Quantum mechanical calculations and experimental measurement of N-terminal charge effects on 1HN and 1HC alpha chemical shifts in peptides.

Nuclear magnetic resonance spectroscopists are increasingly utilizing chemical shifts to characterize the secondary structure of proteins. The present study addresses the effects that the positively charged amino group at the N-terminus of a peptide has on 1HN and 1HC alpha chemical shifts along the chain. This information is necessary for interpreting chemical shift data for proteins and/or for peptides that are used as models for protein structure. The chemical shifts for the 1H resonances of four peptides that differ only in the location of their N-terminii are assigned using two-dimensional nmr spectroscopy. The peptides have sequences derived from the beta subunit of the glycoprotein hormone human chorionic gonadotropin (hCG-beta). Comparison of the 1HN and the 1HC alpha chemical shifts for residues common to all four peptides reveals downfield shifts for 1HN and the 1HC alpha resonances within three residues of the N-terminus compared with chemical shifts in the interior of the peptide. The magnitude of the downfield shift is larger for resonances nearer the N-terminus. Quantum mechanical calculations of the 1HN and 1HC alpha chemical shifts in peptides constructed with six alanine units also predict a significant terminus effect. The calculations agree both qualitatively and quantitatively with the experimental data. The inductive nature of the end effect is confirmed in the calculations by Mulliken population analysis. End effects should be taken into account in determining protein secondary structures from chemical shifts.