Synthesis and Properties of Spin-Labeled Phospholipids

Abstract

The use of phospholipids of different classes, labeled with isotopes, fluorescent or spin labels is an efficient technique in studying lipid-dependent biological systems, particularly of membrane structures [1, 2]. Lipids labeled with stable nitroxyl radicals in their hydrophobic moiety proved to be especially useful in such studies [3–5]. The first investigations in the synthesis and application of spin-labeled phospholipids were published by H. McConnel and coworkers in 1971. Proceeding from egg yolk phosphatidylcholine, the authors synthesized two types of paramagnetic phospholipids comprising a nitroxyl radical either in the polar moiety [6], or in the fatty-acyl residue in position 2 of the glycerol backbone [7]. The schemes of these syntheses are presented in Fig. 1. To incorporate a free radical into the polar choline group (radical I), egg phosphatidylcholine was treated with phospholipase D which cleaves the phosphodiester bond at the choline residue, and the resulting phosphatidylic acid was activated with 2,4,6-triisopropylbenzenesulfochloride to provide a phosphodiester bond with TEMPO-choline [6]. To introduce a radical into the fatty-acyl residues (radical II), egg phosphatidylcholine was treated with phospholipase A2 and the resulting lyso-phosphatidylcholine was acylated by the anhydride of spin-labeled fatty acid containing the nitroxyl in position 5 [7]. In this way, a number of spin-labeled phosphatidylcholine derivatives were prepared, including those on the basis of 6-doxylpalmitic acid [5]. Spin-labeled phospholipids I and II thus produced are semi-synthetic preparations.