New method of blocking lysyl residues of proteins using 4-(4-methoxy-3-sulphophenyl)-2,6-bis-(4-sulphophenyl)py rylium perchlorate

Solution- and Crystal-Phase Covalent Modification of Lysozyme by a Purpose-Designed Organoruthenium Complex. A MALDI-TOF MS Study of its Metal Binding Sites

Study of the reaction between the transition organometallic complex 4-ruthenocenyl 2,6-dimethylpyrylium tetrafluoroborate and the enzyme hen egg white lysozyme (HEWL) in solution and by diffusion in crystals was performed by use of a combination of spectroscopic and chromatographic methods. Conjugation involving the lysine residues of lysozyme appeared to occur readily, yielding very stable ruthenocenyl pyridinium adducts with average degrees of incorporation ranging from 0.2 to 1.8 metal complexes per protein molecule, depending on reaction conditions. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) revealed that the protein conjugates were in fact mixtures of unmodified, mono-, di- and sometimes tripyridinium adducts. In combination with reversed-phased HPLC, we were able to show that six different monoruthenocenyl pyridinium adducts were formed in solution. This result was confirmed by trypsin digestion of a ruthenocenyl pyridinium conjugate and MALDI-TOF MS analysis of the peptide mixture, which showed that lysines 1, 13, 33, 96, 97 and 116 were involved in the reaction with the pyrylium complex, lysines 13, 33 and 116 being the major binding sites. In the tetragonal crystal state, no binding of the ruthenium complex was shown to occur at lysine 116, owing to steric hindrance at this particular position.

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.