Nonlinear van’t Hoff Behavior in the Interaction of Two Water-Soluble Porphyrins with Bovine Serum Albumin (BSA)

Thermodynamic analysis of the binding process of water-soluble negatively charged <i>meso</i>-tetrakis(<i>p</i>-sulfonatophenyl) (TPPS₄) and positively charged <i>meso</i>-tetrakis(4-methylpyridyl) (TMPyP) porphyrins with bovine serum albumin (BSA) at different temperatures was carried out based on the data of BSA quenching fluorescence by porphyrins. The comparison of binding constants (<i>K</i> _b) shows that negatively charged TPPS₄ possesses higher affinity to BSA than positively charged TMPyP. Thermodynamic characteristics of the binding process were obtained in accordance with the van't Hoff theory by processing nonlinear dependences of ln <i>K</i> _b on inverse absolute temperature within the framework of two models: taking into account the dependence or independence of the change in the standard heat capacity (Δ<i>C</i> ⁰) on temperature. A comparison of thermodynamic characteristics with the data obtained from the Förster fluorescence quenching theory and with literature data leads to the conclusion that TPPS₄ is bound to the Sudlow I site (subdomain IIA), while TMPyP is bound to the Heme site (between the subdomains IA and IB). The analysis of Δ<i>C</i> ⁰ changes with temperature demonstrates that binding of TPPS₄ promotes hydration of nonpolar groups in the protein, which increases with the increase of temperature, while binding of TMPyP decreases the hydration of polar groups of the protein, the effect increasing with rising temperature. The obtained information may be useful for elucidating the mechanisms of interaction of porphyrins with albumins and the effect of this interaction upon the effectiveness of porphyrins in photodynamic therapy and in fluorescence diagnostics of cancer.