(14)N quadrupolar coupling of amide nitrogen and Peptide secondary structure as studied by solid-state NMR spectroscopy.

To establish a relationship between the secondary structure of a peptide and the quadrupolar coupling of its amide (14)N, we examined (14)N quadrupolar couplings for eight different polypeptide samples, each of whose secondary structure (alpha-helix or beta-sheet) is known. The (14)N quadrupolar coupling is estimated from indirect observation of a (14)N overtone resonance under magic-angle spinning. From the observed indirect (14)N overtone spectra and calculated (14)N quadrupolar couplings for model molecules by using ab initio calculation (Gaussian03), it is shown that the quadrupolar coupling for the alpha-helix is larger than that for the beta-sheet by a few 100 kHz irrespective of the kind of amino acid residues examined (Ala, Val, Leu).