Experimental and theoretical investigations on the direct interactions between urea and phospholipids in aqueous solutions

Abstract

Experimental and theoretical investigations have been carried out on the interactions of urea and, as a comparison, water with phospholipids. Firstly, Langmuir monolayer technique demonstrated that the presence of urea in the subphase caused a lateral expansion of the molecular monolayer of dipalmitoylphosphatidylcholine (DPPC) and a number of other phospholipids. This is attributed to possible direct interaction between urea and the lipids and thus insertion of urea into the head group region. Secondly, Raman spectra showed that the N–H stretching bands of urea shift to lower wavenumber while the symmetric stretching mode of PO₂ of lipids shift to higher wavenumber, when urea coexists with DPPC in aqueous codispersions. This is consistent with the formation of hydrogen bonds between urea and DPPC molecule. Thirdly, quantum chemical calculations of the first principle demonstrated that the interaction of urea with lipid's amino group, phosphate group, and glycerol backbone are significantly stronger than that with water. Hence it is concluded that urea can interact specifically with the headgroup of phospholipids in an aqueous solution. The results would be helpful to understand the effect of urea on biomembranes and other biomolecular structures.