Transfer energetics of uracil, thymine, cytosine and adenine in aqueous mixtures of lithium chloride, sodium chloride and potassium chloride

Abstract

Standard free energies (<img src='/image/spc_char/delta.gif' border=0> <i>G</i><img src='http://www.niscair.res.in/jinfo/zerot.gif' border=0>) and entropies (<img src='/image/spc_char/delta.gif' border=0> S<img src='http://www.niscair.res.in/jinfo/zerot.gif' border=0>) of transfer of nucleic acid bases viz. uracil (URA), thymine (THY), cytosine (CYT) and adenine (ADE) from water to aqueous mixtures of lithium chloride, sodium chloride and potassium chloride have been evaluated from solubility measurements at different temperatures. The interaction effect of <img src='/image/spc_char/delta.gif' border=0> <i>G</i><img src='http://www.niscair.res.in/jinfo/zerot.gif' border=0>_int, as guided by the combined effects of hydrophilic hydration (H₁H) of hydrophilic parts and hydrophobic hydration (H_bH) of hydrophobic parts of these bases in these ionic cosolvents relative to that in water, have been obtained by subtraction of the cavity effect <img src='/image/spc_char/delta.gif' border=0> <i>G</i>, _cav (i) as computed by SPT formulations from the corresponding <img src='/image/spc_char/delta.gif' border=0> <i>G</i><img src='http://www.niscair.res.in/jinfo/zerot.gif' border=0> values. The composition profiles of <img src='/image/spc_char/delta.gif' border=0> <i>G</i><img src='http://www.niscair.res.in/jinfo/zerot.gif' border=0> and <img src='/image/spc_char/delta.gif' border=0> G_int<img src='http://www.niscair.res.in/jinfo/zerot.gif' border=0> indicate that each of the bases is found to be increasingly desolvated in these aqueous electrolytes save URA and THY at higher compositions of aqueous KCl, their order being KCI < NaCI < LiCI at any of the compositions, as expected from the primary solvation effect of the cations of the ionic cosolvents. The observed T<img src='/image/spc_char/delta.gif' border=0> Sand T<img src='/image/spc_char/delta.gif' border=0>S<img src='http://www.niscair.res.in/jinfo/zerot.gif' border=0><i>, </i> _int (i)-composition profiles are, however, found to be dictated by the relative 3D structure breaking effect as well as the relative H₁H_bH effect, induced by these ionic cosolvents. The observed increased desolvation of the bases reflect that the intramolecular H-bonds between the bases of the double strands of DNA/RNA helices are likely to get strengthened in these ionic cosolvent systems.