Nature of vibrational coupling in helical peptides: An isotopic labeling study

Infrared (IR) and vibrational circular dichroism (VCD) spectra were measured for a series of isotopically (C-13 on two or more amide C=O) labeled, 25 residue, a-helical peptides of the sequence Ac(AAAAK)(4)AAAAY-NH2 that were also studied in the previous paper. Theoretical IR and VCD simulations were performed for correspondingly isotopically labeled Ac-A(24)-NHCH3 constrained to an a-helical conformation by use of property tensor transfer from density functional theory (DFT) calculations on AcA(10)-NHCH3. The simulations predicted and experiments confirmed that the vibrational coupling constants between i, i + 1 and i, i + 2 residues differ in sign, thus leading to a reversal of the C-13 VCD pattern and explaining the large shift in the C-13 amide I frequency as reported in the previous paper. The sign of the coupling constant remained consistent for larger label separation (with the exception of i, i + 4) and for more labels with uniform separation. Such effects confirm that the isotopically labeled group vibrations are essentially only coupled to each other and are effectively uncoupled from those of the unlabeled groups. This development confirms the utility of isotopic labels for site-specific structural studies with vibrational spectra. Observed spectral effects cannot be explained by considering only transition dipole coupling (TDC) between amide oscillators, particularily for smaller label separations, but the TDC and ab initio predicted couplings roughly converge at large separation.