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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...

The laboratory experiments of Al2O3 inclusions modified by calcium treatment in linepipe steels with the S content of 30 ppm and 310 ppm were performed at 1 873 K. Particularly, samples were taken before calcium treatment ( 1 minute before the calcium addition) and at various times (typically 1 minute, 10 minutes and 30 minutes) after calcium treatment to study the transient inclusions evolution during calcium modification. Traditional modification mechanism of Al2O3 inclusions is that Al2O3 inclusion is directly modified by dissolved calcium or indirectly modified via CaO formed from calcium and oxygen. In the current study, after Ca-Si alloy power addition, CaS outer layer were promptly formed on the angular Al2O3 inclusion. With the reaction of the CaS outer layer and the Al2O3 core or [O], the Al2O3 inclusion was modified to a spherical liquid calcium aluminate. Moreover, the equilibrium curve of Ca-O and Ca-S, the stability diagram of inclusions and equilibrium precipitation of inclusions during solidification in linepipe steels were calculated to study the formation of inclusions.

Previous studies from our laboratories have delineated the relationship between the acyl chain asymmetry of mixed-chain phosphatidylcholines, C(X):C(Y)PC, and the effect of ethanol concentration, [EtOH], on the main phase transition temperature, T(m), and the phase structure of the lipid bilayer composed of C(X):C(Y)PC using differential scanning calorimetry and X-ray diffraction techniques [Huang and McIntosh, Biophys. J. 72 (1997) 2702--2709]. In the present work, we have extended these studies to characterize the effect of [EtOH] on the T(m) and the phase structure of the lipid bilayer composed of sn-1 saturated/sn-2 monounsaturated phosphatidylcholines with various positions of the cis double bond. Specifically, five positional isomers of 1-eicosanoyl-2-eicosenoyl-sn-glycero-3-phosphocholines, C(20):C(20:1 Delta(n))PC with n=5, 8, 11, 13 and 17, were synthesized and studied. For C(20):C(20:1 Delta(n))PC with n=5 and 8, results from the calorimetric experiments showed that in response to various concentrations of ethanol, the change in T(m) of the lipid bilayer composed of monounsaturated lipids was characterized by a sigmoidal or biphasic profile in the plot of T(m) versus [EtOH]. In contrast, a continuous depression of the T(m) by ethanol was observed calorimetrically for C(20):C(20:1 Delta(n))PC with n> or =11. The X-ray diffraction experiments further demonstrated that C(20):C(20:1 Delta(5))PC and C(20):C(20:1 Delta(8))PC can undergo the ethanol-induced gel-to-fully interdigitated phase transition at T

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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

In this article, the formation mechanism of transverse corner cracks on a lowcarbon steel continuous-casting slab was investigated. The factors influencing the transverse corner cracks were discussed. The hot ductility of the lowcarbon steel within 600°C and 1250°C was detected using a thermal simulator Gleeble 1500 (Dynamic Systems, Inc., Poestenkill, NY) to determine the embrittling temperature range of the steel. The temperature of the slab varied with time, especially at the slab corner, and it was calculated and discussed. It was found that transverse corner cracks were generated on the ferrite films along grain boundaries, and there was little decarburization layer near the cracks. According to the calculated temperature at slab corner, the cooling water flow rate and cooling strategy were optimized by adjusting the cooling water flow rate at each spray cooling zone to avoid the embrittling temperature range at the bending and straightening segments of the caster. As a result, the transverse corner cracks were successfully weakened.