Differential scanning calorimetry study of mixed-chain phosphatidylcholines with a common molecular weight identical with diheptadecanoylphosphatidylcholine

et al. 1999

In an attempt to investigate systematically the effects of various single and multiple cis carbon-carbon double bonds in the sn-2 acyl chains of natural phospholipids on membrane properties, we have de novo synthesized unsaturated C 20 fatty acids comprised of single or multiple methylene-interrupted cis double bonds. Subsequently, 15 molecular species of phosphatidylethanolamine (PE) with sn-1 C 20 -saturated and sn-2 C 20 -unsaturated acyl chains were semi-synthesized by acylation of C 20 -lysophosphatidylcholine with unsaturated C 20 fatty acids followed by phospholipase D-catalyzed base-exchange reaction in the presence of excess ethanolamine. The gel-to-liquid crystalline phase transitions of these 15 mixed-chain PE, in excess H 2 O, were investigated by high resolution differential scanning calorimetry. In addition, the energy-minimized structures of these sn-1 C 20 -saturated/sn-2 C 20 -unsaturated PE were simulated by molecular mechanics calculations. It is shown that the successive introduction of cis double bonds into the sn-2 acyl chain of C(20):C(20)PE can affect the gel-to-liquid crystalline phase transition temperature, T m , of the lipid bilayer in some characteristic ways; moreover, the effect depends critically on the position of cis double bonds in the sn-2 acyl chain. Specifically, we have constructed a novel T m diagram for the 15 species of unsaturated PE, from which the effects of the number and the position of cis double bonds on T m can be examined simultaneously in a simple, direct, and unifying manner. Interestingly, the characteristic T m profiles exhibited by different series of mixed-chain PE with increasing degree of unsaturation can be interpreted in terms of structural changes associated with acyl chain unsaturation.Most naturally occurring diacyl phospholipids in eukaryotic cell membranes are of a mixed acyl chain variety, meaning that the fatty acids esterified at the sn-1 and sn-2 positions of the glycerol backbone are originated primarily in vivo from saturated and unsaturated fatty acyl-CoA, respectively. Since the chemical composition of fatty acids can vary greatly in terms of the acyl chain length, the degree of unsaturation, and the position of cis carbon-carbon double bonds (⌬-bonds), 1 membrane phospholipids are structurally an extremely diverse group of amphipathic molecules. In a given type of cell, membrane phospholipids may amount to several hundreds of distinctive chemical species. Despite the bewildering diversity, the basic motif of most unsaturated fatty acyl chains is surprisingly simple, viz. in the sn-2 acyl chain, the cis carboncarbon double bonds are invariably separated by a three-carbon unit comprised of a methylene group (-CH 2 -) sandwiched by two olefinic carbons. The biochemical significance of this regular methylene-interrupted interval is, however, not clear.Although it has long been known that mixed-chain diacyl phospholipids in aqueous media can uniquely assemble into a two-dimensional sheet-like structure called the lipid bilayer, progress is non...

Section: Methodsmentioning

confidence: 99%

Effects of Various Numbers and Positions of cisDouble Bonds in the sn-2 Acyl Chain of Phosphatidylethanolamine on the Chain-melting Temperature

Wang

et al. 1999

“…The other 11 molecular species were sn-1-saturated/sn-2-polyunsaturated PE containing three, four, and five cis double bonds at different positions in the sn-2-acyl chain. Initially, 11 molecular species of the corresponding sn-1-saturated/sn-2-polyunsaturated PC were semisynthesized and purified by established procedures (14), with a modified condition that all procedures were carried out as far as possible in an O 2 -free, N 2 atmosphere to minimize autoxidation. This precautious step was also applied to the rest of the PE synthesis.…”

Section: Methodsmentioning

confidence: 99%

“…Subsequent repeated reheatings result in, again, a single endothermic transition at 82.5°C, indicating that the gel-to-liquid crystalline phase transition of the C(20):C(20)PE bilayer is reproducible, and the crystalline-to-gel phase transition is irreversible under the present experimental conditions. Another example pertaining to the thermal history dependence of the phase transition behavior is observed for the aqueous dispersion of C (20):C(20:2⌬ 11,14 )PE. In the first DSC heating scan, a single, relatively sharp transition peaked at 26.4°C is observed; however, the same sample exhibits a broader transition centered at 22.4°C upon immediate reheating (Fig.…”

Section: Methodsmentioning

confidence: 99%

Influence of the Positions of cis Double Bonds in the sn-2-Acyl Chain of Phosphatidylethanolamine on the Bilayer's Melting Behavior

