Model phospholipid interaction with cholesterol and melatonin: Raman spectroscopy and density functional theory study

We present a Raman spectroscopy and quantum chemical computational study for model phospholipid membrane molecule dipalmitoylphosphatidylcholine (DPPC) mixed with cholesterol or melatonin at various concentrations. Utilizing a confocal Raman microspectroscopy setup, we capture a DPPC conformation state by evaluating the number of trans/gauche bonds using I-1064/I-1090 and I-1128/I-1090 vibrational mode intensity ratios. We show that the cholesterol increases the number of trans bonds in DPPC hydrocarbon chains, that is, causing membrane ordering, while melatonin addition introduces gauche conformers, generating disorder. It is noticed that the I-1128/I-1090 intensity ratio is a more suitable parameter for the characterization of DPPC hydrocarbon chain structural properties compared with I-1064/I-1090. We thus clarify our experimental observations by performing careful density functional theory (DFT) calculations of Raman spectra for DPPC tails, containing up to three gauche bonds at different chain positions. The obtained results demonstrate the strong interconnection between 1064 and 1090 cm(-1) vibrational modes originating from their complex splitting and shifting along with continuous Raman activity redistribution upon various gauche bonds introduction, while it is not the case for 1128 and 1090 cm(-1) modes pair.