| Abstract
| - Infrared (IR) and vibrational circular dichroism (VCD) spectra were measured for a series ofisotopically (13C on two or more amide CO) labeled, 25 residue, α-helical peptides of the sequence Ac−(AAAAK)4AAAAY−NH2 that were also studied in the previous paper. Theoretical IR and VCD simulationswere performed for correspondingly isotopically labeled Ac−A24−NHCH3 constrained to an α-helicalconformation by use of property tensor transfer from density functional theory (DFT) calculations on Ac−A10−NHCH3. The simulations predicted and experiments confirmed that the vibrational coupling constantsbetween i, i + 1 and i, i + 2 residues differ in sign, thus leading to a reversal of the 13C VCD pattern andexplaining the large shift in the 13C amide I frequency as reported in the previous paper. The sign of thecoupling constant remained consistent for larger label separation (with the exception of i, i + 4) and formore labels with uniform separation. Such effects confirm that the isotopically labeled group vibrations areessentially 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 vibrationalspectra. 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 predictedcouplings roughly converge at large separation.
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