Thermodynamics investigation of a series of metalloporphyrazine-bovine serum albumin complexes

Abstract

The equilibrium binding of the tetra-cationic complexes ( N , N ′, N ″, N ‴-tetra-methyltetra-2,3-pyridinoporphyrazinato)copper(II), ([ Cu (2,3- TMTPPA )]4+), ( N , N ′, N ″, N ‴-tetra-methyltetra-3,4-pyridinoporphyrazinato)copper(II), ([ Cu (3,4- TMTPPA )]4+), (( N , N ′, N ″, N ‴-tetra-methyltetra-3,4-pyridinoporphyrazinato)cobalt(II), ([ Co (3,4- TMTPPA )]4+) and (( N , N ′, N ″, N ‴-tetra-methyltetra-3,4-pyridinoporphyrazinato)zinc(II), ([ Zn (3,4- TMTPPA )]4+) with bovine serum albumin (BSA) has been studied in phosphate buffer pH = 7.0 and at various temperatures using multi-spectroscopy techniques. The results of resonance light scattering (RLS) studies represent no aggregate formation of porphyrazine in the surface of BSA and low tendency of these porphyrazine for aggregate formation. The binding constants and binding stoichiometries were determined by analyzing of optical absorption spectra of porphyrazine complexes at various concentration of BSA using SQUAD software. The results show that the best fitting corresponds to a 1:1 complex model between BSA and porphyrazines. The thermodynamic parameters were calculated by van't Hoff equation at various temperatures. The data indicate that the process is entropy driven suggesting that hydrophobic interactions play a considerable role in the complex formation. The binding of porphyrazine complexes to BSA quenches fluorescence emission of BSA via a dynamic mechanism and the quenching process obeys a linear Stern-Volmer relationship. The average aggregation number of BSA, which has been calculated from the analysis of fluorescence quenching data, indicates the absence of any porphyrazine induced aggregation of BSA due to its interaction with porphyrazine complexes. Fluorescence studies also indicate that porphyrazine is bound to site I of BSA placed in sub-domain IIA, where tryptophan 214 is located.