Quenching of tryptophan fluorescence in various proteins by a series of small nickel complexesElectronic supplementary information (ESI) available. CCDC reference numbers 830386830389 and 853856. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c2dt12169g

A series of twelve anionic, cationic, and neutral nickel( ii ) complexes have been synthesized and characterized. The interaction of these complexes with bovine serum albumin (BSA), human serum albumin (HSA), lysozyme (Lyso), and tryptophan (Trp) has been studied using steady-state fluorescence spectroscopy. Dynamic and static quenching constants have been calculated, and the role played in quenching by the ligand and complex charge investigated. The nickel complexes showed selectivity towards the different proteins based on the environment surrounding the Trp residue(s). Only small neutral complexes with hydrophobic ligands effectively quenched protein fluorescence via static quenching, with association constants ranging from 10 2 M 1 (free Trp) to 10 10 M 1 (lysozyme), indicating a spontaneous and thermodynamically favorable interaction. The number of binding sites, on average, was determined to be one in BSA, HSA and free Trp, and two in lysozyme. The degree of protein binding and fluorescence quenching by several novel nickel( ii ) complexes has been investigated. Small neutral complexes with hydrophobic ligands bind with the greatest affinity.