Kinetics and dynamics of annealing during sub-gel phase formation in phospholipid bilayers: A saturation transfer electron spin resonance study

Studying Lipid-Protein Interactions with Electron Paramagnetic Resonance Spectroscopy of Spin-Labeled Lipids

Spin label electron paramagnetic resonance (EPR) of lipid-protein interactions reveals crucial features of the structure and assembly of integral membrane proteins. Spin-label EPR spectroscopy is the technique of choice to characterize the protein solvating lipid shell in its highly dynamic nature, because the EPR spectra of lipids that are spin-labeled close to the terminal methyl end of their acyl chains display two spectral components, those corresponding to lipids directly contacting the protein and those corresponding to lipids in the bulk fluid bilayer regions of the membrane. In this chapter, typical spin label EPR procedures are presented that allow determination of the stoichiometry of interaction of spin-labeled lipids with the intramembranous region of membrane proteins or polypeptides, as well as the association constant of the spin-labeled lipid with respect to the host lipid. The lipids giving rise to a so-called immobile spectral component in the EPR spectrum of such samples are identified as the motionally restricted first-shell lipids solvating membrane proteins in biomembranes. Stoichiometry and selectivity are directly related to the structure of the intramembranous sections of membrane-associated proteins or polypeptides and can be used to study the state of assembly of such proteins in the membrane. Since these characteristics of lipid-protein interactions are discussed in detail in the literature (see ref. Marsh, Eur Biophys J 39:513-525, 2010 for a recent review), here we focus more on how to spin label model membranes and biomembranes and how to measure and analyze the two-component EPR spectra of spin-labeled lipids in phospholipid bilayers that contain proteins or polypeptides. After a description of how to prepare spin-labeled model and native biological membranes, we present the reader with computational procedures for determining the molar fraction of motionally restricted lipids when both, one or none of the pure isolated-mobile or immobile-spectral components are available. With these topics, this chapter complements a previous methodological paper (Marsh, Methods 46:83-96, 2008). The interpretation of the data is discussed briefly, as well as other relevant and recent spin label EPR techniques for studying lipid-protein interactions, not only from the point of view of lipid chain dynamics.

Studying lipid-protein interactions with electron paramagnetic resonance spectroscopy of spin-labeled lipids

Spin label electron paramagnetic resonance (EPR) of lipid-protein interactions reveals crucial features of the structure and assembly of integral membrane proteins. Spin label EPR spectroscopy is the technique of choice to characterize the protein-solvating lipid shell in its highly dynamic nature, because the EPR spectra of lipids that are spin labeled close to the terminal methyl end of their acyl chains display two spectral components, those corresponding to lipids directly contacting the protein and those corresponding to lipids in the bulk fluid bilayer regions of the membrane. In this chapter, typical spin label EPR procedures are presented that allow determination of the stoichiometry of interaction of spin-labeled lipids with the intra-membranous region of membrane proteins or polypeptides, as well as the association constant of the spin-labeled lipid with respect to the host lipid. The lipids giving rise to the so-called immobile spectral component in the EPR spectrum of such samples are identified as the motionally restricted first-shell lipids solvating membrane proteins in biomembranes. Stoichiometry and selectivity are directly related to the structure of the intra-membranous sections of membrane-associated proteins or polypeptides and can be used to study the state of assembly of such proteins in the membrane. Since these characteristics of lipid-protein interactions are discussed in detail in the literature [see Marsh (Eur Biophys J 39:513-525, 2010) for a most recent review], here we focus more on how to spin label model and biomembranes and how to measure and analyze the two-component EPR spectra of spin-labeled lipids in phospholipid bilayers that contain proteins or polypeptides. After a description of how to prepare spin-labeled model and native biological membranes, we present the reader with computational procedures for determining the molar fraction of motionally restricted lipids when both, one, or none of the pure isolated-mobile or immobile-spectral components are available. With these topics, this chapter complements a recent methodological paper [Marsh (Methods 46:83-96, 2008)]. The interpretation of the data is discussed briefly, as well as other relevant and recent spin label EPR techniques for studying lipid-protein interactions, not only from the point of view of lipid chain dynamics.

