Water and hydrogen bonds in DMPA and DMPC membranes seen from near infrared spectroscopy

Abstract

By exploiting the sensitivity of the NIR spectrum to hydrogen bonds, particularly in the first overtone of water around 1450/55 nm, the influence of temperature and phospholipid head group on the H-bond network and water polymerization in membranes of DMPA and DMPC was studied. Principal components analysis of the spectra was used to disclose subtle spectral changes that mirror alterations in the vibrational energy of the water O-H bonds, as a measure of the H-bond network. Increasing temperatures drastically diminished the number of strongly H-bonded water molecules and increase the number of weakly H-bonded waters. This dominating effect of temperature was missing after the subtraction of the pure water spectra from the lipid-containing ones, suggesting the presence of two water populations. An intriguing secondary effect of temperature was also revealed. At the water:lipid interface, phospholipids exhibited an effect qualitatively similar to that of the temperature. DMPA, and particularly DMPC, disrupted the H-bond network in the neighboring water, reducing water polymerization and strengthening the water O-H bonds. The type of the polar head affects the H-bonds more than duplicate the concentration of the lipid from 250 to 500 μM. A relation between head group structure and the effect on the H-bonds network and the existence of two populations of water molecules are discussed.