Electron Spin Echo Modulation Experiments Research Articles

Recruitment of a Foreign Quinone into the A1 Site of Photosystem I: II. STRUCTURAL AND FUNCTIONAL CHARACTERIZATION OF PHYLLOQUINONE BIOSYNTHETIC PATHWAY MUTANTS BY ELECTRON PARAMAGNETIC RESONANCE AND ELECTRON-NUCLEAR DOUBLE RESONANCE SPECTROSCOPY

Electron paramagnetic resonance (EPR) and electron-nuclear double resonance studies of the photosystem (PS) I quinone acceptor, A(1), in phylloquinone biosynthetic pathway mutants are described. Room temperature continuous wave EPR measurements at X-band of whole cells of menA and menB interruption mutants show a transient reduction and oxidation of an organic radical with a g-value and anisotropy characteristic of a quinone. In PS I complexes, the continuous wave EPR spectrum of the photoaccumulated Q(-) radical, measured at Q-band, and the electron spin-polarized transient EPR spectra of the radical pair P700(+) Q(-), measured at X-, Q-, and W-bands, show three prominent features: (i) Q(-) has a larger g-anisotropy than native phylloquinone, (ii) Q(-) does not display the prominent methyl hyperfine couplings attributed to the 2-methyl group of phylloquinone, and (iii) the orientation of Q(-) in the A(1) site as derived from the spin polarization is that of native phylloquinone in the wild type. Electron spin echo modulation experiments on P700(+) Q(-) show that the dipolar coupling in the radical pair is the same as in native PS I, i.e. the distance between P700(+) and Q(-) (25.3 +/- 0.3 A) is the same as between P700(+) and A(1)(-) in the wild type. Pulsed electron-nuclear double resonance studies show two sets of resolved spectral features with nearly axially symmetric hyperfine couplings. They are tentatively assigned to the two methyl groups of the recruited plastoquinone-9, and their difference indicates a strong inequivalence among the two groups when in the A(1) site. These results show that Q (i) functions in accepting an electron from A(0)(-) and in passing the electron forward to the iron-sulfur clusters, (ii) occupies the A(1) site with an orientation similar to that of phylloquinone in the wild type, and (iii) has spectroscopic properties consistent with its identity as plastoquinone-9.

Four pulse electron spin echo modulation studies of Cu(II) complexes in MCM-41 silica tube materials

The coordination geometry of Cu(II) complexes with water and ammonia has been studied by four pulse electron spin echo modulation spectroscopy in siliceous (L)Cu-MCM-41 and in aluminum-containing (L)Cu-AlMCM-41 where (L) denotes Cu(II) incorporation by liquid phase ionexchange. An analysis of the proton sum peaks in the echo modulation pattern of the water and ammonia ligands reveals significant differences in the Cu(II) coordination between MCM-41 and AlMCM-41. In the aluminum-containing material (L)Cu-AlMCM-41, Cu(II) coordinates to two molecules of water or ammonia and three framework oxygens in a square-based pyramidal coordination geometry. The base of the pyramid is formed by two adsorbate molecules together with two framework oxygens. A third framework oxygen is located at the apex of the pyramid. The cupric ion site is slightly shifted from the plane of the pyramid base towards the apex resulting in an off-plane position. In the siliceous material (L)Cu-MCM-41, [Cu(H2O)6]2+ and [Cu(NH3)4]2+ complexes are observed. The results of four pulse electron spin echo modulation experiments support a distorted octahedral coordination geometry for the [Cu(H2O)6]2+ complex in (L)Cu-MCM-41.

Electron spin resonance and electron spin echo spectroscopic studies of Cu2+ sites in synthetic cancrinite zeolites

Electron spin resonance (ESR) and electron spin echo modulation (ESEM) studies of Cu2+ sites introduced by ion exchange are reported in two types of synthetic cancrinite zeolites, one of them a more hydrated sample, Na6 Al6Si6O24•8H2O (denoted C1), and the other a less hydrated sample with entrained calcium carbonate, Na6Al6Si6O24•CaCO3•8H2O (denoted C2). ESR spectra reveal that C1 activated at 100 °C contains Cu2+ sites of tetrahedral symmetry with [Formula: see text] and [Formula: see text] while at a higher activation temperature of 400 °C Cu2+ sites with a different symmetry are produced corresponding to [Formula: see text] and [Formula: see text] C2 activated at 100 °C contains almost exclusively Cu2+ sites of trigonal symmetry with [Formula: see text] and [Formula: see text] Two- and three-pulse ESEM experiments at 4 K give 2D and 27Al modulation patterns that corroborate these ESR assignments. Possible locations of the Cu2+ sites in the C1 and C2 structures are proposed.

