Thiols and the diazo group in photoaffinity labels

The photoactivable carbene precursor, 2-diazo-3,3,3-tri-fluoro-propionyloxy group, has been introduced recently for light-induced covalent crosslinking in studies of protein-phospholipid interactions in biomem-branes. The diazo group in this reagent has now been shown to undergo reduction in the dark by a number of thiols (dithiothreitol, 2-mercaptoethanol, cysteine and reduced glutathione) used commonly as protective agents for proteins. In contrast, thioglycolate did not cause significant reduction and, therefore, can be safely used as a protective agent. In vesicles formed from phospholipids containing the above photolabel in the fatty acyl chain, dithiothreitol and 2-mercaptoethanol caused reduction. However, cysteine and reduced glutathione caused insignificant reduction of the diazo group, presumably because of their non-permeant nature.