Water concentration profiles in membranes measured by ESEEM of spin-labeled lipids

Electron spin-echo envelope modulation (ESEEM) spectroscopy of phospholipids spin-labeled systematically down the sn-2 chain was used to detect the penetration of water (D2O) into bilayer membranes of dipalmitoyl phosphatidylcholine with and without 50 mol % cholesterol. Three-pulse stimulated echoes allow the resolution of two superimposed 2H-ESEEM spectral components of different widths, for spin labels located in the upper part of the lipid chains. Quantum chemical calculations (DFT) and ESEEM simulations assign the broad spectral component to one or two D2O molecules that are directly hydrogen bonded to the N-O group of the spin label. Classical ESEEM simulations establish that the narrow spectral component arises from nonbonded water (D2O) molecules that are free in the hydrocarbon chain region of the bilayer membrane. The amplitudes of the broad 2H-ESEEM spectral component correlate directly with those of the narrow component for spin labels at different positions down the lipid chain, reflecting the local H-bonding equilibria. The D2O-ESEEM amplitudes decrease with position down the chain toward the bilayer center, displaying a sigmoidal dependence on position that is characteristic of transmembrane polarity profiles established by other less direct spin-labeling methods. The midpoint of the sigmoidal profile is shifted toward the membrane center for membranes without cholesterol, relative to those with cholesterol, and the D2O-ESEEM amplitude in the outer regions of the chain is greater in the presence of cholesterol than in its absence. For both membrane types, the D2O amplitude is almost vanishingly small at the bilayer center. The water-penetration profiles reverse correlate with the lipid-chain packing density, as reflected by 1H-ESEEM intensities from protons of the membrane matrix. An analysis of the H-bonding equilibria provides essential information on the binding of water molecules to H-bond acceptors within the hydrophobic interior of membranes. For membranes containing cholesterol, approximately 40% of the nitroxides in the region adjacent to the lipid headgroups are H bonded to water, of which ca. 15% are doubly H bonded. Corresponding H-bonded populations in membranes without cholesterol are ca. 20%, of which ca. 6% are doubly bonded.     

Synthesis of 4-Aryl-6-oxo-5-cyano-1,6-dihydropyridine-2-carboxylic Acids and Their Methyl Esters by Reacting Methyl Acylpyruvates with Malononitrile and Cyanoacetamide

Russian Journal of General Chemistry - A series of new methyl 4-aryl-6-oxo-5-cyano-1,6-dihydropyridine-2-carboxylates and 4-aryl-6-oxo-5-cyano-1,6-dihydropyridine-2-carboxylic acids were...     

The CIDNP effect in plastic crystals

The CIDNP effects in the photolysis reactions of some aldehydes and ketones (including linear, aromatic and cyclic) in plastic crystals of cyclohexane have been discovered and studied. In going from liquid to solid solutions, the change of polarization sign is observed for some substances investigated. The CIDNP effect for a simple model for a relative motion of atoms of radicals possessing spin density (translational diffusion in a restricted volume) has been calculated. The results observed are interpreted in the framework of the radical pair mechanism of CIDNP effect formation.     

ESEEM of disordered systems in frequency domain: Analytical formulae in the case of nuclear spin 1/2 and weak hyperfine interaction

The analytical expressions for the spectral density of the dead time free electron spin echo envelope modulation (ESEEM) signal of disordered system are obtained for a paramagnetic center with nuclear spin 1/2 and weak axially symmetric hyperfine interaction. The spectral density is given by the Fourier transformation of the ESE signal averaged over all orientations. The order of the two linear operations may be changed. Fourier transformation of the nonaveraged ESE signal supplies us with the sum of the Dirac δ -functions. Averaging of such a spectrum is a rather trivial operation leading to the spectral densities in the final form.     

Reaction of 1-(2-Oxocyclohexyl)ethane-1,1,2,2-tetracarbonitrile with α,β-Unsaturated Aldehydes

Russian Journal of General Chemistry - The reaction of 1-(2-oxocyclohexyl)ethane-1,1,2,2-tetracarbonitrile with crotonic aldehyde, cinnamic aldehyde, 2-furylacrolein, 3-phenyl-2-propinal,...     

Librational dynamics of nitroxide molecules in a molecular glass studied by echo-detected EPR

Nitroxides 2,2,6,6-tetramethyl-4-piperidone N-oxide (tempone), 3-carboxy-proxyl and potassium peroxylamine disulfonate (Fremy salt) in glycerol solution were studied in a wide temperature range near the glass transition temperatureT _g. The echo-detected (ED) electron paramagnetic resonance (EPR) lineshape reveals strong dependence on the time interval τ between the echo-forming microwave pulses which is readily explained by anisotropic phase relaxation. Employing a librational model of molecular motion and the Redfield relaxation theory, spectra were simulated for the τ’s varying in a large interval. The anisotropic relaxation rate increases with temperature increase and it is larger for nitroxide with a larger molecular size. The mean-squared amplitude of motion, obtained from reduced hyperfine splitting in continuous-wave EPR, near T_g linearly depends on temperature which is characteristic of harmonic solids. For tempone in a host crystal 2,2,4,4-tetramethyl-cyclobutan-1, 3-dione the anisotropic spin relaxation rate decreases with temperature increase so the found feature solely belongs to a glassy state. A new approach is proposed for modeling slow wobbling motion in a restricted angular space.     

MRI study of spatial distribution of photochemical reaction products

Spatial distribution of molecules with chemically induced dynamic nuclear polarization has been studied by nuclear magnetic resonance imaging. It is shown that heating of a system during the photolysis can cause highly nonuniform distribution of reaction products due to a convective effect.     

Weakly coupled radical pairs in solids: ELDOR in ESE structure studies

Possibilities of the structure determination of radical pairs having fixed geometry with the help of ELDOR in ESE technique are considered. It is demonstrated that one can obtain information on relative orientation of paramagnetic centers in weakly coupled pairs in addition to the energy parameters of the spin Hamiltonian. Appropriate requirements for such experiments are formulated.