Contrast between POPC and SC membrane results

Determine whether the observed difference in how a single lag-time-independent diffusivity profile describes projected dynamics in a POPC lipid bilayer versus a stratum corneum (SC) lipid membrane arises from stronger memory effects, different coupling of the solute position coordinate along the membrane normal to orthogonal degrees of freedom, or differences in local structural relaxation, in order to delineate the validity range of reduced one-dimensional diffusion (Smoluchowski) models for membrane transport.

Background

In this study, the residence-time approach (RTA) was introduced to estimate position-dependent diffusivities from biased molecular dynamics, and its performance was assessed for oxygen in a hexadecane/water slab, water in a POPC bilayer, and multiple solutes in a stratum corneum (SC) membrane. Propagator-level validation revealed that in POPC no single lag-time-independent diffusivity profile fully captures projected dynamics across the entire lag-time range, whereas in the more ordered and heterogeneous SC membrane the RTA performed consistently well across solutes.

This contrasting behavior raises a specific mechanistic question: whether POPC’s greater difficulty reflects stronger non-Markovian memory, different coupling of the projected coordinate to other degrees of freedom, or different local structural relaxation relative to SC. Resolving this will help define when reduced one-dimensional Smoluchowski models are adequate for membrane transport predictions.