Spectroscopic properties of chymotrypsin and model compounds indicate
that a low-barrier hydrogen bond participates in the mechanism of seri
ne protease action. A low-barrier hydrogen bond between N delta 1 of H
is(57) and the beta-carboxyl group of Asp(102) in chymotrypsin can fac
ilitate the formation of the tetrahedral adduct, and the nuclear magne
tic resonance properties of this proton indicate that it is a low-barr
ier hydrogen bond. These conclusions are supported by the chemical shi
ft of this proton, the deuterium isotope effect on the chemical shift,
and the properties of hydrogen-bonded model compounds in organic solv
ents, including the hydrogen bond in cis-urocanic acid, in which the i
midazole ring is internally hydrogen-bonded to the carboxyl group.