Ernst Ohmes

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...

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.

Pulsed EPR detection of light-generated nuclear coherences in photosynthetic reaction centers

Abstract Light generation of nuclear coherence in spin-correlated radical pairs has been established as a novel mechanism employing a pulsed EPR technique. At the instant of the laser flash, the radical pair intermediates of the primary events of photosynthesis are created in a singlet state, which is not an eigenstate of the corresponding spin Hamiltonian. This implies formation of zero quantum electron and single quantum nuclear coherence between specific eigenstates of the radical pair. Analytical model calculations indicate that light generation of nuclear coherence is associated with high nuclear spin polarization.

Molecular dynamics studied by transient esr nutation spectroscopy of photoexcited triplet states: Chlorophyll a in liquid crystalline matrix

Abstract Molecular order and dynamics of chlorophyll a triplets in liquid crystals are studied by transient ESR following pulsed laser excitation. The time evolution of the transverse magnetization is monitored at various static and microwave magnetic fields. Analysis of the ESR transients is achieved by employing a relaxation model based on the stochastic Liouville equation. Computer simulations provide the orientational order parameters and motional correlation times varying from 10 −8 to 10 −3 s.

Electron spin resonance of symmetrical three-spin systems in nematic liquid crystals

An E.S.R. line-shape model is developed for fast tumbling three-spin systems in nematic liquid crystals. The line positions are calculated from a spin hamiltonian, which considers Zeeman, exchange, dipolar and hyperfine interactions of the three unpaired electrons. The dominant spin relaxation process, determining the line-widths, is assumed to result from the anisotropy of the zero-field splitting coupled to the molecular motion. The predictions of the theory are tested by comparison with the temperature-dependent E.S.R. spectra of trisverdazyl radicals in 4,4′-azoxydianisole. Good agreement is found between experimental and simulated spectra. A detailed analysis provides values for the solute order parameters [Pbar] 2 and [Pbar] 4. They correspond surprisingly well to predictions of the molecular-field theories of nematic liquid crystals.

Electron paramagnetic resonance of slowly tumbling triradicals: Investigation of ΔM =1, ΔM = 2, and ΔM = 3 transitions

Abstract The EPR lineshapes of symmetrical triradicals undergoing slow tumbling motions are investigated with particular reference to Δ M = 2 and Δ M = 3 transitions. The density matrix equation of motion including a term for rotational diffusion is solved using the finite-difference approximation. Theory predicts broadening of the Δ M = 2 transitions with increasing rotational motion but practically no broadening of the Δ M = 3 transitions. The predictions are tested by comparison with the temperature-dependent X-band spectra of trishydrazyl radicals in s-butylbenzene. Good agreement is found between experimental and simulated spectra. A detailed analysis provided values for the rotational correlation times. They are used to determine the hydrodynamic radius of the trishydrazyl radicals.

Magnetic-field-induced orientation of photosynthetic reaction centers as revealed by time-resolved W-band EPR of spin-correlated radical paris: Development of a molecular model

Spin-correlated radical pairs are the short-lived intermediates of the primary energy conversion steps of photosynthesis. In this paper, we develop a comprehensive model for the spin-polarized electron paramagnetic resonance (EPR) spectra of these systems. Particular emphasis is given to a proper treatment of the alignment of the photosynthetic bacteria by the field of the EPR spectormeter. The model is employed to analyze time-resolved W-band (94 GHz) EPR spectra of the secondary radical pair P700+A1− in photosystem I formed by photoexcitation of the deuterated and15N-substituted cyanobacteriumSynechococcus lividus. Computer simulations of the angular-dependent EPR spectra of P700/+A1/− provide values for the order parameter of the cyanobacterial cells and for the orientation of the membrane normal in a molecular reference system. The order parameter from EPR compares favorably with corresponding data from electron microscopy obtained for theS. lividus cells under similar experimental conditions. It is shown that high-field EPR of a magnetically aligned sample in combination with the study of quantum beat oscillations represents a powerful structural tool for the short-lived radical pair intermediates of photosynthesis.

Quantum Beats as Probes of the Primary Events in Bacterial Photosynthesis

Light-induced radical pairs in fully deuterated iron-containing bacterial reaction centers of Rhodobacter sphaeroides have been studied by transient EPR following pulsed laser excitation. Quantum beat oscillations are observed in the transverse magnetization at low temperatures. Due to its spin-correlated generation, the secondary radical pair P+ [QA Fe2+]- is expected to start out in a coherent superposition of eigenstates, as observed experimentally. Thorough investigation of light-induced nuclear coherences, detected in the transverse electron magnetization, can provide detailed information on the electronic structure of the primary donor, which is essential for a better understanding of the primary events of bacterial photosynthesis.

Orientational order of triradical spin probes in nematic liquid crystals

The trigonal symmetry of organic triradicals makes them important candidates for spin probes of liquid crystals. Here we develop a lineshape model for such spin probes, within the limit of fast motion, and show how the E.S.R. spectra depend on the orientation of the director with respect to the magnetic field. The theory is employed to study the lineshapes of trisverdazyl radicals in 4,4′-dimethoxyazoxybenzene, aligned by a high frequency electric field. An analysis of the angular-dependent linewidths provides reliable values for the order parameter P 4. They are in good agreement with statistical theories of nematic liquid crystals.

Spin Probe Studies of Oriented Liquid-Crystalline Polymers

The spin probe technique is employed to study liquid-crystalline side chain polymers [1], oriented in a high frequency electric field [2]. Temperature and angular dependent electron spin resonance spectra are analyzed, using a comprehensive lineshape model [3]. Computer simulations provide the order parameters P2 and rotational correlation times t of the nitroxide probes [4]. They are related to the structure and dynamics of the nematic polymer: