Hybrid density functional quantum mechanical calculations were used to study the strength of the hydrogen bond between His(57)N(delta1) and Asp(102) O-delta1 in chymotrypsin and how it changes along the reaction coordinate. Comparison of experimental shifts with the results of chemical shift calculations on a variety of small molecules, including species containing very strong hydrogen bonds, has validated the overall approach and provided the means for calibrating and correcting the calculated values. Models of the active site of chymotrypsin in its resting state and tetrahedral intermediate state were derived from high-resolution X-ray structures. The distance between His(57)N(delta1) and Asp(102) O-delta1 in each model was varied between 2.77 Angstrom (weak hydrogen bond) and 2.50 Angstrom (extremely strong hydrogen bond), and the one-dimensional potential energy surface of the hydrogen-bonded proton (or deuteron/triton) was determined. The zero-point energy, probability distribution, and chemical shift were determined for each distance. Calculated values for NMR chemical shifts, NMR chemical shift differences between H-1 and H-3, and H-2/H-1 fractionation factors were compared with published experimental values. Energies provided by the calculations indicated that the hydrogen bond between His(57)N(delta1) and Asp(102) O-delta1 in the chymotrypsin active site increases in strength by 11 kcal mol-l in going from the resting state of the enzyme to the tetrahedral intermediate state. This result confirms the hypothesis that the strengthened hydrogen bond plays an important role in lowering the energy of the transition state and, hence, in the catalytic efficiency of the enzyme. Models of the transition state that best fit the experimental data are consistent with a "strong" hydrogen bond between His(57)N(delta1) and Asp(102) O-delta1 but apparently not a "low-barrier" or "very strong" hydrogen bond.