Exploring membrane viscosity at the headgroup region utilizing a hemicyanine-based fluorescent probe

Abstract

Membrane viscosity is an important determinant for several cellular functions. In spite of having vast literature on the interfacial and acyl chain region viscosity of the membrane, there is hardly any report on the membrane viscosity at the headgroup region. In this present work, we have exploited the photophysical properties of a hemicyanine dye covalently linked with a C16 alkyl chain (HC16) to explore the membrane viscosity at the headgroup region. The chromophoric group of HC16 locates at the headgroup region due to its overall positive charge. Methanol-glycerol mixture of known composition and viscosity were used to develop the standard curve of apparent rotational correlation time against viscosity, and utilized this standard curve to evaluate viscosity of dimyristoylphosphatidylcholine (DMPC) membrane as a function of temperature. The result exhibited that there is a 4-fold change in headgroup viscosity of the DMPC membrane between gel and fluid phase; with a sharp change during the phase transition. Moreover, we have monitored the effect of cholesterol on the viscosity of the headgroup region of a biologically relevant lipid, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), keeping in mind the role of cholesterol on membrane organization, dynamics, and cellular functions. Interestingly, the headgroup viscosity of POPC membrane does not show a significant change in presence of various concentrations of cholesterol, though literature showed remarkable change in interfacial and acyl chain region viscosity in presence of similar concentrations of cholesterol.