Interaction of some water-soluble metalloporphyrazines with human serum albumin

The thermodynamic of the binding of N,N0,N00,N000-tetramethyltetra-2,3-pyridinoporphyrazinatocopper(II) ([Cu(2,3-tmtppa)]4þ), N,N0,N00,N000-tetramethyltetra-3,4-pyridinoporphyrazinatocopper(II) ([Cu(3,4-tmtppa)]4þ), N,N0,N00,N000-tetramethyltetra-3,4-pyridinoporphyrazinatocobalt( II) ([Co(3,4-tmtppa)]4þ), and N,N0,N00,N000-tetramethyltetra-3,4-pyridinoporphyrazinatozinc(II) ([Zn(3,4-tmtppa)]4þ), complexes to human serum albumin (HSA) has been studied. The binding constants (K) were obtained by analysis of optical absorption spectra of mentioned complexes at various HSA concentrations using SQUAD software. The values of K were obtained 9.77 £ 104 M21 for [Cu(2,3-tmtppa)]4þ and, 1.35 £ 105, 6.92 £ 105 and 2.00 £ 105 M21 for [Cu(3,4-tmtppa)]4þ, [Zn(3,4-mtppa)]4þ, and [Co(3,4-tmtppa)]4þ, respectively, at 27 8C. The higher affinity of Cu-3,4-isomer towards HSA with respect to the 2,3-isomer can be attributed to favorable positioning of the cationic charges, which enables superior interaction with HSA. The thermodynamic parameters were calculated by van’t Hoff equation. The enthalpy and entropy changes were 42.15 kJ mol21 and 236.02 J mol21 K21 for [Cu(2,3-tmtppa)]4þ, 35.56 kJ mol21 and 216.75 J mol21 K21 for [Cu(3,4-tmtppa)]4þ, 32.60 kJ mol21, 270.97 J mol21 K21 for [Zn(3,4-tmtppa)]4þ and 47.75, 210.15 J mol21 K21 for [Co(3,4-tmtppa)]4þ, respectively. The results indicate that the process is entropy driven suggesting that hydrophobic interactions are the main driving forces for complex formation. Binding of porphyrazine complexes quenches fluorescence emission of HSA. The quenching process obeys linear Stern–Volmer relationship. The values of Stern–Volmer constants ðKSVÞ and quenching rate constants ðkqÞ were determined by Stern–Volmer relationship. The values of KSV and kq were determined nearly 106 M21 and 1015 M21 S21. Fluorescence studies also indicate that porphyrazine is probably bound to site I of HSA placed in sub-domain IIA, where tryptophan 214 is located. A competitive reaction by using phenyl putazone, a well-known site I marker, confirms this suggestion. The presence of hydrophobic cavity in the IIA sub-domain suggests that the interaction between HSA and porphyrazine is predominantly hydrophobic. q 2004 Published by Elsevier B.V