FT-IR observation of covalent labelling of lysozyme crystals by organometallic complexes of transition metals

Electrophilic complexes of iron and chromium which have been reported to react with proteins in solution have been reacted with hen-egg white lysozyme (HEWL) in both the solution and crystal phases under similar pH and buffer conditions. This work was carried out with a view to developing novel side-chain selective heavy metal derivatives for protein X-ray crystallographic studies. Reaction of HEWL with a tricarbonyldienyliron cation (1) in aqueous solution led to modification of the sole histidine residue with concurrent reversible modification of other protein residues. Reaction of (1) with crystalline HEWL showed no covalent binding and only a build up of a hydrolysis product in the water channels of the crystal was observed. Reactions with a series of tricarbonylarylchromium pyrylium salts (2) led to the formation of stable covalent HEWL derivatives in solution. Chromatographic and IR spectroscopic studies showed that binding took place specifically at the epsilon-amino group of lysine residues to give a series of mono- and di-substituted products. When crystals of HEWL were soaked with the chromium reagents covalent binding to some of the lysine residues was also observed. In contrast, HEWL crystals which had their lysine side chains disabled did not bind any of the chromium reagents.