Hydration Analysis on Atp Hydrolysis by Microwave Dielectric Spectroscopy

Abstract

Recent theoretical works reveiled that the hydration energies of nucleotides and inporganic phosphates are one of important factors in the ATP hydrolysis energy. Here we have measured the hydration states of the reactants and the products of ATP hydrolysis reaction by precesion microwave dielectric relaxation spectroscopy. The complex dielectric spectra of hydrated solutes, such as sodium salts of ATP4-, ADP3- , HPO42-, were measured. We detected constrained water and hyper-mobile water around nucleotides and phosphates. Hydration change upon the neutralization reaction of phosphate, H2PO4- + OH- ◊ HPO42- + H2O, was investigated by measuring high-resolution complex dielectric spectra of mono- and di-sodium phosphate solutions at 20°C to understand the hydration effect on the thermodynamics of phosphate buffer reaction. From each solution spectrum the dielectric spectrum of a spherical volume containing each hydrated solute in water was derived based on a suspension theory. Each spectrum was decomposed into a bulk water (fcw ∼ 17 GHz) component and two Debye dispersion components, assigned as constrained water (fc2 ∼ 6.4GHz fcw), respectively. The dielectric dispersion of hyper-mobile water was about five times stronger than the constrained one. The strengths of these two Debye dispersions decreased by 20% (ΔN2 = −7) for the constrained water number and by 10% (ΔN1 = −15) for the hyper-mobile water number upon the neutralization reaction, while those decrements were compensated by an increase of dispersion strength of bulk water. It is thought that the entropy changes corresponding to the number increases of constrained water and hyper-mobile water molecules are negative and positive, respectively. So the present result provides us a physical explanation of the small effect of hydration change on the total entropy change upon the reaction.