Y. Tomita et al., A METHOD FOR DIHEDRAL ANGLE MEASUREMENT IN SOLIDS - ROTATIONAL RESONANCE NMR OF A TRANSITION-STATE INHIBITOR OF TRIOSE PHOSPHATE ISOMERASE, Journal of the American Chemical Society, 116(19), 1994, pp. 8766-8771
In rotational resonance, a solid-state NMR technique for measuring dip
olar couplings, the spinning speed, upsilon(r), is adjusted such that
Delta nu(iso) = n upsilon(r) where Delta nu(iso) is isotropic chemical
shifts difference and n (an integer) is referred to as the order. At
higher orders of the rotational resonance the magnetization exchange r
ate depends strongly upon the relative orientations of the two carbons
, particularly if both sites have broad chemical shift anisotropy. Exp
erimental exchange curves and computer simulations for crystalline [C-
13(2)]glycolic acid at n = 4 demonstrate that this effect can be used
to measure dihedral angles. Pur data indicate that the hydroxyl group
and the two carboxyl oxygen atoms are in the same plane, in agreement
with crystallography. Phosphoglycolic acid (PGA) is a transition-state
analog inhibitor of triose phosphate isomerase (TIM). The n = 4 excha
nge curves for [C-13(2)]PGA bound to TIM indicate an ''in-plane'' conf
ormation for the phosphate with respect to the carboxy group. This con
formation had been previously proposed to explain the lack of phosphat
e elimination during catalysis and had been suggested also on the basi
s of crystallographic results. The chemical shift anisotropy of the ca
rboxy group of PGA indicates that it is deprotonated when bound to the
enzyme. We discuss the scope of applications of rotational resonance
for measuring dihedral angles in other systems.