Nuclear relaxation and critical fluctuations in membranes containing cholesterol

Self-consistent mean-field model for palmitoyloleoylphosphatidylcholine-palmitoyl sphingomyelin-cholesterol lipid bilayers

The connection between membrane inhomogeneity and the structural basis of lipid rafts has sparked interest in the lateral organization of model lipid bilayers of two and three components. In an effort to investigate anisotropic lipid distribution in mixed bilayers, a self-consistent mean-field theoretical model is applied to palmitoyloleoylphosphatidylcholine (POPC)--palmitoyl sphingomyelin (PSM)--cholesterol mixtures. The compositional dependence of lateral organization in these mixtures is mapped onto a ternary plot. The model utilizes molecular dynamics simulations to estimate interaction parameters and to construct chain conformation libraries. We find that at some concentration ratios the bilayers separate spatially into regions of higher and lower chain order coinciding with areas enriched with PSM and POPC, respectively. To examine the effect of the asymmetric chain structure of POPC on bilayer lateral inhomogeneity, we consider POPC-lipid interactions with and without angular dependence. Results are compared with experimental data and with results from a similar model for mixtures of dioleoylphosphatidylcholine, steroyl sphingomyelin, and cholesterol.

Multiphoton polarization imaging of steady-state molecular order in ternary lipid vesicles for the purpose of lipid phase assignment

We have investigated lipid acyl chain order parameters of giant unilamellar vesicles (GUVs) using multiphoton fluorescence microscopy. We compare two widely used models of lipid acyl chain order parameters: the "wobble-on-a-cone" model and the Gaussian distribution model. For the first time, we systematically address a ternary system for which the phase diagram encompassing both composition and temperature space has been mapped in order to determine tie-line directions and thus phase assignment. In addition, because miscibility and chain melting transitions can be observed directly and simultaneously with multiphoton microscopy, our technique is applicable to determining the extent of the coupling between chain order and miscibility; thus, it provides a more robust platform for comparison with theory.

Adventures in physical chemistry

My research has included chemical physics, electron and NMR spectroscopy, membrane biophysics, and immunology. This research was curiosity driven as well as problem and technique oriented. A theoretical equation was developed for relating nuclear hyperfine splittings to electron spin distributions in free radicals. Another equation was developed to relate NMR spectra to chemical reaction rates. Early evidence for the liquid-like properties of cell membranes was obtained through the use of paramagnetic probes (spin labels). Spin labels were used in measurements of lateral as well as transverse diffusion of phospholipids in bilayer membranes. Liquid-liquid phase separations were discovered in monolayer membranes containing phospholipids and cholesterol. In the area of immunology, it was shown that antigenic peptides bind to reconstituted class II MHC molecules in membranes and trigger specific T-helper cells.

Lateral organization of complex lipid mixtures from multiscale modeling

The organizational properties of complex lipid mixtures can give rise to functionally important structures in cell membranes. In model membranes, ternary lipid-cholesterol (CHOL) mixtures are often used as representative systems to investigate the formation and stabilization of localized structural domains ("rafts"). In this work, we describe a self-consistent mean-field model that builds on molecular dynamics simulations to incorporate multiple lipid components and to investigate the lateral organization of such mixtures. The model predictions reveal regions of bimodal order on ternary plots that are in good agreement with experiment. Specifically, we have applied the model to ternary mixtures composed of dioleoylphosphatidylcholine:18:0 sphingomyelin:CHOL. This work provides insight into the specific intermolecular interactions that drive the formation of localized domains in these mixtures. The model makes use of molecular dynamics simulations to extract interaction parameters and to provide chain configuration order parameter libraries.