Abstract
A series of eight water soluble anionic, cationic, and neutral zinc(II) complexes were synthesized and characterized. The interaction of these complexes with bovine serum albumin (BSA), human serum albumin (HSA), lysozyme, and free tryptophan (Trp) was investigated using steady-state fluorescence spectroscopy. Static and dynamic fluorescence quenching analysis based on Stern-Volmer kinetics was conducted, and the decrease in fluorescence intensity of the Trp residue(s) can be ascribed predominantly to static quenching that occurs when the Zn complex binds to the protein and forms a nonfluorescent complex. The role played by the nature of the ligand, the metal, and complex charge in quenching Trp fluorescence was investigated. The binding association constants (Ka) ranged from 104to 1010 M−1and indicate that complexes with planar aromatic features have the strongest affinity for globular proteins and free Trp. Complexes with nonaromatic features failed to interact with these proteins at or in the vicinity of the Trp residues. These interactions were studied over a range of temperatures, and binding was found to weaken with the increase in temperature and was exothermic with a negative change in entropy. The thermodynamic parameters suggest that binding of Zn complexes to the proteins is a highly spontaneous and favorable process.
Highlights
Zinc(II) is one of the most important metal ions found in proteins and metalloenzymes, including alcohol dehydrogenase, carbonic anhydrase, carboxypeptidase A, matrix metalloproteinase, and thermolysin [1,2,3,4,5]
All characterization data, including IR spectroscopy, UV-vis spectroscopy, and elemental microanalysis are consistent with the formulations and structures given for each complex
The Zn complexes possessing planar aromatic ligands exhibit exceptionally strong interactions and much greater fluorescence quenching, with the Trp residues located in hydrophobic environments
Summary
Zinc(II) is one of the most important metal ions found in proteins and metalloenzymes, including alcohol dehydrogenase, carbonic anhydrase, carboxypeptidase A, matrix metalloproteinase, and thermolysin [1,2,3,4,5]. The most common donor protein-derived ligands tend to be His, Cys, Asp, Glu, and water molecules This hydrophilic shell is often embedded within a larger shell made up of hydrophobic side chains [7,8,9]. Several small Zn(II) complexes with varying O, N, and S donor ligands, including some that are novel, were synthesized and characterized The interaction of these complexes with BSA, HSA, lysozyme, and free tryptophan (Trp) was investigated using steady-state fluorescence spectroscopy to gain information on the binding mode, binding order, affinity, efficiency, and mechanism of fluorescence quenching. The interaction of Zn complexes with lysozyme is included since it is a very well characterized protein, containing six Trp residues that are located in the substrate binding sites and are essential for enzymatic activity. The experimental results have allowed us to discuss the type, degree, and nature of the interactions taking place between the protein and the Zn complexes, with particular consideration to polarity, hydrophobicity, and molecular charge
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