intensity responses were measured with IR absorption, vibrational circular dichroism (VCD), and Raman spectroscopies and simulated with density functional theory (DFT) based computations. Band fitting the spectral components and correlating the results to the computed coupling between selected labeled positions were used to determine coupling constant signs and to estimate their magnitudes for specific sequences. The observed frequency and intensity patterns, and their variation between IR and VCD with label position in the sequence, follow the theoretical predictions to a large degree, but are complicated by end effects that alter the local force field (FF) for some residues in these short peptides. These FF variations were overestimated in the theoretical models which may be evidence of structural variations not included in the model. By analyzing the simulations with different coupling models, the coupling constants were determined to lie in a range (positive) +3−5 cm−1 for sequential residues (i,i+1) and with (negative) −3 cm−1 as an upper bound for alternate ones (i,i+2). The sequential amide coupling for 310-helices is weaker than for α-helices but has the same sign and is larger than and oppositely signed as compared to 31-, or poly-(Pro) n type-II, helices.