Synthesis of carbamyl and ether analogs of phosphatidylcholines

Probing the role of C-1 ester group in Naja naja phospholipase A2-phospholipid interactions using butanetriol-containing phosphatidylcholine analogues

To understand the role of the ester moiety of the sn-1 acyl chain in phospholipase A2-glycerophospholipid interactions, we introduced an additional methylene residue between the glycerol C1 and C2 carbon atoms of phosphatidylcholines, and then studied the kinetics of hydrolysis and the binding of such butanetriol-containing phospholipids with Naja naja phospholipase A2. Hydrolysis was monitored by using phospholipids containing a NBD-labelled sn-2 acyl chain and binding was ascertained by measuring the protein tryptophan fluorescence. The hydrolysis of butanetriol-containing phospholipids was invariably slower than that of the glycerol-containing phospholipids. In addition, the enzyme binding with the substrate was markedly decreased upon replacing the glycerol residue with the 1,3,4-butanetriol moiety in phosphatidylcholines. These results have been interpreted to suggest that the sn-1 ester group in glycerophospholipids could play an important role in phospholipase A2-phospholipid interactions.

Glycerol backbone conformation in phosphatidylcholines is primarily determined by the intramolecular stacking of the vicinally arranged acyl chains

To analyse the effect of the altered glycerol backbone structure on the glycerophospholipid conformation, we have replaced the glycerol moiety by the rac-1,2,4-butanetriol residue in 1,2-diacyl-sn-glycero-3-phosphocholines (PC), and then analysed the resulting 1,2-dialkanoyloxy-rac-but-4-yl-[2-(trimethylammonium)ethyl] phosphates (1,2-bPC) and 1,3-dial-kanoyloxy-rac-but-4-yl-[2-(trimethylammonium)ethyl] phosphates (1,3-bPC) by high-resolution 1H- and 13C-NMR spectroscopy in both CDCI3 and D2O. The preferred conformation about the C1-C2 glycerol bond in PC was almost completely preserved in 1,2-bPC, but it was completely random in case of 1,3-bPC. Out of the three C-C bonds present in the butanetriol backbone of 1,3-bPC, only the C2-C3 bond experienced a restricted rotation. However, the conformational preference about this bond was virtually similar to that observed for the C1-C2 bond in PC. These results clearly demonstrate that the preferred conformation of the glycerol backbone is determined primarily by the intramolecular acyl chain stacking which essentially requires a vicinal arrangement of the acyl chains in glycerophospholipids.

1-O-benzyl-2-O-methyl-rac-glycerol: a key intermediate for synthesis of ether glycerolipids

A new scheme for synthesis of saturated 1-O-alkyl-2-O-methyl-rac-glycerolipids is described. The reductive cleavage of 2-O-methyl-1,3-O,O-benzylideneglycerol with BH3.THF complex leads to 1-O-benzyl-2-O-methyl-rac-glycerol, which is a key in intermediate for the facile preparation of 1-O-alkyl-2-O-methyl-rac-glycerol and derivatives.

Chirospecific syntheses of 2H- and 13C-labeled 1-O-alkyl-2-O-alkyl'-sn-glycero-3-phosphoethanolamines and 1-O-alkyl-2-O-alkyl'-sn-glycero-3-phosphocholines

A convenient sequence for the synthesis of 1-O-alkyl-2-O-alkyl'-sn-glycero-3-phospholipids was demonstrated starting from 2,3-O-isopropylidene-sn-glycerol, which was first alkylated with 1-bromohexadecane, then converted to the corresponding benzylidene analog. Other less convenient methods to prepare 2,3-O-benzylidene-1-O-hexadecyl-sn-glycerol were also investigated. The key step in the synthesis was the reduction of 2,3-O-benzylidene-1-O-hexadecyl-sn-glycerol with lithium aluminum hydride-aluminum chloride to give 3-O-benzyl-1-O-hexadecyl-sn-glycerol as the major product in 79% yield. The syntheses of 1-O-hexadecyl-2-O-hexadecyl-(1',1'-d2,-sn-glycero-3-phosphoethanolamine and 1-O-hexadecyl-2-O-hexadecyl-(1'-13C)-sn-glycero-3-phosphoethanolamine as well as the correspondingly labeled sn-glycero-3-phosphocholine analogs were then performed. The optical purities of the synthetic intermediates and the ether lipids were established by a novel 1H-NMR method.

Carbamyl phosphatidylcholine--cholesterol interactions in unilamellar vesicles

Small unilamellar vesicles formed from 1-palmitoyl-2-O-(N-(heptadec-8-cis-enyl)carbamyl)-sn-glycero-3-pho sphocholine (CMPC) or 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) in the presence of varying amounts of cholesterol have been studied using fluorescence polarization and NMR (1H and 13C) techniques. The fluorescence polarization and 1H-NMR data clearly indicate that the phospholipid packing order in CMPC bilayers is significantly greater than that in the POPC bilayers. The 13C-NMR chemical shift measurements show that this difference between the two phospholipids possibly arises due to the intramolecular hydrogen-bond formation between the -NH and the phosphate residues in the CMPC molecule. It is further shown that unlike POPC, the CMPC packing order is not much affected by including cholesterol in the phospholipid bilayers. These results demonstrate that introduction of one -NH residue adjacent to the C-2 carbonyl carbon in the POPC molecule could make its structure more ordered in the vesicles bilayer, and also would alter its interactions with cholesterol.

Effect of phospholipid structure on stability and survival times of liposomes in circulation

The phosphatidylcholine (PC) component of liposomes was structurally modified by replacing its C-1, or both C-1 and C-2, ester linkage(s) with an ether and/or carbamyl bond(s) or by changing its steric configuration. Small unilamellar liposomes were formed from PC, traces of the corresponding 14C-labeled PC and cholesterol in the presence of 6-carboxyfluorescein (02.M) by sonication, and purified by centrifugation. These liposomes were administered intravenously to rats, and their stability in blood as well as the rate of their clearance from the circulation were determined. Stability and survival times of liposomes were markedly increased by modifying both the C-1 and the C-2 ester linkages in PC. A similar but quantitatively smaller effect was observed when only the C-1 ester linkage was modified. However, the stability remained unaffected by changing the steric configuration of PC, but this modification influenced the clearance rate of liposomes from the circulation. These results demonstrate that both stability in blood and the clearance rate from circulation can be modulated by structurally modifying the ester linkages in the phospholipid component of liposomes.

Influence of the phospholipid structure on the stability of liposomes in serum

The effect of serum on the structural integrity of liposomes consisting of ether and/or carbamyl analogs of 1,2-diester phosphatidylcholine (PC) has been evaluated by measuring both the efflux of the entrapped 6-carboxyfluorescein and the lipid transfer to serum proteins, and the results have been compared with the egg PC liposomes. Replacement of the C-1 ester bond in PC by an ether linkage did not significantly enhance the liposome stability, but it was markedly increased upon introducing further structural changes in the C-2 ester region of the resulting 1-ether-2-ester PC. However, the stability was not influenced by altering the steric configuration of the latter phospholipid. These results strongly suggest that lysis of liposomes in serum can be prevented by structurally modifying the ester bond(s) in the phospholipid component of liposomes.