Chain-length dependence of the hydration properties of saturated phosphatidylcholines as revealed by FTIR spectroscopy

Phospholipids - as biomembrane constituents - are an interesting class of molecules, and among them, phosphatidylcho-lines (PCs) or lecithins are most abundant in eucaryotic organisms. We have used FTIR spectroscopy to explore hydration-induced phenomena in symmetric, even-numbered, saturated diacyl PCs with varying chain length, n, of 10-22. These compounds were studied at room temperature as multilamellar films dependent on water activity which was regulated via the ambient relative humidity (RH). The PCs were compared in terms of water-uptake capacity by analyzing adsorption isotherms obtained from IR spectra. As demonstrated in the data, this parameter is governed by the phase type adopted in the lipid assemblies rather than by the sheer chain length. Short-chain lipids with n less than or equal to 14 which are shown to undergo lyotropic main transitions at room temperature and to adopt the lamellar liquid-crystalline phase can imbibe considerably more water than the other PCs maintaining a rigid phase at any RH. Chain melting is not systematically correlated with spectral features due to bands assigned to polar IR-active groups (phosphate, carbonyl). (C) 2001 Elsevier Science B.V. All rights reserved.