Reply to Nagle et al.: The universal stiffening effects of cholesterol on lipid membranes
Rana Ashkar, Milka Doktorova, Frederick A. Heberle, Haden L. Scott, Francisco N. Barrera, John Katsaras, George Khelashvili, Michael F. Brown
Abstract
Based on neutron spin-echo (NSE), solid-state NMR, and molecular dynamics (MD) simulations, we report clear evidence that cholesterol stiffens 1,2-dioleoyl- sn -glycero-3-phosphocholine (DOPC) membranes (1). Contrary to statements by Nagle et al. (2), the relaxations measured by NSE and NMR directly relate to elastic membrane constants (3, 4). While both NSE and NMR spectroscopy measure the time-averaged 𝜏 -decay, 〈 A ( t ) A ( t + τ ) 〉 , the data analysis methods separate the mean-square amplitude (MSA) from membrane dynamics, providing information about equilibrium and dynamical bilayer properties (4). In the case of isotropic systems (simplest case), the MSA = 〈 A t 2 〉 and the time average 〈 A ( t ) 〉 is zero. However, for ordered systems such as lipid bilayers, the time average is nonzero … [↵][1]1To whom correspondence may be addressed. Email: ashkar{at}vt.edu or [email protected]. [1]: #xref-corresp-1-1