Cation Binding Induced Changes in ¹⁵ N CSA in a Membrane-Bound Polypeptide

Cation binding to the monovalent cation selective channel, gramicidin A, is shown to induce changes in the dipolar and chemical shift observables from uniformly aligned samples. While these changes could be the result of structural or dynamic changes, they are shown to be primarily induced by through-bond polarizability effects when cations are solvated by the carbonyl oxygens of the peptide backbone. Upon cation binding partial charges are changed throughout the peptide plane, inducing large changes in the ¹³ C ₁ chemical shifts, smaller changes in the ¹⁵ N chemical shifts, and even smaller effects for the ¹⁵ N- ¹³ C ₁ and ¹⁵ N- ² H dipolar interactions. These conclusions are substantiated by characterizing the ¹⁵ N chemical shift tensors in the presence and absence of cations in fast-frozen lipid bilayer preparations of gramicidin A.