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

Thermodynamic analysis of the binding process of water-soluble\nnegatively charged <i>meso</i>-tetrakis(<i>p</i>-sulfonatophenyl) (TPPS<sub>4</sub>) and positively charged <i>meso</i>-tetrakis(4-methylpyridyl) (TMPyP) porphyrins with bovine\nserum albumin (BSA) at different temperatures was carried out based\non the data of BSA quenching fluorescence by porphyrins. The comparison\nof binding constants (<i>K</i><sub>b</sub>) shows that negatively\ncharged TPPS<sub>4</sub> possesses higher affinity to BSA than positively\ncharged TMPyP. Thermodynamic characteristics of the binding process\nwere obtained in accordance with the van’t Hoff theory by processing\nnonlinear dependences of ln <i>K</i><sub>b</sub> on inverse\nabsolute temperature within the framework of two models: taking into\naccount the dependence or independence of the change in the standard\nheat capacity (Δ<i>C</i><sup>0</sup>) on temperature.\nA comparison of thermodynamic characteristics with the data obtained\nfrom the Förster fluorescence quenching theory and with literature\ndata leads to the conclusion that TPPS<sub>4</sub> is bound to the\nSudlow I site (subdomain IIA), while TMPyP is bound to the Heme site\n(between the subdomains IA and IB). The analysis of Δ<i>C</i><sup>0</sup> changes with temperature demonstrates that\nbinding of TPPS<sub>4</sub> promotes hydration of nonpolar groups\nin the protein, which increases with the increase of temperature,\nwhile binding of TMPyP decreases the hydration of polar groups of\nthe protein, the effect increasing with rising temperature. The obtained\ninformation may be useful for elucidating the mechanisms of interaction\nof porphyrins with albumins and the effect of this interaction upon\nthe effectiveness of porphyrins in photodynamic therapy and in fluorescence\ndiagnostics of cancer.