Figure 1.

(a) Vibrational spectra of unbound (P) and complexed (PW) bovine pancreatic trypsin inhibitor (cannot be distinguished on this scale). The vibrational frequencies ν_i correspond to the normal modes of the energy-minimized structure (20) obtained by diagonalizing the mass-weighted matrix of the second derivatives of the energy (21). (b) Cumulative change of the vibrational entropy ΔS(n) = ∑_i=1ⁿ ΔS_i on water binding, where ΔS_i = (S_i^PW − S_i^P) is the contribution of the ith vibration mode. The Inset shows ΔS_i for the first 100 modes. S_i = [hν_i/T/(e^hν_i/kT − 1) − kln(1 − e^−hνi/kT)] is the entropic content of one vibration mode i (12).