The enhancement of the spin-lattice relaxation rate for nuclear spins in a ligand bound to a paramagnetic metal ion [known as the paramagnetic relaxation enhancement (PRE)] arises primarily through the dipole-dipole (DD) interaction between the nuclear spins and the electron spins. In solution, the DD interaction is modulated mostly by reorientation of the nuclear spin-electron spin axis and by electron spin relaxation. Calculations of the PRE are in general complicated, mainly because the electron spin interacts so strongly with the other degrees of freedom that its relaxation cannot be described by second-order perturbation theory or the Redfield theory. Three approaches to resolve this problem exist in the literature: The so-called slow-motion theory, originating from Swedish groups [Benetis et al., Mol. Phys. 48, 329 (1983); Kowalewski et al., Adv. Inorg. Chem. 57, (2005); Larsson et al., J. Chem. Phys. 101, 1116 (1994); T. Nilsson et al., J. Magn. Reson. 154, 269 (2002)] and two different methods based on simulations of the dynamics of electron spin in time domain, developed in Grenoble [Fries and Belorizky, J. Chem. Phys. 126, 204503 (2007); Rast et al., ibid. 115, 7554 (2001)] and Ann Arbor [Abernathy and Sharp, J. Chem. Phys. 106, 9032 (1997); Schaefle and Sharp, ibid. 121, 5387 (2004); Schaefle and Sharp, J. Magn. Reson. 176, 160 (2005)], respectively. In this paper, we report a numerical comparison of the three methods for a large variety of parameter sets, meant to correspond to large and small complexes of gadolinium(III) and of nickel(II). It is found that the agreement between the Swedish and the Grenoble approaches is very good for practically all parameter sets, while the predictions of the Ann Arbor model are similar in a number of the calculations but deviate significantly in others, reflecting in part differences in the treatment of electron spin relaxation. The origins of the discrepancies are discussed briefly.

Chloroplast thylakoid membranes isolated in the presence of EDTA retain high rates of 0 evolution (>340 #mol-hl'-mg chlorophyll'l) but contain no Mn1+ that is detectable by electron paramagnetic resonance (EPR) at room temperature, The total Mn2+ content of these preparations is 4.6 per 400 chlorophylls; 0.6 Mn2+ can be released by addition of Ca2 , a treatment that does not affect 02 evolution.The remaining Mn2+ (4 per 400 chlorophylls) appears to be functionally associated with 02 evolution activity. Inhibition by Tris, NH20H, or heat will release a small fraction of Mn2+ from these membranes (=25% with Tris, for example). Addition of Ca2+ further enhances Mn2+ release so that for Tris and for NH20H, 2 and 3, respectively, Mn2 per 400 chlorophylls are extracted from the 02-evolving complex. Based on the microwave power-saturation properties of the EPR MATERIALS AND METHODSThylakoid membranes were isolated from market spinach by a variety ofprocedures. Salt/EDTA membranes were isolated as described (5). Sucrose/MgCl2 and sucrose/EDTA chloroplasts were prepared by grinding leaves in sucrose buffer (0.4 M sucrose/20 mM Hepes, pH 7.5/15 mM NaCl)/2 mM MgCl2 or 1 mM EDTA. The pellets from these initial steps were washed with sucrose buffer/I mM EDTA and then with sucrose buffer/ 2 mM MgCl2 and then suspended in sucrose buffer. Finally, membranes were isolated by grinding leaves in sucrose buffer, pelleting the membranes, and suspending the pellet in sucrose buffer without further washing. The final suspensions [2.4-4.5 mg of chlorophyll (Chl)/ml] were either stored at -350C or used immediately. When assayed for activity, these preparations gave gramicidin-uncoupled rates of 02 evolution >300.mol-hr-'-mg Chl'-; one exception, the sucrose buffer-isolated and washed preparation, is below. The possibility that residual amounts of EDTA might contaminate these preparations was examined by addition of MnCl2 to the membrane suspensions before and after additional washes with 150 mM NaCl/4 mM MgCl2 or sucrose buffer. The levels of EPR-detectable Mn2+ found in these experiments indicated that the concentration of residual EDTA was <0.5 AuM. For Tris inactivation, a thylakoid suspension (0.6 ml) was mixed with 0.2 ml of 3.2 M Tris,-pH 8 (at 250C), allowed to stand in room light for 20 min at 30C, and then transferred to the EPR flat cell. When NH20H was the inactivating reagent, the suspension (0.6 ml) was mixed with 6Al of a 500 mM stock solution of NH20H in 0.01 M HC1 and then incubated in the dark for 20 min. Heat inactivation at 570Cwas carried out in an EPR flat cell as described (15). Inactivation of 02 evolution by these inhibitory treatments was assessed by the appearance of signal lIf or by direct assay of 02 evolution activity (or both).Abbreviation: Chl, chlorophyll. t To whom reprint requests should be addressed. 7507The publication costs ofthis article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U. S. C. §1734 solely t...

