Dielectric Relaxation Study of Dynamic Properties of Hydrated Phospholipid Bilayers
ABSTRACT The dynamical properties of hydrated di-myristoyl-phosphatidylcholin were studied at microwave frequencies. The relaxation frequency and the energy of bound water were determined: the properties of bound water vary quasi-continuously with the amount of bound water. Water is stronger bound in the fluidlike than in the gel phase. For the first time we could measure directly the trans-gauche isomerization rotation of the alcyl chains. It was observed that two mechanisms are separately operating at the phase transition of the hydrated bilayers: 1. a change of hydration coupled with a change of headgroup conformation and 2. the chain melting. The binding or release of water is preceding the chain melting or freezing by 1 K. It was found that hysteresis effects of the phase transition depend strongly on the rate of hydration.
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ABSTRACT: Modulating the relative humidity (RH) of the ambient gas phase of a phospholipid/water sample for modifying the activity of phospholipid-sorbed water [humidity-controlled osmotic stress methods, J. Chem. Phys. 92 (1990) 4519 and J. Phys. Chem. 96 (1992) 446] has opened a new field of research of paramount importance. New types of phase transitions, occurring at specific values of this activity, have been then disclosed. Hence, it is become recognized that this activity, like the temperature T, is an intensive parameter of the thermodynamical state of these samples. This state can be therefore changed (phase transition) either, by modulating T at a given water activity (a given hydration level), or, by modulating the water activity, at a given T. The underlying mechanisms of these two types of transition differ, especially when they appear as disorderings of fatty chains. In lyotropic transitions, this disordering follows from two thermodynamical laws. First, acting on the activity (the chemical potential) of water external to a phospholipid/water sample, a transbilayer gradient of water chemical potential is created, leading to a transbilayer flux of water (Fick's law). Second, water molecules present within the hydrocarbon region of this phospholipid bilayer interact with phospholipid molecules through their chemical potential (Gibbs-Duhem relation): the conformational state of fatty chains (the thermodynamical state of the phospholipid molecules) changes. This process is slow, as revealed by osmotic stress time-resolved experiments. In thermal chain-melting transitions, the first rapid step is the disordering of fatty chains of a fraction of phospholipid molecules. It occurs a few degrees before the main transition temperature, T(m), during the pretransition and the sub-main transition. The second step, less rapid, is the redistribution of water molecules between the different parts of the sample, as revealed by T-jump time-resolved experiments. Finally, in lyotropic and thermal transitions, hydration and conformation are linked but the order of anteriority of their change, in each case, is probably not the same. In this review, first, the interactions of phospholipid submolecular fragments and water molecules, in the interfacial and hydrocarbon regions of phospholipid/water multibilayer stacks, will be described. Second, the coupling of the conformational states of phospholipid and water molecules, during thermal and lyotropic transitions, will be demonstrated through examples.Biochimica et Biophysica Acta 06/2004; 1663(1-2):19-51. · 4.66 Impact Factor