Chemical Models for Aqueous Biodynamical Processes

Abstract

The proton inventory method was applied to the study of three processes: the viscous flow of water, the neutral hydrolysis of esters, and the exchange reaction between aqueous sodium ion and the carboxylic exchanger Amberlite CG-50®. The fluidity (1/viscosity; of mixtures of protium and deuterium oxides at 298.16 K can be described by the equation $ = $ (1 ~ n + 0.90n) J ^ n o This result indicates that two protons contribute equally to the overall isotope effect. The proposed mechanism for momentum transfer involves an interstitial water molecule linked to "flowing" molecular sheaths. The rate constants of neutral hydrolysis of ethyl trifluoroacetate at 298.16 K can be described by the equation k = k (l-n+0.47n) (l-n+0. 78«) n o . This result suggests a catalytic entity with 67% hydronium ion character which we have called the poena -hydronium ion. The rate of exchange of the reaction Na(aq)+ + Arnberlite CG-50® 4. was found to be independent of a) [Na ] / (resin) ; b) hydrodynamic condi- 4- -j- -ftions; c) nature of cation (Li , Na , K ) . The solvent isotope effect for Na and K exchange was found to be 3.06 ± 0.17 and 3.31 i 0.38, respectively. These results suggest that diffusion of the newly exchanged hydronium ion through the polymeric matrix is rate determining. The reaction rate constants for sodium exchange in mixtures of H20~D20 at 298.16 K can be described by the equation k = k (1 - n + 0. 70n)3 which n o supports the proposed mechanism. Fractionation factors were measured for four geminal hydroxylic compounds as models for the tetrahedral intermediate in the hydrolysis of carboxylic acid derivatives. The values are 1.23 ± 0.08 for chloral hydrate; 1.24 ± 0.20 for ninhydrin; 1-28 ± 0.17 for D-glucose; and 1.23 ± 0.02 for D-fructose. These values indicate that hydrogens are more tightly bound in these molecules than in bulk water. A theoretical calculation of deuterium isotope effects was made on a gem-diol model. The isotope effect K.JK^ increases with an increase in the hydroxylic D li torsional force constant. An extensive table of isotopic fractionation factors is given. vi