Photoactive yellow protein (PYP) has a characteristic hydrogen bond (H bond) between p-coumaric acid chromophore
and Glu46, whose OH bond length has been observed to be 1.21 Å by the neutron diffraction technique
[Proc. Natl. Acad. Sci. 106, 440–4]. Although it has been expected that such a drastic elongation of the OH bond
could be caused by the quantum effect of the hydrogen nucleus, previous theoretical computations including
the nuclear quantum effect have so far underestimated the bond length by more than 0.07 Å. To elucidate the origin
of the difference, we performed a vibrational analysis of the H bond on potential energy curve with O…O distance
of 2.47 Å on the equilibrium structure, and that with O…O distance of 2.56 Å on the experimental crystal
structure. While the vibrationally averaged OH bond length for equilibrium structure was underestimated, the
corresponding value for crystal structure was in reasonable agreement with the corresponding experimental
values. The elongation of the O…O distance by the quantum mechan