A Solvent Model of Nucleotide–Protein Interaction—Partition Coefficients of Phosphates Between Water and Organic Solvent

Abstract

In attempt to experimentally evaluate hydration/solvation of phosphoric compounds in aqueous solution, partitioning of phosphoric compounds from an aqueous solution to an organic solvent has been quantified. Transfer of phosphates from an aqueous solution into octanol was greatly enhanced by addition of alkylamine as an amphiphilic extractant. This alkylamine/octanol system exhibited amine basicity, and thus the pH of solution was controlled by equilibration with buffer. Further, enthalpy changes of the transfers of ATP and ADP from water to the alkylamine/octanol were estimated from van’t Hoff analysis, and these enthalpy changes depended on ionization enthalpy of buffer. This result suggests that the transfers are accompanied with protonation of phosphoric ions and deprotonation of alkylamine. Finally, the partition coefficients of ATP, ADP, AMP, and P_i were estimated under the pH-controlled condition at 25 °C. The partition coefficients depended on the pH of aqueous phase and the net charge of phosphoric compounds. Therefore, the transfer is likely to be determined by electrostatic interaction between phosphoric ion and amine. The solvent system with a nucleotide-uptake capacity may partly mimic the function of ATP-binding proteins.