Huang

et al. 1997

In an attempt to examine the effects of different numbers and positions of cis double bonds in the sn-2-acyl chain of phosphatidylethanolamine (PE) on the bilayer's melting behavior, 21 molecular species of PE were first semisynthesized, and their T m and ⌬H values were subsequently determined by high resolution differential scanning calorimetry. In the plot of T m versus the number of the cis double bond, some characteristic profiles were observed for the various series of PEs. In recent years, there has been increasing evidence suggesting the existence of a close correlation between some vital functions of cells and the various degrees of unsaturation in the sn-2-acyl chains of membrane phospholipids (1-3). Consequently, detailed and systematic investigation of the properties of sn-1-saturated/sn-2-unsaturated phospholipids in the organized membrane structure such as the lipid bilayer should be recognized and pursued. Furthermore, now is the time for studying unsaturated phospholipids, since sophisticated computer-based molecular modeling techniques from the hardware and the software fields have come together to provide valuable structural and energetic information about self-assembled biomolecules within a reasonable time frame. Results obtained with experimental and modeling studies of sn-1-saturated/sn-2-unsaturated phospholipids could eventually help illuminate the specific roles played by various natural phospholipids in many membrane-related cell functions or diseases.The first detailed and systematic studies of the thermotropic phase behavior of phospholipids containing various numbers of cis double bonds in their sn-2-acyl chains were performed by Keough and co-workers (4 -6). By using two series of phosphatidylcholines (PCs) 1 and by applying the differential scanning calorimetry (DSC) techniques, they have shown that the introduction of a cis double bond into the sn-2-acyl chain at or near the center of the chain has a marked reducing effect on the phase transition temperature (T m ) associated with the gel-toliquid crystalline phase transition of the lipid bilayer. A second cis double bond incorporated at the methylene-interrupted position toward the methyl end also reduces the T m , but by a smaller amount. Interestingly, the introduction of a third cis double bond between the second cis double bond and the methyl end can cause a slight increase in T m . This calorimetric work has been confirmed more or less by other investigators using PCs with different chain lengths (7,8).Recently, we have studied systematically the effect of different positions of single cis double bond in the sn-2-acyl chain on the phase transition behavior of lipid bilayers composed of sn-1 saturated/sn-2-monounsaturated PCs or phosphatidylethanolamines (PEs) by high resolution DSC and computer-based molecular mechanics (MM) simulations (9 -11). The position of the single cis double bond (⌬ n ) is found in both lipid classes to exert a characteristic influence on the gel-to-liquid crystalline

“…Semisynthesis of Monounsaturated Phosphatidylcholines-Isomerically pure (Ͼ98 mol %) monounsaturated phosphatidylcholines were semisynthesized at room temperature by acylation of CdCl 2 adducts of lysophosphatidylcholine, in dry chloroform, with cis-monounsaturated fatty acid anhydride that was prepared in situ from unsaturated fatty acid and dicyclohexylcarbodiimide, in the presence of catalyst 4-pyrrolidinopyridine, according to the modified procedure of Mena and Djerassi (8) as described previously (9). All reactions were carried out under an N 2 atmosphere to avoid lipid oxidation.…”

Section: Methodsmentioning

confidence: 99%

“…All reactions were carried out under an N 2 atmosphere to avoid lipid oxidation. The synthesized lipids were purified by silica gel column chromatography as described elsewhere (9). All monounsaturated fatty acids with different chain lengths and distinct position of the double bond were purchased from Sigma or Nu Chek Prep, Inc. (Elysian, MN).…”

Section: Methodsmentioning

confidence: 99%

Phosphatidylcholines with sn-−1 Saturated and sn-−2 cis-Monounsaturated Acyl Chains

Wang

et al. 1995

Recently, we have shown by high resolution differential scanning calorimetry that the position of a cis double bond (⌬-bond) in a series of 1-stearoyl-2-octadecenoyl-phosphatidylcholines can affect the phase transition temperature (T m ) or enthalpy (⌬H) of the gelto-liquid crystalline phase transition of this series of lipids in the following manner. The value of T m (or ⌬H) is minimal when the ⌬-bond is positioned at C(11) in the sn-2 acyl chain; in addition, this value increases steadily as the ⌬-bond migrates toward either end of the acyl chain, resulting in a symmetrical, inverted bell-shaped profile (Wang, Z.-q., Lin, H.-n., Li, S., and Huang, C. (1995) J. Biol. Chem. 270, 2014 -2023). In this communication, we have further demonstrated the inverted bell-shaped profile of T m using 1-arachidoyl-2-eicosenoyl-phosphatidylcholines. In addition, we have extended the lipid series of 1-stearoyl-2-octadecenoyl-phosphatidylcholines to include 1-arachidoyl-2-octadecenoyl-phosphatidylcholines and 1-behenoyl-2-octadecenoyl-phosphatidylcholine, each series with a ⌬-bond at varying carbon position of 6, 7, 9, 11, 12, and 13. Calorimetric results obtained with these three series of lipids indicate that the inverted bell-shaped curve shifts toward higher temperatures in a nonuniform manner as the saturated sn-1 acyl chain length increases from 17 to 19 and then to 21 C-C bond lengths. Specifically, the T m (or ⌬H) values are nearly identical for these cis-monoenoic lipids when their ⌬-bonds are positioned at C(13). Based on the height of the rotational energy barrier obtained with molecular mechanics calculations, it is evident that the rotational flexibility of the single C-C bond adjacent to the ⌬-bond in 1-stearoyl-2-octadecenoyl-phosphatidylcholine increases as the ⌬-bond migrates from C(9) to C(13). The differential scanning calorimetry results obtained with the three series of lipids can thus be attributed to an increase in the rotational flexibility of the short chain segment succeeding the C(14) atom in the sn-2 octadecenoyl chain. In this communication, we also propose that in the gel-state bilayer of sn-1 saturated/ sn-2 cis-monounsaturated phosphatidylcholine the entire length of the shorter segment of the sn-2 acyl chain acts as a structural perturbing element; hence, it is mainly responsible for the large lower T m of the monoenoic lipid relative to the saturated counterpart. Finally, two general equations relating T m with the structural parameters of cis-monoenoic phosphatidylcholines are presented. These equations, formulated primarily on the assumption that the short segment of the sn-2 acyl chain acts as a perturbing element, are shown to have strong predictive power in estimating the T m values of the gel-to-liquid crystalline phase transitions for sn-1 saturated/sn-2 cis-monounsaturated phosphatidylcholines.Phosphatidylcholines isolated from the plasma membrane of eukaryotic cells are a structurally diverse group of phospholipids. The bewildering variety of membrane phosphatidylcholines originates fro...