Interaction of some water-soluble metalloporphyrazines with human serum albumin

The thermodynamic of the binding of N,N',N,N'''-tetramethyltetra-2,3-pyridinoporphyrazinatocopper(II) ([Cu(2,3-tmtppa)](4+)), N,N',N,N'''-tetramethyltetra-3,4-pyridinoporphyrazinatocopper(II) ([Cu(3,4-tmtppa)](4+)), N,N',N,N'''-tetramethyltetra-3,4-pyridinoporphyrazinatocobalt(II) ([Co(3,4-tmtppa)](4+)), and N,N',N,N'''-tetramethyltetra-3,4-pyridinoporphyrazinatozinc(II) ([Zn(3,4-tmtppa)](4+)), 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 X 10(4) M-1 for [Cu(2,3-tmtppa)](4+) and, 1.35 X 10(5), 6.92 X 10(5) and 2.00 X 10(5) M-1 for [Cu(3,4-tmtppa)](4+), [Zn(3,4-mtppa)](4+), and [Co(3,4-tmtppa)](4+), respectively, at 27degreesC. 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 mol(-1) and 236.02 J mol(-1) K-1 for [Cu(2,3-tmtppa)](4+), 35.56 kJ mol(-1) and 216.75 J mol(-1) K-1 for [Cu(3,4-tmtppa)](4+), 32.60 kJ mol(-1), 270.97 J mol(-1) K-1 for [Zn(3,4-tmtppa)](4+) and 47.75, 210.15 J mol(-1) K-1 for [Co(3,4-tmtppa)](4+), 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 (K-SV) and quenching rate constants (k(q)) were determined by Stern-Volmer relationship. The values of K-SV and k(q) were determined nearly 10(6) M-1 and 10(15) M-1 S-1. 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. (C) 2004 Published by Elsevier B.V.