Cholesterol is a critical component of eukaryotic cell membranes and a key molecule in controlling membrane fluidity, packing state, and various membrane functions. It also plays a regulatory role in antibiotic drug resistance by augmenting the mechanical properties of its host membrane against structural damage. Structurally, cholesterol exhibits a universal condensing effect on fluid lipid membranes (saturated and unsaturated), manifested in increased membrane thickness and smaller area per lipid. However, the effects of cholesterol on membrane mechanics have, so far, been presumed to be non-universal. While cholesterol is widely accepted to have a strong stiffening effect on saturated lipid membranes, its effects on unsaturated lipid membranes remain controversial and, in some cases, challenge our understanding of the mechanisms governing membrane mechanics. For instance, previous studies on di-monounsturated lipids, such as DOPC, showed that cholesterol has virtually no effect on the bending rigidity modulus, κ, despite clear evidence of a condensing effect. In contrast, we recently showed, using neutron spin echo (NSE) spectroscopy, that cholesterol results in a non-trivial stiffening of DOPC membranes over NSE length/time scales. Direct NSE measurements of bending dynamics in DOPC membranes with increasing cholesterol content yielded increasing trends in κ up to ~ 3-fold increase at 50 mol% cholesterol. Here, we report similar results from solid state 2H-NMR spectroscopy which unambiguously show a gradual decrease in the square-law dependence of spin lattice relaxation rates (R1Z) on segmental order parameters (SCD), indicating cholesterol-induced membrane stiffening. Analysis of these results in the context of fluctuation-dissipation theorem yields the same trends in κ as observed by NSE. Parallel studies on membranes with different lipid unsaturation (e.g. POPC vs. DOPC) indicate that the degree of cholesterol-induced stiffening depends on chain unsaturation. Our dynamical results, as well as structural SANS/SAXS and ESR data, are in excellent agreement with real-space fluctuation analysis from molecular dynamics simulations on cognate DOPC-cholesterol bilayers and with standard theories of membrane mechanics. These findings have significant biological implications and encourage a reassessment of mechanical measurements of cholesterol-containing membranes over length and time scales that are relevant to biological processes.

Cholesterol stiffens saturated and unsaturated phosphocholine membranes

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.