Photochemistry and Photobiology, zyxwvuts 2006, 82: 274-280 zyxwvu Comparison of Phosphatidylcholine Vesicle Properties Related to Geometrical Isomerism Carla Ferreri’, Silvia Pierotti’, Andrea Barbieri’, Laura Zambonin2, Laura Landi2, Silvia R a ~ i * ~ , Pier Luigi LuisiS3, Francesco Barigelletti’ and Chryssostomos Chatgilialoglu*’ ’ISOF, Consiglio Nazionale delle Ricerche, Via P. Gobetti 101, 40129 Bologna, Italy *Dipartimento di Biochimica zyxwvuts “G. Moruzzi”, Universita di Bologna, Via lrnerio 48, 401 26 Bologna, Italy 3ETH Zentrum, Institute of Polymers, Universitat Strasse 6, Zurich, CH-8092, Switzerland Received 31 May 2005; accepted 18 August 2005; publishd online 23 August 2005 DOI: 10.1562/2005-06-01-RA-559 ABSTRACT zyxwvutsrq Glycerophosphatidylcholine containing trans-unsaturated fatty acid residues was prepared by reaction of the corre- sponding naturally occurring cis lipid with photochemically generated thiyl radicals. This modified lipid was chosen as the simplest model for gaining some insights of the complex scenario of membrane formation, in connection with the role of lipid geometry and the predominance of cis lipids in eukaryotic cells. The critical aggregation concentration for the spontaneous formation of vesicles was determined for cis and trans isomers with cis-parinaric acid used as a fluores- cent probe and it was found to be similar for both lipids. Vesicle dimensions were investigated by light scattering and electron microscopy, and the type of fatty acid residues influenced the vesicle diameter, with a decrease along the series cis > trans > saturated. Fluorescence measurement of dye release from trans and cis vesicles showed also a differ- ent permeability. A picture emerged of the geometrical isomer preference in cells as a process driven by natural selection during the life evolution of different organisms, both in terms of compartment dimensions and membrane functionality. * zyxwvutsrqpon To whom correspondence should be addressed: ISOF, Consiglio Nazionale delle Ricerche, Via P. Gobetti 101, 40129 Bologna. Fax: + 39 051 6398349; e-mail: chrys@isof.cnr.it 7 This paper is part of a special issue dedicated to Professor J. C. (Tito) Scaiano on the occasion of his 60th birthday. 1 Present address: Pevion Biotech Ltd, Rehhagstrasse 79, Bern, CH-3018, Switzerland. 1 Present address: Dipartimento di Biologia, Universita degli Studi di Roma Tre, Viale Marconi 446, 00146 Roma, Italy. zyxwvutsrq Abbreviations: CAC, critical aggregation concentration; CF, 5-(and-6)- carboxyfluorescein; cis-PnA, cis-parinaric acid; DPPC, 1,2-dipalmitoyl phosphatidylcholine; DSC, differential scanning calorimetry; FA, fatty acid; FAME, fatty acid methyl ester; GC, gas chromatography; HEPES, 4-(2-hydroxyethyI)-l-piperazineethanesulfonic acid; LUVET, large uni- lamellar vesicle by extrusion technique; MOPS, 3-(N-Morpholino)pro- panesulfonic acid; PEPC, 1-palmitoyl-2-elaidoyl phosphatidylcholine; POPC, 1 -palmitoyl-Zoleoyl phosphatidylcholine. zyxwvuts 0 2006 American Society for Photobiology 003 1-8655/06 INTRODUCTION Unsaturated fatty acid residues of glycerol-based phospholipids in eukaryotes generally have the naturally occurring cis double- bond geometry, which is strictly controlled during biosynthesis by the regiospecific and stereoselective enzymatic activity of desaturases (1,2). Membrane physical and functional properties, such as chain melting transition temperature (T,), “fluidity” and permeability, have a fine balance, which is also regulated by the appropriate ratio between saturated and cis unsaturated fatty acids (FA). Considering the variety of glycerophospholipids, which are the most abundant lipids in cell membranes, those containing fatty acid residues with the trans configuration of double bonds are not natural in eukaryotes. They have attracted attention because of their growing relevance in nutrition and health. The effect of trans fatty acid intake has been the subject of several investigations, which evidenced how trans isomers can be metabolized and become constituents of membrane phospholipids. Epidemiological studies have also linked this intake with health complications (3-7). In an effort to understand the potential harmful effects of trans phospholipids in membranes, some recent articles reinvestigated the different membrane properties induced by cis and trans geometries (8,9). Recently, it has also been shown that trans lipid geometry influences the affinity of membranes for cholesterol incorporation and the activity of eco-Ri protein (10). These works, together with previous data (1 1,12), contributed to a comprehensive picture of the lipid geometry strictly involved with the organization and functioning of cell membranes. On the other hand, trans geometry of membrane phospholipids is natural for some bacteria and means survival. In fact, unsaturated fatty acid moieties are essential for a short-term adaptation response to increases in the ambient temperature or high concentrations of toxic substances (1 3,14). They are enzymatically converted from cis to trans isomers, without shifting the double- bond position. Analogous enzymatic pathways for the cis-trans isomerization in mammals are unknown. We became interested in the biological meaning of trans lipid geometry (15.16) in the course of our studies on the thiyl radical- catalyzed cis-trans isomerization of phospholipids in model membranes (17-19). This process occurs by the addition of RS’ radicals to the double bond and subsequent B-fragmentation of the radical adduct, as shown in Fig. 1, wherein the equilibrium is shifted to the right toward the more stable geometrical trans isomer. 274