Phase Behavior of Aqueous Na-K-Mg-Ca-CI-NO3 Mixtures: Isopiestic Measurements and Thermodynamic Modeling

A comprehensive model has been established for calculating thermodynamic properties of multicomponent aqueous systems containing the Na{sup +}, K{sup +}, Mg{sup 2+}, Ca{sup 2+}, Cl{sup -}, and NO{sub 3}{sup -} ions. The thermodynamic framework is based on a previously developed model for mixed-solvent electrolyte solutions. The framework has been designed to reproduce the properties of salt solutions at temperatures ranging from the freezing point to 300 C and concentrations ranging from infinite dilution to the fused salt limit. The model has been parameterized using a combination of an extensive literature database and new isopiestic measurements for thirteen salt mixtures at 140 C. The measurements have been performed using Oak Ridge National Laboratory's (ORNL) previously designed gravimetric isopiestic apparatus, which makes it possible to detect solid phase precipitation. Water activities are reported for mixtures with a fixed ratio of salts as a function of the total apparent salt mole fraction. The isopiestic measurements reported here simultaneously reflect two fundamental properties of the system, i.e., the activity of water as a function of solution concentration and the occurrence of solid-liquid transitions. The thermodynamic model accurately reproduces the new isopiestic data as well as literature data for binary, ternary and higher-order subsystems. Because of its high accuracy in calculating vapor-liquid and solid-liquid equilibria, the model is suitable for studying deliquescence behavior of multicomponent salt systems

Applications of ESE spectroscopy in the study of electron spin polarization in bacterial photosynthesis

In photosynthesis the photon energy of light is converted into chemical potential in thc so-called Reaction Center (RC), a pigmented membrane protein that upon excitation gives rise to a charged radical pair. The radicals are spin correlated and in an external magnetic field the interactions (dipolar, exchange) between the two electron spins give rise to non-Boltzmann populations of their magnetic sublevels (spin polarization), which is probed by time-resolved EPR.An overview is given of polarization phenomena in RC of bacterial origin, and recent theoretical interpretations highlighted. It is demonstrated that in all likelihood the exchange interaction of the primary radical pair in the RC of purple bacteria is positive. The implications of this result for our understanding of the initial processes of solar energy conversion are discussed

Relative orientation of the optical transition dipole and triplet axes of the photosystem II primary donor: a magnetophotoselection study

A magnetophotoselection (MPS) study on the triplet state of the primary donor in the D1-D2-cytochrome b559 complex is performed at 6 K. For this purpose a novel double-modulation technique was developed, resulting in a 2-fold better signal-to-noise ratio, compared to the conventional technique. The MPS data were found to depend on the excitation wavelength, The angles calculated from the MPS at the longest wavelength are representative for the a,transition moment of P680, phi(x) = 55 degrees +/- 1 degrees, phi(y) = 30 degrees +/- 4 degrees, phi(z) = 90 degrees +/- 12 degrees, where phi(i) is the angle with the ith triplet axis, and yield a positive sign for E. The angles found for P680 differ from those reported for monomeric chlorophyll a in solution (Vrieze, J.; Hoff, A. J. Chem. Phys. Lett. 1995, 237, 493). This difference is discussed in terms of the structure of P680. Assuming C-2 symmetry, with the 2-fold axis perpendicular to the membrane plane, a model is presented in which P680 is an excitonically-coupled dimer of chlorophyll a molecules, whose triplet state at 6 K resides on one of the constituent monomers. The angles of the triplet magnetic axes of this monomer with the membrane plane correspond to those determined in an EPR study on oriented samples (Van Mieghem, F. J. E.; Satoh, K.; Rutherford, A, W. Biochim. Biophys. Acta 1991, 1058, 379-385). The ($) over right arrow Q(y) transition moments of the two chlorophylls of P680 are close to antiparallel, and the ($) over right arrow Q(y)((-)) axis of the lower excitonic component lies in the membrane plane. The Chi porphyrin planes make an angle of 60 degrees, and their intercenter distance is estimated to be 11 Angstrom

