Photogenerated reagents for membrane labeling. 1. Phenylnitrene formed within the lipid bilayer

Phenylnitrene generated photochemically from phenyl azide that is bound to artificial phospholipid vesicles labels the fatty acid chains of the lipids in low yield. The labeling yield varies from approximately 3.3% with soybean lecithin (which is highly unsaturated) to approximately 0.25% with dimyristoyllecithin (which is completely saturated). Labeling is largely eliminated by reduced glutathione in the aqueous phase. Nitrenes are evidently unsatisfactory reagents for the labeling either of lipids or by analogy of the hydrophobic portions of membrane proteins. This is mainly because the long lifetimes and electrophilic character of nitrenes will lead to the preferential labeling of extrinsic membrane components. Phenyl azide itself is further compromised as a lipophilic reagent by its rather low partition coefficient into lipid bilayers, as measured by equilibrium dialysis.