Sites of intermolecular crosslinking of fatty acyl chains in phospholipids carrying a photoactivable carbene precursor

Sonicated vesicles of l-fatty acyl-2-w-(2-diazo-3.3,3-trifluoropropionoxy) fatty acyl sn-glycero-3-phosphorylcholines were shown recently to form intermolecular crosslinks by insertion of the photogenerated carbene into a C-H bond of a neighboring hydrocarbon chain. We now report that photolysis of multilamellar dispersions gives a second series of products in which carbene insertion is accompanied by elimination of a molecule of hydrogen fluoride. The sites of crosslinking in the latter compounds have been studied by mass spectrometry using phospholipids with varying chain lengths of the fatty acyl groups carrying the carbene precursor. The patterns observed show that the point of maximum crosslinking is consistent with the recent conclusion that in phospholipids the sn-2 fatty acyl chain trails the sn-1 chain by 2-4 atoms.

Carbamyl Analogs of Phosphatidylcholines Synthesis, Interaction With Phospholipases and Permeability Behavior of their Liposomes

A novel class of phospholipase-resisting phosphatidylcholine analogs, in which the C-2 ester group or both C-1 and C-2 ester groups have been replaced by carbamyloxy functions (-NH-C-O-), have been synthesized. These lipids were not degraded by phospholipase A2 while complete hydrolysis occurred with phospholipase C. Ultrasonic irradiation of the aqueous dispersions of the phospholipids in the presence as well as in the absence of cholesterol resulted in the formation of closed bilayer structures as evidenced by negative staining electron microscopy and also by their ability to entrap [14C]glucose. The leakage rates of glucose at 37°C from liposomes of these compounds have also been measured. Liposomes consisting of 1,2-dipentadecanylcarbamyloxy-sn-glycero- 3-phosphorylcholine were found to be more leaky (2.1 %/h) as compared to the liposomes of 1-palmitoyl-2-pentadecanylcarbamyloxy-sn -glycero-3-phosphoryl- choline (O.5%/h). Moreover, inclusion of cholesterol (33 mol%) into the bilayers of the former phospholipid had no effect on the leakage rate (2.4%/h) while it effectively reduced permeability of the latter (O.22%/h). These phosphatidylcholines are useful for studying the possible role of phospholipases in the capture and lysis of liposomes in vivo.