Thermodynamic analysis of proton- and urea-induced dissociation of tobacco mosaic virus: stoichiometry, common ion effect, cooperativity, heterogeneity of subunits and the effect of urea as a homogenizer

The urea- and pressure-induced dissociation of tobacco mosaic virus (TMV) and proton involvement were investigated using a thermodynamic approach that considered the extent of reaction for protons and urea. The stoichiometry of these effectors was determined, as were the Gibbs free energy of dissociation (Δ G D ), the volume change associated with dissociation (Δ V ) and the Hill coefficient ( H ). At low pressure, the proton release ( μ_H^ + ) was 1.9 and 1.4 mol H + /mol of TMV at pH 4.8 and 6.8, respectively. Below pH 4.5 and at pH 6.2, there was proton absorption ( μ_H^ + < 0). The urea stoichiometry at atmospheric pressure ( µ U ) indicated urea uptake throughout the pH range examined, with values of µ U from − 1.8 to − 2.2 mol U/mol of TMV subunit at a urea concentration (expressed in pU) of − 0.18 and − 0.55, respectively. The presence of urea resulted in a much smoother dissociation profile, as indicated by the values for Δ G D , Δ V and µ U . This finding suggested an important role for urea in promoting homogenization, as previously reported in the literature. Overall, compared to available methods, the thermodynamic procedure described here allowed a much more detailed analysis of the thermodynamic parameters involved and the detection of properties associated with homogenization by urea.