A Needless but Interesting Controversy with Pedagogical Overtones

Abstract

A decade ago four independent groups, using four independent methods, reported the unanticipated and still surprising result that the bending modulus KC of bilayers of DOPC, unlike other standard lipids, does not increase with addition of cholesterol. A recent paper, using neutron spin echo (NSE) and nuclear magnetic resonance relaxation times (NMRτ), reports that the bending modulus of DOPC increases three fold with increasing cholesterol concentration up to 50%. It would appear that this has become a controversial topic. This controversy is needless because NSE and NMRτ do not measure the bending modulus as it has been defined for decades. Each measures important, complementary, properties that could enrich our understanding of bilayer properties, notably membrane viscosity and location of the neutral plane employed in membrane mechanics. The resolution of this controversy rests on elucidating the difference between measurements on a system in equilibrium that lead directly to equilibrium properties like the bending modulus, and measurements on the same equilibrium system that lead to more complex results involving both static moduli and transport properties. While the difference between static and non-equilibrium quantities is well understood in the physical sciences and statistical thermodynamics, this emerging controversy seems to warrant a pedagogical reminder.