Transition measurements, lipid phase

Phase transitions of the system such as chain ordering transitions of lipids, appear in the isotherm as regions of constant pressure in the case of first order phase transitions involving the coexistence of two phases, or as a kink in the isotherm corresponding to a second order phase transition. These kinds of surface measurements are highly sensitive to impurities and must be carried out using very pure water and sample materials. 

It should be mentioned that DSC and NMR do not measure the same parameters, and in this way, these techniques are complementary. DSC is a dynamic method, which gives information about the transitions between different phases of lipids, whereas NMR allows quantitation of liquid and solid phases at equilibrium. Indeed, NMR and DSC methods give different values for the solid fat index (SFI) (Walker and Bosin, 1971 Norris and Taylor, 1977) NMR values are much lower than those given by DSC below 20°C. For example, for milk fat at 5°C, DSC and NMR indicate 78.1% and 43.7% solid fat, respectively. The observed difference can be explained by the presence of an amorphous phase which, due to its melting enthalpy, is seen as a solid by the DSC method. Using time-domain NMR, Le Botlan et al. (1999) showed that in milk fat samples, an intermediate component exists between the solid and liquid phases, constituting about 6% of an aged milk fat. 

The dependence of D on lipid phase state was also reported [492], It was notable that the first measurable diffusion in the DMPC and DMPE thin films occurred at temperatures near the point at which the transition to the liquid crystalline La phase in bulk solution was observed. However, this is not the case for the negatively charged DMPG, for which diffusion (D = 610 8 cm2 s 1) was detected at two temperatures (15 and 20°C) where gel Plt and/or La phase may exist. 

Analysis of the results and comparison with the lipid phase transition observed iq the bulk lipid/water systems allows to conclude that the lowest temperature during heating at which measurable diffusion occurred correlates with the onset of formation of the lamellar Ln liquid crystalline phase of the given phospholipid. Therefore, the data support a correlation between the surface and bulk phase transitions. This was confirmed in recent studies where the lipid surface phase transition was successfully measured for the first time in foam film by independent means involving the detailed investigations of the temperature dependences of the W(C) curve for the foam film and its thickness. 

Mention should be made here of the recently developed technique of pressure perturbation calorimetry (PPC), which measures the temperature-dependent volume change of a solute or colloidal particle in aqueous solution. PPC can also be used to detect thermotropic phase transitions in lipid model membranes and to characterize the accompanying volume changes and the kinetics of the phase transition. PPC essentially measures the heat change that results from small pressure changes at a constant temperature in a high-sensitivity isothermal calorimeter. For an excellent recent review on PPC as applied to lipid systems, the reader is referred to Heerklotz (19). 

Quinn PJ. Measurement of kinetics and mechanisms of phase transitions in lipid-water systems. J. Appl. Crystallogr. 1997 30 733-738. 

A review of these disparate but related investigations is presented beginning with a description of the use of time-resolved X-ray diffraction (TRXRD) to study lipid phase transition kinetics and mechanism in Sect. 1. It is the enormous intrinsic intensity of synchrotron radiation that enables TRXRD measurements to be made. However, this advantage brings with it the hazards of radiation damage. This critical issue is addressed in Sect. 2 along with recommendations for minimizing the effect. 

In summary, there are many good reasons for studying lipid mesomorphism and the dynamics and mechanism of lipid phase transitions. The dynamic measurements enable limiting transition rates to be established and provide a basis for formulating, evaluating and refining transition mechanisms. Such measurements also reveal details of molecular structure and overall composition that influence phase stability and modulate transition rates and mechanism. The ultimate goal is to understand phase behavior and transition mechanism at a fundamental level and, thereby, effect control over lipid phase relations in vivo and in reconstituted, model and formulated systems. 

The first TRXRD study of phase transitions occurring in membranes and membrane lipid extracts was described in 1972 [45]. It is interesting to note that these time-resolved measurements were realized through a technological innovation in the form of a linear, position-sensitive proportional X-ray counter. In the past decade, considerable interest in lipid phase transition kinetics has developed in response to the emergence of new technologies, the most important of which include the synchrotron radiation source, and X-ray optics and detectors. The increased interest level is reflected in a growing literature. 

As was mentioned in previous sections, mixing lipids, and particularly sterols, often has the effect of broadening the phase transitions. Intrinsic proteins have the same effect. Thus it is not surprising that cell membranes, containing highly complex mixtures of lipids and proteins, do not usually exhibit measurable thermotropic phase transitions. With a few exceptions, transitions in cell membranes can only be seen with high-sensitivity DSC instruments. 

The significance of measurement of specific volume versus temperature (dilatometry) for studies of lipid phase transitions was discussed in Section 8.5, and there are numerous dilatometric studies of fat systems. 

The results presented here show that electrical pulsation causes reversible enhancement of peptide transport through human skin in vitro. In all cases, only intact peptide was measured. Moreover, the enhanced flux cannot be accounted for by increased current, and it is most likely due to increased ionic mobility of the peptide within the skin. Thus, electrical pulsation reversibly alters the ion-transport properties of skin. The rate-limiting barrier to skin transport of ionized compounds is the lipids of the stratum comeum. Alteration of stratum comeum lipids results in a significant increase in skin transport (Golden et al, 1987 Potts and Francoeur, 1990). For example, heating the skin to temperatures just above the stratum comeum lipid phase-transition temperature results in a 100-fold increase in sodium-ion conductivity. The sodium-ion conductivity returns to pretreatment values when the skin is cooled (Oh et al, 1993). Similarly, alteration of stratum comeum lipid stmcture with chemical perturbants such as oleic acid increases ion conductivity (Potts et al, 1992). The application of a transient electrical pulse to other lipid-based membranes creates a high-permeability state associated with the reversible formation of pores within the membrane (electroporation). Thus, it seems likely that electrical pulsation of human skin results in the formation of transient pores within stratum comeum lipids. 

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