EPR spectroscopy and molecular dynamics modelling: a combined approach to study liquid crystals

This review outlines the recent theoretical and computational developments for the prediction of motional electron paramagnetic resonance spectra with introduced spin probes from molecular dynamics simulations. The methodology is illustrated with applications to thermotropic and lyotropic liquid crystals at different phases and aggregate states

Assessing limitations to the two-level approximation in nonlinear optics for organic chromophores by ab initio methods

The use of a two-level approximation to simply characterize the nonlinear optical properties of organic materials is well known. Usually only electronic ground states are significantly populated; higher levels are engaged only in the capacity of virtual states, and it is frequently assumed that just one such state dominates in determining the response. Calculating nonlinear optical susceptibilities on this basis, excluding all but the ground and one excited state in a sum-over-states formulation, is a technique widely deployed in the calculation and analysis of nonlinear optical properties. However, the necessity for such an approach is diminishing as, particularly within the last decade, the accuracy of ab initio calculations has reached unprecedented levels. This offers new opportunities for a vigorous test of existing models using real molecular structures. Here we report the results of our recent work on testing the general validity of two-level calculations in nonlinear optics. Firstly, through the extension of approximation to a three-level model we demonstrate that the neglect of additional excited states can lead to substantially erroneous results for the hyperpolarizability elements. Secondly, using high levels of theory and basis set we report the results of ab initio calculations for both ground and electronically excited states of the optimised structures, for selected merocyanine dyes. The results are used for the calculation of hyperpolarizabilities by a rigorous sum-over-states formulation. A systematic comparison with the two-level approach provides a means for identifying the limits of the model and the criteria for its validity

Electron Paramagnetic Resonance Spectra Simulation Directly from Molecular Dynamics Trajectories of a Liquid Crystal with a Doped Paramagnetic Spin Probe

We report simulation of EPR spectra directly and entirely from trajectories generated from molecular dynamics simulations. Results are reported for a model 3β-DOXYL-5α-cholestane spin probe in a coarse-grained solvent representing a 5CB nematic host. The results are in excellent agreement with the experimental spectra. The calculated order parameters associated with the paramagnetic probe show strong correlation with the order parameter of 5CB mesogens and are in agreement with those reported in the literature. Simulation of EPR spectra entirely from molecular dynamics of real structures provides direct correlation between molecular motions and the features observed in the spectra, allowing unambiguous interpretation of the spectra. This method opens the possibility for “computer engineering” of spin-labeled materials with the desired properties, such as spin-labeled proteins, prior to experiment

Angular dependences of perpendicular and parallel mode Electron Paramagnetic Resonance of oxidised beef heart cytochrome c oxidase

Cytochrome c oxidase catalyzes the reduction of oxygen to water with a concomitant conservation of energy in the form of a transmembrane proton gradient. The enzyme has a catalytic site consisting of a binuclear center of a copper ion and a heme group. The spectroscopic parameters of this center are unusual. The origin of broad electron paramagnetic resonance (EPR) signals in the oxidized state at rather low resonant field, the so-called g' = 12 signal, has been a matter of debate for over 30 years, We have studied the angular dependence of this resonance in both parallel and perpendicular mode X-band EPR in oriented multilayers containing cytochrome c oxidase to resolve the assignment. The "slow" form and compounds formed by the addition of formate and fluoride to the oxidized enzyme display these resonances, which result from transitions between states of an integer-spin multiplet arising from magnetic exchange coupling between the five unpaired electrons of high spin Fe(III) heme a(3) and the single unpaired electron of Cu-B. The first successful simulation of similar signals observed in both perpendicular and parallel mode X-band EPR spectra in frozen aqueous solution of the fluoride compound of the closely related enzyme, quinol oxidase or cytochrome bo(3), has been reported recently (Oganesyan et at., 1998, J. Am. Chem. Sec. 120:4232-4233). This suggested that the exchange interaction between the two metal ions of the binuclear center is very weak (\J\ approximate to 1 cm(-1)), with the axial zero-field splitting (D approximate to 5 cm(-1)) of the high-spin heme dominating the form of the ground state. We show that this model accounts well for the angular dependences of the X-band EPR spectra in both perpendicular and parallel modes of oriented multilayers of cytochrome c oxidase derivatives and that the experimental results are inconsistent with earlier schemes that use exchange coupling parameters of several hundred wavenumbers.</p