Pressure effects on the structure, dynamics and phase transitions of phospholipid vesicles

PRESSURE EFFECTS ON THE STRUCTURE, DYNAMICS AND PHASE TRANSITIONS OF PHOSPHOLIPID VESICLES 

Phosphatidylcholines are the most prominent components of biological membranes and therefore often serve as model biomembrane systems in biophysical studies.Due to their amphiphathic character, phospholipids have a strong tendency to spontaneously form bilayer structures when 

Besides these thermotropic phase transitions, a variety of pressure-induced phase transformations can be observed, and it has been demonstrated that temperature and pressure have non-congruent effects on the structural and phase behaviour of these systems. 

The reason why one chose to follow the main liquid-crystalline to gel phase transition in DPPC by monitoring the linewidth of the various or natural abundance resonance is evident when we consider the expressions for the spin-lattice relaxation time (Ti) and the spin-spin relaxation time T2). The first one is given by 1/Ti oc [/i(ft o) + 72(2ft o)] where Ji coq) is the Fourier transform of the correlation function at the resonance frequency o o and is a constant related to internuclear separation. The relaxation rate l/Ti thus reflects motions at coq and 2coq. In contrast, the expression for T2 shows that 1/T2 monitors slow motions IjTi oc. B[/o(0) -I- /i(ft o) + /2(2u o)], where /o(0) is the Fourier component of the correlation function at zero frequency. Since the linewidth vi/2 (full-width at half-maximum intensity) is proportional to 1 / T2, the changes of linewidth will reflect changes in the mobility of various carbon atoms in the DPPC bilayer. 

The choline head group maintains considerable mobility, even in the gel phase, because it is exposed to water, and its methyl groups can still rotate quite freely. 

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