Molecular dynamics of hydrogen bonds in protein-D2O: the solvent isotope effect.
We suggest that the H-bond in proteins not only mirrors the motion of hydrogen in its own atomistic setting but also finds its origin in the collective environment of the hydrogen bond in a global lattice of surrounding H2O molecules. This water lattice is being perturbed in its optimal entropic configuration by the motion of the H-bond. Furthermore, bonding interaction with the lattice drop the H-bond energy from some 5 kcal/mol for the pure protein in the absence of H2O, to some 1.6 kcal/mol in the presence of the H2O medium. This low value here is determined in a computer experiment involving MD calculations and is a value close to the generally accepted value for biological systems. In accordance with these computer experiments under ambient conditions, the H-bond energy is seriously depressed, hence confirming the subtle effect of the H2O medium directly interacting with the H-bond and permitting a strong fluxional behavior. Furthermore, water produces a very large change in the entropy of activation due to the hydrogen bond breakage, which affects the rate by as much as 2 orders of magnitude. We also observe that there is an entire ensemble of H-bond structures, rather than a single transition state, all of which contribute to this H-bond. Here the model is tested by changing to D2O as the surrounding medium resulting in a substantial solvent isotope effect. This demonstrates the important influence of the environment on the individual hydrogen bond.
Sheu
Sheh-Yi
SY
Department of Life Sciences, National Yang-Ming University, Taipei 112, Taiwan. sysheu@ym.edu.tw
Schlag
E W
EW
Selzle
H L
HL
Yang
Dah-Yen
DY
D003198:Computer Simulation; D017666:Deuterium Oxide / Q000737:chemistry*; D006860:Hydrogen Bonding; D007554:Isotopes / Q000737:chemistry; D008958:Models, Molecular; D044367:Phase Transition; D017434:Protein Structure, Tertiary; D011506:Proteins / Q000737:chemistry*; D012997:Solvents / Q000737:chemistry*; D013816:Thermodynamics
D007554:Isotopes; D011506:Proteins; D012997:Solvents; D017666:Deuterium Oxide