So. Shan et D. Herschlag, THE CHANGE IN HYDROGEN-BOND STRENGTH ACCOMPANYING CHARGE REARRANGEMENT - IMPLICATIONS FOR ENZYMATIC CATALYSIS, Proceedings of the National Academy of Sciences of the United Statesof America, 93(25), 1996, pp. 14474-14479
The equilibrium for formation of the intramolecular hydrogen bond (K-H
B) in a series of substituted salicylate monoanions was investigated a
s a function of Delta pK(a), the difference between the pK(a) values o
f the hydrogen bond donor and acceptor, in both water and dimethyl sul
foxide. The dependence of log K-HB upon Delta pK(a) is linear in both
solvents, but is steeper in dimethyl sulfoxide (slope = 0.73) than in
water (slope = 0.05). Thus, hydrogen bond strength can undergo substan
tially larger increases in nonaqueous media than aqueous solutions as
the charge density on the donor or acceptor atom increases. These resu
lts support a general mechanism for enzymatic catalysis, in which hydr
ogen bonding to a substrate is strengthened as charge rearranges in go
ing from the ground state to the transition state; the strengthening o
f the hydrogen bond would be greater in a nonaqueous enzymatic active
site than in water, thus providing a rate enhancement for an enzymatic
reaction relative to the solution reaction. We suggest that binding e
nergy of an enzyme is used to fix the substrate in the low-dielectric
active site, where the strengthening of the hydrogen bond in the cours
e of a reaction is increased.