Morphological Classification and Structural Characterization of DMPC/Detergent Mixtures by SAXS, Laurdan Fluorescence, and 31P NMR 6 views

Bicelles are disc-shaped assemblies formed by mixing a bilayer-forming lipid with a micelle-forming detergent, widely used as membrane mimetics for structural studies of membrane proteins. The classical bicelle model predicts that the lipid forms a segregated planar core while the detergent caps the high-curvature rim, but whether this structural picture holds beyond the canonical DMPC/DH6PC system has not been systematically tested. This dissertation characterizes the morphology, internal organization, and thermal behavior of DMPC mixed with seven structurally diverse detergents drawn from three classes: diacyl short-chain phosphatidylcholines (DH6PC, DH7PC), lysophosphatidylcholines (LPC8, LPC10, LPC12), and alkylphosphocholines (FC10, FC12). Small-angle X-ray scattering (SAXS), Laurdan generalized polarization (GP) fluorescence, and ³¹P NMR were applied across a range of lipid-to-detergent ratios (q-values) and temperatures spanning the DMPC gel-to-fluid transition. The combined data reveal that bicelle formation is not a binary outcome but a continuum, and that the seven systems fall into three categories. Five systems (DH6PC, DH7PC, FC12, LPC10, LPC12) are phase-responsive, undergoing a composition-dependent transition from mixed micelles toward bilayer-like assemblies and a cooperative structural response to DMPC melting near its Tm. FC10 is phase-insensitive: SAXS and GP detect a structural transition, but ³¹P NMR shows the FC10 headgroup environment remaining distinct from DMPC at all temperatures, identifying a rim-confined detergent that does not integrate into the bilayer core even in the fluid phase. LPC8 fails to solubilize DMPC by all three measures. Total hydrophobic chain length is the primary determinant of where each system falls along the mixing-to-segregation continuum; at the 10-carbon boundary, the ester carbonyl of LPC10 facilitates partial bilayer integration absent in the alkyl-linked FC10. The three techniques converge on a consistent morphology for most systems, and the divergence of ³¹P NMR is what identifies FC10's phase-insensitivity. No system fully satisfies the classical segregated disc model, and FC10 at high q-values is the closest approximation to it.

PHD (Doctor of Philosophy)

bicelle; membrane mimetics; lipid-detergent mixtures; 31P NMR spectroscopy; Laurdan GP; Small-Angle X-ray Scattering; lipid phase transition

English

2026-07-29