SMART, a general purpose pulse experiment simulation program using numerical density matrix calculations

A general purpose pulse experiment simulation program, SMART (simulation of magnetic resonance pulse experiments in the time domain), is described generating time-domain signals. It features a Cartesian product-operator interpreter to handle arbitrary Hamiltonians without limitations on the spin quantum number for simulations in a variety of situations. A high-level pulse experiment simulation language is tailored to a wide variety of one- or two-dimensional homo- and heteronuclear magnetic resonance experiments using radio-frequency pulses, delays, and phase-cycling schemes. Illustrative simulations of 2D NMR experiments (E.COSY and time reversal of scalar J couplings) with a strongly coupled three-spin system and electron spin-echo modulation experiments are presented.

Selective excitation in electron spin-echo modulation experiments

Electron spin-echo spectroscopy (1-6) has become a powerful method for the study of hyperfine and quadrupole interactions in inhomogeneously broadened systems. Both twoand three-pulse experiments have been used to measure the echo modulation envelope as a function of pulse separation. Modulation effects are caused by interactions which do not change sign under a 7r pulse and necessarily involve a violation of the high-field assumption for nuclear resonance under the particular measurement condition (1, 2). It is now widely accepted that weak selective microwave pulses cause the echo envelope modulation to disappear (4, 5). Reduction of modulation depth due to partial excitation has recently been analyzed in detail by Barkhuijsen et al. (7). On the other hand we have recently found that weak extended-time excitation reproduces the entire echo envelope modulation in a single experiment, provided that the weak irradiation is followed by a short and strong refocusing pulse (8). In this communication we give further experimental evidence for the fact that a weak selective excitation pulse followed by a strong refocusing pulse produces the same envelope modulation as obtained with two strong pulses. We also present an explanation for this finding. The experimental results are summarized in Fig. 1. Figure la gives the echo envelope modulation for a Cu(II)-aquo complex at 4.2 K using short microwave pulses both for excitation (10 ns) and refocusing (20 ns) with nominal flip angles of (a/2) and K, respectively. The Fourier transform of this pattern consists of two peaks at 14 and 28 MHz corresponding to the proton Zeeman frequency and its second harmonic. When the short excitation pulse is replaced by a long (210 ns) weak pulse, no change in the modulation pattern is observed (Fig. lb). However, a loss of signal amplitude occurs caused by the reduced bandwidth of excitation, not visible in Fig. lb due to normalized amplitudes. In Fig. lc, the short refocusing x pulse is also replaced by a long (2 10 ns) and weak pulse. In contrast to the first two experiments the echo modulation pattern disappears completely. By increasing the amplitude of the two pulses part of the echo modulation may be recalled (Fig. Id). The pattern observed, however, does not fully correspond to

Effect of micellar interface modifications on the reactivity of embedded photoproduced cations

Lifetime distributions of photoproduced N,N,N',N',-tetramethylbenzidine cation radicals (TMB/sup +/) in micellar solutions of dodecyltrimethylammonium chloride (DTAC) and sodium dodecyl sulfate (SDS), with and without NaCl or 1-butanol added to modify the micellar interface potential, were determined for inverse Laplace transforms of the decay curves adequately approximated by (TMB/sup +/)/(TMB/sup +/)/sub 0/ = exp(-(t/tau/sub 0/))/sup ..cap alpha../. The constant ..cap alpha.., determining the width of the TMB/sup +/ lifetime distribution, was found to be insensitive to these micellar interface modifications, while the variation of the effective lifetime tau/sub 0/ was found to correlate with TMB/sup +/-water interactions measured by electron spin echo modulation experiments. 13 references, 4 figures.