Melatonin induces opposite effects on order and dynamics of anionic DPPG model membranes

The temperature and concentration induced effects of melatonin on anionic dipalmitoyl phosphatidylglycerol (DPPG) multilamellar liposomes (MLVs) were investigated by using Fourier transform infrared (FTIR) spectroscopy and differential scanning calorimetry (DSC). The results show that melatonin does not perturb the phase transition profile, while a decrease in the main transition temperature (Tm) is noticed at high melatonin concentrations (15, 24 and 30 mol %). Low concentrations of melatonin (3, 6 and 9 mol %) decrease the frequency of the CH2 stretching mode, implying an ordering effect, whilst high concentrations of melatonin disorders system both in the gel and liquid crystalline phases. Furthermore, at low and high concentrations, melatonin also causes opposite effect on membrane dynamics. The bandwidth of the CH2 stretching modes decreases at low concentrations, implying a decrease in the dynamics, while increasing it at high concentrations. Furthermore, it causes significant decrease in the frequency of the CO stretching and PO2- antisymmetric double bond stretching bands of DPPG for all concentrations both in the gel and liquid crystalline phases, which indicates strong hydrogen bonding around these functional groups.