Gerd Kothe

Chain configuration and flexibility gradient in phospholipid membranes. Comparison between spin-label electron spin resonance and deuteron nuclear magnetic resonance, and identification of new conformations.

The electron spin resonance spectra of 1-myristoyl-2-[n-(4,4-dimethyloxazolidine-N-oxyl)myristoyl]-sn- glycero-3-phosphocholine spin-label positional isomers (n = 6, 10, and 13) have been studied in oriented, fully hydrated bilayers of dimyristoylphosphatidylcholine, as a function of temperature and magnetic field orientation. The spectra have been simulated using a line-shape model which incorporates chain rotational isomerism, as well as restricted anisotropic motion of the lipid molecules as a whole, and which is valid in all motional regimes of conventional spin-label electron spin resonance (ESR) spectroscopy. At least one component of the lipid motion is found to lie in the slow-motion regime for all label positions, even in the fluid liquid crystalline phase, well above the phase transition. In the gel phase, the chain isomerism lies in the slow-motional regime, and the overall motions are at the rigid-limit. In the fluid phase, the chain isomerism is in the fast-motional regime, and the chain axis motions are in the slow regime. This indicates that the commonly used motional-narrowing theory is not appropriate for the interpretation of spin-label spectra in biological membranes. The simulation parameters yield a consistent description for the chain order and dynamics for all label positions. The correlation times and order parameters for the overall motion are the same at all positions down the chain, whereas the chain conformation and trans-gauche isomerism rate display a characteristic flexibility gradient, with increasing motion towards the terminal methyl end of the chain. Significantly, it is found that all six distinct tetrahedral orientations of the hyperfine tensor at the labeled segment are required for a consistent description of the chain isomerism. For the C-6 segment only the 0 degree (trans) and two 60 degrees (gauche) orientations are significantly populated, for the C-10 position two further 60 degrees orientations are populated, and for the C-13 position all orientations have non-vanishing populations. Detailed comparisons have been made with the results of 2H nuclear magnetic resonance (NMR) measurements on dimyristoylphosphatidylcholine labeled at the same position in the sn-2 chain, and using an identical motional model. The parameters of overall reorientation, both order parameter and correlation times, have very similar values as determined by ESR and NMR. The major difference between the results from the two methods lies in the conformational populations at the labeled chain segment and the trans-gauche isomerization rate in the gel phase. The conformational order is much lower for the spin-labeled chain segments than for the corresponding deuterium-labeled segments, and the isomer interconversion rates in the gel phase(although displaying a mobility gradient in both cases) are found to be much slower in the former case. In addition the spin-label measurements provide information on the macro order (chain tilt), which is only available from oriented samples. These results are consistent between the different spin label positions and are in agreement with the findings from x-ray diffraction.

Multipulse dynamic nuclear magnetic resonance of phospholipid membranes

Multipulse dynamic NMR has been employed to study molecular order and dynamics of deuteron (2H) labeled phospholipid membranes. Variation of pulse sequence and pulse separation provides the large number of independent experiments necessary for a proper molecular characterization of the systems. Analysis of these experiments is achieved by employing a density matrix formalism, based on the stochastic Liouville equation. Arbitrary relaxation rates and line shapes of single and multiple quantum transitions are considered.The various 2H NMR experiments of macroscopically unoriented bilayers of 1,2‐dimyristoyl‐sn‐glycero‐3‐phosphocholine (DMPC), specifically deuterated at the 6‐ and 14‐position of the 2‐chain, are faithfully reproduced by the model. Computer simulations provide the orientational distributions and conformations of the hydrocarbon chains and the correlation times of the various motions. In the Lα phase the correlation times τR∥ and τR⊥ for chain rotation and chain fluctuation are of the order of...

Multipulse dynamic NMR of liquid crystal polymers

Theorie de la matrice de densite; experiences choisies: polymeres deuteries; mesures RMN; analyse des donnees; resultats representatifs; discussion generale; conclusions

Transient EPR of light-induced radical pairs in plant photosystem I: observation of quantum beats

Abstract Electron spin polarization of light-induced radical pairs in plant photosystem I is studied by transient EPR following pulsed laser excitation. The time evolution of the transverse magnetization is monitored at various static and microwave magnetic fields. Quantum beat oscillations are observed at early times after light excitation of fully deuterated whole algae Synechococcus lividus . Model calculations for the time profiles, based on the correlated radical pair mechanism, provide information on the spin—spin coupling and lifetime of the secondary radical pair.

Collective lipid motions in bilayer membranes studied by transverse deuteron spin relaxation

The anisotropy and pulse frequency dispersion of the spin–spin relaxation time TCP2E from Carr–Purcell–Meiboom–Gill pulse sequences is employed to evaluate the major contribution to transverse 2H spin relaxation in bilayer membranes. Analysis of the experiments is achieved in terms of a density operator formalism, employing the stochastic Liouville approach. From a comparison of the observed angular and frequency dependences of TCP2E with those predicted for order director fluctuations, we conclude that collective lipid motions constitute the dominant transverse relaxation process. Computer simulations provide the viscoelastic parameters of the lipid membranes. For 1,2‐dimyristoyl‐sn‐glycero‐3‐phosphocholine bilayers at T=318 K an average elastic constant of K=2×10−11 N and an effective viscosity of η=0.1 P have been determined. Using the experimentally accessible value for the long wavelength cutoff of the elastic modes, one obtains the mean square amplitude of the director fluctuations 〈θ20〉=0.04. This ...