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The enhancement of nuclear magnetic resonance ͑NMR͒ relaxation rates produced by paramagnetic solutes is physically rather different for electron spin Sϭ1/2 paramagnetic species than for Sу1 species due to the presence of zero-field splitting interactions in the electron spin Hamiltonians of the latter. When the zfs energy is larger than the electronic Zeeman energy, the electron spin precessional motion is spatially quantized with respect to the molecule-fixed principal axis system ͑PAS͒ of the zfs tensor rather than along the external laboratory magnetic field. An analytical theory of the orthorhombic zfs limit has been derived in which the motion of the electron spin variables is described in the zfs-PAS and that of the nuclear spin variables in the laboratory coordinate frame. The resulting theoretical expressions are simple in form and suggest a physically transparent interpretation of the experiment. The NMR relaxation enhancement R 1p results from additive contributions, R 1x , R 1y , and R 1z , arising from the molecular-frame Cartesian components of the time-dependent electron spin magnetic moment operator r (t). Each Cartesian component R 1r depends on the dipolar power density at the nuclear Larmor frequency that is produced by the corresponding Cartesian component of r (t). The theory displays the dependence of the relaxation enhancement on the variables of molecular structure in a very simple and physically transparent form: R 1r ϰr Ϫ6 ͓1ϩ P 2 (cos r )͔, where r is the interspin distance and cos r is the direction cosine of the interspin vector with the rth principal axis of the zfs tensor. New experimental data are presented for the model Sϭ1 complex ͓trans-Ni͑II͒͑acac͒ 2 ͑H 2 O͒ 2 ͔ (acacϭacetylacetonato) in dioxane solvent. The magnetic field dependence of the proton T 1 of the axial water ligands has been measured over the range 0.15-1.5 T, the lower end of which corresponds to the zfs limit. The experimental data have been analyzed using the new analytical theory for the zfs-limit regime in conjunction with spin dynamics simulations in the intermediate regime. Dipolar density power plots are presented as graphical devices which clearly exhibit the physical information in the experiment, and which permit a rapid differentiation of the sensitive and insensitive parameters of theory. The data analysis depends strongly on the zfs parameter ͉E͉ and on the electron spin relaxation time S,z along the zfs-PAS z-axis, but only very weakly on the other parameters of theory. A fit of the data to theory provided the values ͉E͉ϭ1.8Ϯ0.1 cm Ϫ1 and S,z ϭ8.0Ϯ0.3 ps.

