where the θ CD is the angle between the bilayer normal (i.e., the z-axis) and the carbondeuterium vector, with the deuterium atom position predicted. The order parameters were calculated by averaging five 100 ns blocks, and the resulting block averages were subsequently used to derive the final average and standard deviation. The average angle between the tail vector (C20-C7, C35-C21) and the bilayer normal was determined using gmx gangle (see Figure for atom numbering). Given the planar nature of the membranes, the normal of the bilayer was assumed to be the z-axis. The angle between the headgroup and the tail vectors extending from the central carbon atom to the terminal atom (C5-C20, C5-C35) was calculated in a similar way. Tail splay angles were calculated with gmx gangle by selecting 1) second tail carbon atoms C9 and C22 (Figure ) and 2) terminal carbon atoms C20 and C35 (Figure ). In both cases, the same central carbon atom, C5, was selected. In addition, we computed the tail rotational autocorrelation function (ACF rot ) using a vector extending from the first carbon atom to the terminal carbon atom (C7-C20, C21-C35). The equation for calculating the autocorrelation function (ACF) of a given property f is defined as