Membrane lipids calorimetric studies

The interactions of TDZ (6) with model membranes composed of different phospholipids were also studied by the same group [78]. Calorimetric studies demonstrated that TDZ (6) altered the thermotropic properties of negatively charged DMPC membranes to a larger extent than of zwitterionic phospholipids (PC and PE). The character of the drug-induced changes of the transition parameters of all studied lipids indicated that TDZ (6), similarly to other phenothiazine derivatives, was likely to be localized close to the po-lar/apolar interface of the bilayers. Experiments in which fluorescent probe 1,6-diphenyl-1,3,5-hexatriene (DPH) was employed revealed that TDZ (6) reduced the mobility of lipid molecules in a concentration-dependent manner and thus decreased membrane fluidity. The influence of TDZ (6) on isolated... 

McElhaney, R. N. Differential scanning calorimetric studies of lipid-protein interactions in model membrane systems. Biochimica et Biophysica acta 564 361-421, 1986. 

Gennis RB (1989) Biomembranes molecular structure and function. Springer, New York Yeagle P (1992) The structure of biological membranes. CRC Press, Boca Raton McElhaney RN (1986) Differential scanning calorimetric studies of lipid-protein interactions in model membrane systems. Biochim Biophys Acta 864(3-4) 361-42l HianikT, Passechnik VI (1995) Bilayer lipid membranes structure and mechanical properties. Kluwer, Netherlands... 

In this contribution, there will be described the principal type of calorimetric experiments which can be perfonned to elucidate the behavior of lipid model membranes. Most of the examples come from work performed in the author s laboratory. Numerous other groups are involved with calorimetric studies on tliese model membrane systems and there exists a vast amount of literature, because calorimetry has become a standard analytical method in the last ten to twenty years. It is our goal to present the principles of the method and not an extensive review over the current literature. Therefore, this chapter is necessarily biased towards work from the author s laboratory and apologies are extended to all those whose work is not adequately represented here. 

Non-calorimetric studies of effects of hydrostatic high pressure on plant sources are more common. In general, pressure effects on plant metabolic rates are small, but some distinct changes have been noted in growth, photosynthesis, temperature responses, and plant structure [48]. Interpretation of the role of pressure on plant metabolism remains uncertain. Hypotheses that have been framed to explain pressure effects are generally written in terms of volume changes and structural transitions in chloroplasts and in lipid membranes, but there are equally tenable alternative explanations such as pressure effects on equilibria. 

Lewis RNAH, Winter I, Kriechbaum M et al (2001) Studies of the structure and organization of cationic lipid bilayer membranes calorimetric, spectroscopic, and x-ray diffraction studies of linear saturated P-O-ethyl phosphatidylcholines. Biophys J 80 1329-1342... 

Lewis RNAH, McElhaney RN (2000) Surface charge markedly attenuates the nonlamellar phase-forming propensities of lipid bilayer membranes calorimetric and P-31-nuclear magnetic resonance studies of mixtures of cationic, anionic, and zwitterionic lipids. Biophys J 79 1455-1464... 

On the other hand, studies with three-dimensional isotropic lamellar matrices have shown that Azone is a weakly polar molecule, which can occupy the interfacial region as well as the hydrocarbon interior of bilayers [86,87]. The contrasting observations of Azone promoting the assembly of reversed-type liquid-crystal phases (e.g., reversed hexagonal and reversed micellar) in simple model lipid systems [88-90], while also favoring the formation of lamellar structures in one of these mixtures [91], adds further confusion to the discussion [92]. This notwithstanding, the studies by Schiickler and co-workers [91] emphasize the differences in the calorimetric profiles of intact human stratum comeum (HSC) and model SC lipid mixtures Although these systems are clearly useful and versatile, extrapolation of inferences from model lipids to the intact membrane must be performed with caution. 

Although studies of the thermotropic phase behavior of singlecomponent multilamellar phospholipid vesicles are necessary and valuable, these systems are not realistic models for biological membranes that normally contain at least several different types of phospholipids and a variety of fatty acyl chains. As a first step toward understanding the interactions of both the polar and apolar portions of different lipids in mixtures, DSC studies of various binary and ternary phospholipid systems have been carried out. Phase diagrams can be constructed by specifying the onset and completion temperatures for the phase transition of a series of mixtures and by an inspection of the shapes of the calorimetric traces. A comparison of the observed transition curves with the theoretical curves supports... 

The occurrence of cholesterol and related sterols in the membranes of eukaryotic cells has prompted many investigations of the effect of cholesterol on the thermotropic phase behavior of phospholipids (see References 23-25). Studies using calorimetric and other physical techniques have established that cholesterol can have profound effects on the physical properties of phospholipid bilayers and plays an important role in controlling the fluidity of biological membranes. Cholesterol induces an intermediate state in phospholipid molecules with which it interacts and, thus, increases the fluidity of the hydrocarbon chains below and decreases the fluidity above the gel-to-liquid-crystalline phase transition temperature. The reader should consult some recent reviews for a more detailed treatment of cholesterol incorporation on the structure and organization of lipid bilayers (23-25). 

Synthetic lipid A analogues were used in FTTR investigations to study possible intermolecular conformations of neighboring molecules [79]. There is good evidence that the bisphosphory-lated /3-(l—>6)-linked GlcN disaccharide backbone of lipid A is inclined (20—40° relative to the membrane normal), thus, the phosphate linked to 01 is reaching to the outside and that at 04 is buried in the membrane. This model is supported by data from transition temperature measurements and calorimetric experiments. However, it should be noted that other authors reported the reverse conformation with the 04 phosphate on the surface and the 01 phosphate in the membrane [80,81]. 

Seto, G. W., Marwaha, S., Kobewka, D. M., et al. (2007) Interactions of the Australian tree frog antimicrobial peptides aurein 1.2, citropin 1.1 and maculatin 1.1 with lipid model membranes Differential scanning calorimetric and Fourier transform infrared spectroscopic studies. Biochimica et Biophysica Acta, 1768, 2787-2800. 

These results have been extended and confirmed by numerous further studies. When intrinsic proteins interact with lipid mixtures that are partially immiscible, e.g. l-palmitoyl-2-oleoyl PC (POPC) and POPE, the calorimetric data suggest that the packing defects at domain boundaries could facilitate the direct interaction of the transmembrane regions of integral proteins with the lipid hydrophobic chains. Thus lipid domain formation would help in the membrane insertion of proteins. 

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