67Zn QCPMG Solid-State NMR Studies of Zinc Complexes as Models for Metalloproteins

Abstract

Zinc plays a key role in the active binding site for a range of important metalloproteins. For example, Zn{sup 2+} is important for the function of pencillamine, insulin, carboxypeptidase A, thermolysin, and phospholipase C. To understand the enzymatic function of these metalloproteins, it is of interest to study the Zn{sup 2+} coordination environment with a variety of ligands, i.e., N, O, and S donor atoms. Information on Zn{sup 2+} complexation may potentially be obtained from liquid-state {sup 67}Zn NMR (isotropic chemical shifts, {delta}{sub iso}; T{sub 1} and T{sub 2} relaxation). However, the large {sup 67}Zn line width and poor receptivity will prevent useful data from being obtained on biological compounds via liquid-state NMR. Furthermore, the Zn{sup 2+} coordination is particularly reflected in the {sup 67}Zn (I = 5/2) quadrupole coupling, an interaction which may be obtained only indirectly from liquid-state relaxation studies. Solid-state {sup 67}Zn NMR is a more direct and informative probe for the local structure but is unfortunately associated with broad line shapes due to a large quadrupole moment. In this paper the authors demonstrate that {sup 67}Zn QCPMG NMR represents a feasible approach to study Zn{sup 2+} coordination in model complexes for metalloenzymes.