Anomalous viscosity behavior of a bicelle system with various molar ratios of short- and long-chain phospholipids

Viscosity versus temperature curves were obtained for five DMPC/DHPC/H ₂Obilayer systems with DMPC/DHPC molar ratios of 3.2, 2.8, 2.5, 2.0, and 1.5. The curves showed viscosity maxima that shifted to higher temperatures as the mole fraction of DHPC increased. The maxima are extremely high for the first three systems and are estimated to be no less than 600 Pa·s. The shift to higher temperature is explained in terms of decrease in the coalescence of bicelles as the mole fraction of DHPC increases. The maxima are attributed to two competing effects; the normal decrease in viscosity with temperature is counterbalanced at low to moderate temperatures with an increase in viscosity resulting from coalescence. Such coalescence increases as the DMPC/DHPC molar ratio increases.