Exploring the interactions between phosphane-based platinum(II) dichlorides and cytochrome c by electrospray ionization high resolution mass spectrometry

Reactions of platinum drugs with proteins are attracting growing attention for their important biological implications. Several studies pointed out that electrospray ionization mass spectrometry constitutes an excellent technique to study metallodrug-protein interactions. We report here the analysis of the reaction of two diphosphane platinum(II) dichlorides, namely bis(trimethylphosphane) platinum(II) dichloride and bis(triethylphosphane) platinum(II) dichloride, with horse heart cytochrome c (cyt c) monitored through electrospray ionization high resolution mass spectrometry. A remarkable selectivity in terms of adduct stoichiometry was highlighted for both compounds: for 3:1 metal:protein ratio, cyt c platination was almost complete in contrast to what we observed for “classical” platinum(II) drugs cisplatin, carboplatin and oxaliplatin that preferably give monoadducts under the same experimental conditions. In order to shed light on the nature of the three possible binding sites, digestion experiments, using endoproteinase Asp-N and trypsin, were carried out on bis(triethylphosphane) platinum(II) dichloride incubated with cyt c. The results of digestion experiment with endoproteinase Asp-N proved that the three binding sites were within the G1-T49 cyt c segment and the experiment with trypsin supported the involvement of histidines H18, H26 and H33 as preferential binding sites. In light of these results the phosphane ligand seems to confer to the platinum(II) center a pronounced selectivity for histidine coordination on cyt c. This is somewhat surprising since the “classical” platinum(II) drugs have preferred binding sites on cyt c at M65, H26 and H33, the first site being the most prominent one.