Dynamics and ordering of lipid spin-labels along the coexistence curve of two membrane phases: An ESR study

Abstract

An analysis of electron spin resonance (ESR) spectra from compositions along the liquid-ordered (L o ) and liquid-disordered (L d ) coexistence curve from the brain-sphingomyelin/ dioleoylphosphatidylcholine/cholesterol (SPM/DOPC/Chol) model lipid system was performed to characterize the dynamic structure on a molecular level of these coexisting phases. We obtained 200 continuous-wave ESR spectra from glycerophospholipid spin-labels labeled at the 5, 7, 10, 12, 14, and 16 carbon positions of the 2 nd acyl chain, a sphingomyelin spin-label labeled at the 14 carbon position of the amide-linked acyl chain, a headgroup-labeled glycerophospholipid, a headgroup-labeled sphingomyelin, and the cholesterol analogue spin-label cholestane all within multi-lamellar vesicle suspensions at room temperature. The spectra were analyzed using the MOMD (microscopic-order macroscopic-disorder) model to provide the rotational diffusion rates and order parameters which characterize the local molecular dynamics in these phases. The analysis also incorporated the known critical point and invariant points of the neighboring threephase triangle along the coexistence curve. The variation in the molecular dynamic structures of coexisting L o and L d compositions as one moves toward the critical point is discussed. Based on these results, a molecular model of the L o phase is proposed incorporating the "condensing effect" of cholesterol on the phospholipid acyl chain dynamics and ordering and the "umbrella model" of the phospholipid headgroup dynamics and ordering.

Molecular modeling of lipid probes and their influence on the membrane

“…This is in agreement with simulations on a different spin-label with labeling in the tail region [78] where the label stayed also in the hydrophobic region. The low density of the probe lipids does, however, not lead to an a overall change of the membrane thickness in at least partial agreement with experimental data on a similar headgroup labeled lipid [83]. The experiments speak for a location of the headgroup close to the interface but it is not conclusive how close this localization is exactly.…”

Toward increased concentration sensitivity for continuous wave EPR investigations of spin-labeled biological macromolecules at high fields

“…Structure and structure-function relationships of macromolecules are areas of intense EPR effort [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. Coupled with site-directed spin-labeling (SDSL), EPR is oftentimes used to characterize protein and nucleic acid structures and dynamics, conformational changes, molecule folding, macromolecule complexes, and oligomeric structures [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]17].…”

“…Coupled with site-directed spin-labeling (SDSL), EPR is oftentimes used to characterize protein and nucleic acid structures and dynamics, conformational changes, molecule folding, macromolecule complexes, and oligomeric structures [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]17]. The majority of biomolecules do not contain unpaired electrons from which one can obtain an EPR signal; therefore, spin-labeling approaches have been developed [15,[18][19][20][21][22][23][24][25] where site-specific persistent radicals or paramagnetic metal-probes are incorporated at specific locations within a biomolecule.…”

Conformational dynamics and distribution of nitroxide spin labels

“…The MOMD model is also applicable to DOXYL-labeled lipid molecules in lipid bilayers [69]. In the MOMD model, the local diffusion frame M of the nitroxide may move with respect to this C' frame, necessitating a time-dependent transformation C'M.…”