H atom positions and nuclear magnetic resonance chemical shifts of short H bonds in photoactive yellow protein.

Recent neutron diffraction studies on photoactive yellow protein (PYP) proposed that the H bond between protonated Glu46 and the chromophore-ionized p-coumaric acid (pCA) is a low-barrier H bond (LBHB) mainly because the H atom position was assigned at the midpoint of the O(Glu46)-O(pCA) bond. However, the (1)H nuclear magnetic resonance (NMR) chemical shift (δ(H)) was 15.2 ppm, which is lower than the values of 17-19 ppm for typical LBHBs. We evaluated the dependence of δ(H) on an H atom position in the O(Glu46)-O(pCA) bond in the PYP ground state by using a quantum mechanical/molecular mechanical (QM/MM) approach. The calculated chemical shift unambiguously suggested that a δ(H) of 15.2 ppm for the O(Glu46)-O(pCA) bond in NMR studies should correspond to the QM/MM geometry (δ(H) = 14.5 ppm), where the H atom belongs to the Glu moiety, rather than the neutron diffraction geometry (δ(H) = 19.7 ppm), where the H atom is near the midpoint of the donor and acceptor atoms.