Transient EPR of radical pairs in photosynthetic reaction centers: prediction of quantum beats

Abstract Transient nutation EPR of light-induced radical pairs in photosynthetic reaction centers is studied theoretically using the stochastic Liouville equation. The spin Hamiltonian employed considers Zeeman, exchange and dipolar interactions of the spin-correlated radical pair. Particular emphasis is given to the slow-motional regime, where anisotropic magnetic interactions dominate. The calculated dependence of the transverse magnetization on the static magnetic field agrees well with previous simulations of the transient spectra. Quantum-beat oscillations are predicted for the time evolution of the transverse magnetization. The beat frequency is determined by the spin-spin coupling and g-factor difference of the radical pair.

Orientation-dependent deuteron spin-lattice relaxation times in bilayer membranes: characterization of the overall lipid motion

Abstract The anisotropy of the 2 H spin-lattice relaxation time T 1 Z in oriented bilayer membranes is used to evaluate the major relaxation pathways in the megahertz regime. Analysis of the experiments is achieved in terms of a density operator formalism, employing the Redfield approximation. From a comparison of the observed angular dependences of T 1 Z with those predicted for various motional modes we conclude that restricted rotational diffusion of individual molecules represents the dominant relaxation process. Computer simulations provide the orientational order parameters and motional correlation times of the lipid molecules.

The symmetry of the nematic phase of a thermotropic liquid crystal : biaxial or uniaxial ?

Abstract The symmetry of the thermotropic nematic phase of 2,3,4-tri- n -hexyloxycinnamic acid is investigated using deuterium NMR spectroscopy of the nematogen selectively deuterated in the ethylenic bond. In our experiments the sample was spun about an axis orthogonal to the magnetic field in order to produce a random distribution of the director in two dimensions. The resultant NMR powder pattern is characteristic of a partially averaged quadrupolar tensor with cylindrical symmetry and hence of a uniaxial nematic phase. Simulation of the powder patterns reveals that the upper limit to the biaxiality parameter is approximately 0.1 which is in marked contrast to the large values found for lyotropic biaxial nematics. Our result is not, however, necassarily inconsistent with conoscopic measurements which indicated a small optical biaxiality for this thermotropic nematic.

Deuteron N.M.R. relaxation studies of phospholipid membranes

Abstract Bilayers of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), specifically deuteriated at various positions of the sn-2-chain, have been studied by N.M.R. relaxation methods. Analysis of the experiments, employing a density matrix treatment based on the stochastic Liouville equation, provides new information about the dynamic organization of the different membrane phases (liquid-crystalline, intermediate and gel phases). The complex molecular dynamics are characterized by a super-position of inter- and intramolecular motions, comprising overall reorientation of phospholipid molecules and trans-gauche isomerization of individual chain segments. In addition, there is evidence for two-site rotational jumps of the sn-2-chains in the plane of the membrane. The results clearly demonstrate the particular advantage of N.M.R. relaxation studies in characterizing complex chemical and biological systems.

Exploring the photoexcited triplet states of aluminum and tin corroles by time-resolved Q-band EPR

The photoexcited triplet states of three 5,10, 15-tris(pentafluorophenyl)corroles (tpfc), hosting Sn(IV) and Al(III) in their core, namely, Sn(Cl)(tpfc), Al(pyr)2(tpfc) and Al(pyr)2(tpfc-Br8), were studied by time-resolved electron paramagnetic resonance (TREPR) spectroscopy in the nematic liquid crystal E7. Only two of these metallocorroles, namely, Sn(Cl)(tpfc) and Al(pyr)2(tpfc-Br8), exhibit TREPR spectra following pulsed laser excitation. This result is rationalized in terms of a very low quantum yield of triplet formation in Al(pyr)2(tpfc). Analysis of the spin polarized Q-band (34 GHz) EPR spectra of Sn(Cl)(tpfc) and Al(pyr)2(tpfc-Br8) provides detailed information on the magnetic and kinetic parameters of the triplet states as well as on the molecular ordering of the complexes in the liquid crystal. With the assignment of the zero-field splitting parameterD<0 for the Sn(Cl)(tpfc) and Al(pyr)2(tpfc-Br8), one can evaluate the dominant intersystem crossing path for these metallocorroles. Analysis reveals that in Sn(Cl)(tpfc) the in-plane triplet sublevels are preferentially populated, i.e.,AX, AY≫AZ. This can be rationalized in terms of weak electronic interactions between the Sn(IV) ion and the corrole π-system, consistent with the domed structure of Sn(Cl)(tpfc). In Al(pyr)2(tpfc-Br8), however, the out-of-plane triplet sublevel is predominantly populated, i.e.,AZ>AX, AY, which is attributed to a large increase in the spin-orbit coupling strength arising from the peripheral bromine atoms on the corrole skeleton.