Effects of monovalent anions on a temperature-dependent heat capacity change for Escherichia coli SSB tetramer binding to single-stranded DNA

Abstract

ABSTRACT: We have previously shown that the linkage of temperature-dependent protonation and DNA base unstacking equilibria contribute significantly to both the negative enthalpy change (¢Hobs) and the negative heat capacity change (¢Cp,obs) for Escherichia coli SSB homotetramer binding to single-stranded (ss) DNA. Using isothermal titration calorimetry we have now examined ¢Hobs over a much wider temperature range (5-60 °C) and as a function of monovalent salt concentration and type for SSB binding to (dT)70 under solution conditions that favor the fully wrapped (SSB)65 complex (monovalent salt concentration g0.20 M). Over this wider temperature range we observe a strongly temperature-dependent ¢Cp,obs. The ¢Hobs decreases as temperature increases from 5 to 35 °C (¢Cp,obs <0) but then increases at higher temperatures up to 60 °C (¢Cp,obs>0). Both salt concentration and anion type have large effects on ¢Hobs and ¢Cp,obs. These observations can be explained by a model in which SSB protein can undergo a temperature- and salt-dependent conformational transition (below 35 °C), the midpoint of which shifts to higher temperature (above 35 °C) for SSB bound to ssDNA. Anions bind weakly to free SSB, with the preference Br-> Cl-> F-, and these anions are then released upon binding ssDNA, affecting both ¢Hobs and ¢Cp,obs. We conclude that the experimentally measured values of ¢Cp,obs for SSB binding to ssDNA cannot be explained solely on the basis of changes in accessible surface area (ASA) upon comple