An engineered microbial biofilm barrier capable of reducing aquifer hydraulic conductivity while simultaneously biodegrading nitrate has been developed and tested at a field-relevant scale. The 22-month demonstration project was conducted at the MSE Technology Applications Inc. test facility in Butte, Montana, which consisted of a 130 ft wide, 180 ft long, 21 ft deep, polyvinylchloride (PVC)-lined test cell, with an initial hydraulic conductivity of 4.2 × 10 −2 cm/s. A flow field was established across the test cell by injecting water upgradient while simultaneously pumping from an effluent well located approximately 82 ft down gradient. A 30 ft wide biofilm barrier was developed along the centerline of the test cell by injecting a starved bacterial inoculum of Pseudomonas fluorescens strain CPC211a, followed by injection of a growth nutrient mixture composed of molasses, nitrate, and other additives. A 99% reduction of average hydraulic conductivity across the barrier was accomplished after three months of weekly or bi-weekly injections of growth nutrient. Reduced hydraulic conductivity was maintained by additional nutrient injections at intervals ranging from three to ten months. After the barrier was in place, a sustained concentration of 100 mg/l nitrate nitrogen, along with a 100 mg/l concentration of conservative (chloride) tracer, was added to the test cell influent over a six-month period. At the test cell effluent the concentration of chloride increased to about 80 mg/l while the effluent nitrate concentration varied between 0.0 and 6.4 mg/l.

Spin dynamics ͑SD͒ methods have been developed to compute NMR paramagnetic relaxation enhancements ͑NMR-PRE͒ produced by solutes with electron spin Sу1 in solution. The spin dynamics algorithms, which are implemented as the computer program SpinDyn.f, are similar in spirit to molecular dynamics calculations in statistical mechanics, except that the spin motion is propagated numerically in time using quantum mechanical equations of motion of the spin operators, rather than Newtonian equations of motion of the molecular degrees of freedom as in MD simulations. SD simulations as implemented in SpinDyn.f provide accurate, flexible, and rapid calculations of NMR-PRE phenomena with few of the assumptions or limitations of previous analytical theories. The program calculates inter-and intramolecular NMR-PRE phenomena for both integer and half-integer spin systems processing under arbitrary Zeeman and zfs Hamiltonians in the presence of Brownian reorientation. Isotropic Brownian reorientation is simulated by means of a finite-step algorithm with adjustable step size. Simulations computed by SpinDyn.f have been used in a systematic study aimed at better understanding the influence of Brownian reorientation on the NMR-PRE of an Sϭ1 ion in a non-Zeeman-limit physical situation. Conditions required for the validity of zfs-limit analytical theory are given. SpinDyn.f has also been used to assess quantitatively the effects of molecular reorientation on a prior analysis of NMR-PRE data for the model Sϭ2 complex ion ͓tris-͑acetylacetonato͒manganese͑III͔͒ in acetone solution; this system was found to be well described by zfs-limit analytical theory.

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Dual-species microbial interactions have been extensively reported for batch and continuous culture environments. However, little research has been performed on dual-species interaction in a biofilm. This research examined the effects of growth rate and substrate concentration on dual-species population densities in batch and biofilm reactors. In addition, the feasibility of using batch reactor kinetics to describe dual-species biofilm interactions was explored. The scope of the research was directed toward creating a dual-species biofilm for the biodegradation of trichloroethylene, but the findings are a significant contribution to the study of dual-species interactions in general. The two bacterial species used were Burkholderia cepacia PR1-pTOM(31c), an aerobic organism capable of constitutively mineralizing trichloroethylene (TCE), and Klebsiella oxytoca, a highly mucoid, facultative anaerobic organism. The substrate concentrations used were different dilutions of a nutrient-rich medium resulting in dissolved organic carbon (DOC) concentrations on the order of 30, 70, and 700 mg/L. Presented herein are single- and dual-species population densities and growth rates for these two organisms grown in batch and continuous-flow biofilm reactors. In batch reactors, planktonic growth rates predicted dual-species planktonic species dominance, with the faster-growing organism (K. oxytoca) outcompeting the slower-growing organism (B. cepacia). In a dual-species biofilm, however, dual-species planktonic growth rates did not predict which organism would have the higher dual-species biofilm population density. The relative fraction of each organism in a dual-species biofilm did correlate with substrate concentration, with B. cepacia having a greater proportional density in the dual-species culture with K. oxytoca at low (30 and 70 mg/L DOC) substrate concentrations and K. oxytoca having a greater dual-species population density at a high (700 mg/L DOC) substrate concentration. Results from this research demonstrate the effectiveness of using substrate concentration to control population density in this dual-species biofilm.