| Abstract: | The dialysis data of Pitha, Huang, and Ts'o for the simultaneous binding of adenosine and guanosine to polyuridylic acid are analyzed here using a grand-partition function method described previously. The conclusion that the predominant mode of guanosine-binding cannot be a competition with adenosine for the primary hydrogen-bonding sites on the 2-polyuridylic acid complex emerges from this analysis. By setting a reasonable upper limit to the amount of competitive binding that might occur, it is found that the difference in standard free energies for the binding of guanosine and adenosine must be at least F G ? F A = 2400 cal/mole, provided the stacking energies for A ? A, A ? G, G ? G interactions are all equal. This difference in binding free energies implies a specificity of at least 80: 1 in favor of A on the primary sites at 5°C. Since this is a lower limit, the actual binding specificity may well be much greater. The desirability of achieving specificity through repulsion of incorrect bases, rather than via attraction of correct bases, is discussed. |