Conformational distributions and hydrogen bonding in gel and frozen lipid bilayers: a high frequency spin-label ESR study

The ESR parameters of PC spin labels in frozen membranes do not simply represent the membrane polarity or water penetration profile. Instead, they show a distribution between hydrogen-bonded (HB) and non-hydrogen-bonded (non-HB) states, which is affected by a number of factors in the membrane composition. Similar to the exclusion of solutes from crystallizing solvents, the pure bulk gel phase excludes nitroxides, forcing acyl chains to take bent conformations. In these conformations, the nitroxide is hydrogen-bonded. Furthermore, upon gradual cooling in the supercooled gel, PC labels undergo slow lateral aggregation, resulting in a broad background signal. However, if the sample is instantly frozen, this background is replaced by the HB component. In membranes with cholesterol, the observed HB/non-HB ratio can best be described by a partition-like equilibrium between nitroxides located in defects of lipid structure within the hydrophobic core and those close to the membrane surface.

Intramembrane polarity by electron spin echo spectroscopy of labeled lipids

The association of water (D(2)O) with phospholipid membranes was studied by using pulsed-electron spin resonance techniques. We measured the deuterium electron spin echo modulation of spin-labeled phospholipids by D(2)O in membranes of dipalmitoyl phosphatidylcholine with and without 50 mol% of cholesterol. The Fourier transform of the relaxation-corrected two-pulse echo decay curve reveals peaks, at one and two times the deuterium NMR frequency, that arise from the dipolar hyperfine interaction of the deuterium nucleus with the unpaired electron spin of the nitroxide-labeled lipid. For phosphatidylcholine spin-labeled at different positions down the sn-2 chain, the amplitude of the deuterium signal decreases toward the center of the membrane, and is reduced to zero from the C-12 atom position onward. At chain positions C-5 and C-7 closer to the phospholipid headgroups, the amplitude of the deuterium signal is greater in the presence of cholesterol than in its absence. These results are in good agreement with more indirect measurements of the transmembrane polarity profile that are based on the (14)N-hyperfine splittings in the conventional continuous-wave electron spin resonance spectrum.

New ordered metastable phases between the gel and subgel phases in hydrated phospholipids

Formation of low-temperature ordered gel phases in several fully hydrated phosphatidylethanolamines (PEs) and phosphatidylcholines (PCs) with saturated chains as well as in dipalmitoylphosphatidylglycerol (DPPG) was observed by synchrotron x-ray diffraction, microcalorimetry, and densitometry. The diffraction patterns recorded during slow cooling show that the gel-phase chain reflection cooperatively splits into two reflections, signaling a transformation of the usual gel phase into a more ordered phase, with an orthorhombic chain packing (the Y-transition). This transition is associated with a small decrease (2-4 microl/g) or inflection of the partial specific volume. It is fully reversible with the temperature and displays in heating direction as a small (0.1-0.7 kcal/mol) endothermic event. We recorded a Y-transition in distearoyl PE, dipalmitoyl PE (DPPE), mono and dimethylated DPPE, distearoyl PC, dipalmitoyl PC, diC(15)PC, and DPPG. No such transition exists in dimyristoyl PE and dilauroyl PE where the gel L(beta) phase transforms directly into subgel L(c) phase, as well as in the unsaturated dielaidoyl PE. The PE and PC low-temperature phases denoted L(R1) and SGII, respectively, have different hydrocarbon chain packing. The SGII phase is with tilted chains, arranged in an orthorhombic lattice of two-nearest-neighbor type. Except for the PCs, it was also registered in ionized DPPG. In the L(R1) phase, the chains are perpendicular to the bilayer plane and arranged in an orthorhombic lattice of four-nearest-neighbor type. It was observed in PEs and in protonated DPPG. The L(R1) and SGII phases are metastable phases, which may only be formed by cooling the respective gel L(beta) and L(beta') phases, and not by heating the subgel L(c) phase. Whenever present, they appear to represent an indispensable intermediate step in the formation of the latter phase.