Stability and thermodynamics of polyuridylic acid–deoxyadenosine complexes in aqueous neutral salt solutions

✍ Scribed by Thomas Schleich; Gene R. Gould

• Publisher: Wiley (John Wiley & Sons)
• Year: 1974
• Tongue: English
• Weight: 604 KB
• Volume: 13
• Category: Article
• ISSN: 0006-3525
• DOI: 10.1002/bip.1974.360130208

✦ Synopsis

Using the thermodynamic analysis and methodology of Hill (Biopolymers, 12, 257 (1973)) for the treatment of optical thermal transition data the effects of various neutral salt additives on the stability and thermodynamics of the polyu-deoxyadenosine interactions that lead to the formation of triple-stranded helical polymer-monomer complexes have been studied. In order of increasing molar effectiveness as polyU-deoxyadenosine complex stability perturbants (pH 7 and in the presence of 1 M NaCl), the various ions may be ranked:

additives (e.g., NaClO4) caused a decrease in the absolute magnitude of the apparent enthalpy and entropy of binding relative to the values determined in the presence of NaCl. By contrast, stabilizing additives (e.g., Na804) had the opposite effect on these parameters. Along a melting curve the apparent differential heat of complex formation calculated for the binding of deoxyadenosine to polyU in 1 M NaCl appeared to vary linearly with e, the extent of fractional binding. For such a linear dependence it can be shown that the integral heat (usually determined calorimetrically) equals the differential heat at e = 0.5. Correcting the apparent differential heat calculated a t e = 0.5 for ligand a+ tivity resulted in values for the integral heat of binding of deoxyadenosine to polyu in 1 M NaCl of -13 to -16 kcal/mol. Binding isotherms determined in the presence of different inorganic electrolytes could be superimposed provided that different temperatures were compared. However, the additive (CH3)4NCl, which has been shown to interact preferentially with A-T rich regions of DNA (Shapiro, Stannard, and Felsenfeld, Biochemistry, 8, 3233 (1969)) resulted in a considerably broadened binding isotherm indicating less cooperativity.