The analysis of mixed associations of aqueous nucleoside solutions

Abstract

Two similar chemical interactions were studied: (1) that between two naturally occurring nucleosides found in deoxyribonucleic acids, 2'-deoxyadenosine and thymidine; and (2) between the nucleoside 2'-deoxyadenosine and the nucleoside analog 5-fluoro-2'-deoxyuridine. Any difference in the interactions which may be found can be attributed to one chemical substituent: a methyl on thymidine where a fluorine is on 5-fluoro-2'-deoxyuridine. The individual solution behavior of the nucleosides and the nucleoside analog was determined by the evaluation of the concentration dependence of the measured apparent molarities. The apparent molarities were measured by vapor pressure osmometry at 37°C in aqueous solutions. The solution behavior of the individual solutes was found to be: 2'-deoxyadenosine--a non-ideal indefinite self-association with sequential, equal equilibrium constant of 12±2 M^-1 and standard free energy change of -1.5±.5 kcal/mole; thymidine--a non-ideal indefinite self-association with sequential, equal equilibrium constant of 0.75±0.06 M^-1 and standard free energy change of 0.18±0.02 kcal/mole; and 5-fluoro-2'-deoxyuridine--an ideal non-associating solute. The mixed solution behavior of 2'-deoxyadenosine with thymidine and 2'-deoxyadenosine with 5-fluoro-2'-deoxyuridine was then examined using measurements of the concentration dependent apparent molarities of aqueous solutions of various blends of the two chemicals at 37°C by vapor pressure osmometry. For 2'-deoxyadenosine with thymidine the solution behavior was evaluated as a non-ideal one-to-one interaction with a mixed association constant of 117±9 M^-1 and standard free energy change of -2 -9±1-3 kcal/mole. For 2'-deoxyadenosine with 5-fluoro-2'-deoxyuridine the evaluation of the solution behavior gave a non-ideal one-to-one interaction with a mixed association constant of 175±18 and a standard free energy change of -3.2±1.8 kcal/mole.