Time-resolved EPR study of the primary donor triplet in D1-D2-cyt b559 complexes of photosystem II: temperature dependence of spin−lattice relaxation

The temperature dependence of the EPR spectrum and kinetics of the primary donor triplet state (3)P680 are measured with direct-detection CW EPR and electron spin echo (ESE) spectroscopy, respectively. The EPR spectrum was recorded up to 230 K; kinetics could be traced up to 70 K. The observed anisotropy of the temperature dependence of the EPR spectrum recorded within 1 mu s after a laser flash is explained by anisotropic spin relaxation in the precursor primary radical pair. The ESE-detected decay kinetics of the Z-canonical peak of (3)P680 is close to its optical lifetime for T 30 K, due to a rapid acceleration of spin relaxation with temperature. This relaxation is not caused by the presence of oxygen or the paramagnetic heme iron in the b559 cytochrome; it is attributed to slow tripler hopping in the P680 dimer itself. Comparison of the low-temperature kinetics with that of chlorophyll a in solution confirmed that the central Mg ion in the triplet-bearing Chi of (3)P680 is pentacoordinated

The primary acceptor quinone QA in reaction centers of Rhodobacter sphaeroides R26 is hydrogen bonded to the Nσ(1)-H of His M219. An electron spin echo study of QA−•

An electron spin echo envelope modulation (ESEEM) study is performed on the reduced primary electron-accepting ubiquinone-10 (Q(A)(-.)) in Zn-substituted reaction centers of the photosynthetic bacterium Rhodobacter sphaeroides R26. The ESEEM spectra showed hyperfine and quadrupolar couplings of Q(A)(-.) to nitrogens in the protein matrix. Simulation of the spectra revealed the following N-14 coupling parameters: hyperfine interaction: A(iso)=1.85 MHz, T-11=0.32, alpha=0(0), beta=45(0); nuclear quadrupole interaction: e(2)qQ/h=1.52 MHz, eta=0.82. Comparison of the quadrupole values with data in the literature shows that Q(A)(-.) is coupled to the N-delta(1)-H group of the M219 heterocycle, most probably through a hydrogen bond with the 4-C carbonyl group of the quinone

Control of radical pair lifetimes by microwave irradiation: application to photosynthetic reaction centres

Radicals produced by illumination or ionizing radiation are often produced in pairs, which quickly decay by recombination or by diffusion and subsequent reactions. For maximizing the yield of products, and for facilitating the study of reaction pathways, it is desirable to minimize the probability of radical pair recombination. We present a way of controlling the radical pair lifetime through the application of a pulse of resonant microwaves in the presence of a magnetic field. Herewith, two radical pair triplet states are coherently populated, from which the pair cannot recombine directly to the singlet ground state because of spin conservation. We illustrate the method with a photosynthetic photochemical reaction, where we have achieved an increase in the radical pair lifetime of up to two orders of magnitude

Magnetic interaction between QA−. and the triplet state of the primary donor in modified reaction centers of the photosynthetic bacterium Rhodobacter sphaeroides R26

The interaction between the reduced primary acceptor quinone (Q(A)(-.)), and the triplet state of the primary donor D-3 is investigated with time-resolved continuous-wave EPR, The trough at high-field in the Q(A)(-.) electron-spin polarized X-band EPR-spectrum at early delay times after the laser flash [De Groot et al. (1985) Biochim. Biophys. Acta 808, 13-20] is studied as a function of temperature and of the delay, in zinc-reconstituted reaction centers, with and without carotenoid. In all cases investigated a decrease in D-3 concentration is accompanied by a simultaneous attenuation of the high-field trough in the Q(A)(-.) EPR spectrum. These observations confirm the hypothesis that the line-shape of Q(A)(-.) at short delay times is influenced by a magnetic interaction with D-3. The line-shape of the Q(A)(-.) electron-spin polarized EPR spectrum directly after the laser flash, could be very well simulated using an extension of the model of Hore et al. [Here, P.J. et al. (1993) Biochim. Biophys. Acta 1141, 221-230], with a dipolar coupling between Q(A)(-.) and D-3 Of -0.125 mT