X-band EPR Spectra Research Articles

Stabilization of the T-state of human hemoglobin by proflavine, an antiseptic drug.

The effect of proflavine (3,6-diaminoacridine), an antiseptic drug, on the spectroscopic and oxygen binding properties of ferrous human adult hemoglobin (Hb) has been investigated. Upon binding of proflavine to the nitric oxide derivative of ferrous human adult hemoglobin (HbNO), the X-band EPR spectrum displays the characteristics which have been attributed to the T-state of the ligated tetramer. In parallel, oxygen affinity for the deoxygenated derivative of ferrous human adult Hb decreases in the presence of proflavine. The effect of proflavine on the spectroscopic and ligand binding properties of ferrous human adult Hb is reminiscent that of 2,3-D-glycerate bisphosphate, the physiological modulator of Hb action.

Synthesis and characterization of ReV, ReVI and ReVII complexes of the [α2-P2W17O61]10– isomer

Rhenium-(V), -(VI) and -(VII) complexes of the [α2-P2W17O61]10– isomer, a mono-lacunary derivative of the [α-P2W18O62]6– (Wells–Dawson) ion have been prepared and characterized by multinuclear NMR spectroscopy, electrospray mass spectrometry and electron paramagnetic resonance spectroscopy, among other techniques. The molecules have the formulation [α2-ReOP2W17O61]n–, where n = 7,6,5 for ReV, ReVI and ReVII, respectively. 183W NMR spectroscopy for the ReV and ReVII analogs shows that the molecules have Cs symmetry, as expected for substitution in the α2 site. Simulations of the X-band and Q-band EPR spectra of the ReVI analog, using Cs symmetry, allow determination of the g, hyperfine and quadrupole coupling tensors. X-Band, Q-band and W-band EPR spectroscopy show extreme variations in linewidths due to random strains or distortions of the complex.

X-Band EPR Studies of Ferroelectric Lead Titanate (PT), Piezoelectric Lead Magnesium Niobate (PMN), and PMN/PT Powders at 10 and 85 K

X-band EPR spectra of lead titanate (PT) and lead magnesium niobate (PMN) powders prepared by different synthetic methods and a PMN/PT powder of the composition 0.9 PMN/0.1 PT were obtained at 85 and 10 K. Several EPR signals due to adventitious Fe3+ ion impurities, a signal due to the Ti3+ ion, and a signal due to the Pb3+ ion are observed for PT, PMN, and PMN/PT powders. The EPR signals observed at g = 2.0 and 6.0 are assigned to Fe3+ ions in the B-sites of the perovskite lattice structure of lead titanate with axial symmetry. The EPR signals observed at g = 1.99 and 4.25 are assigned to Fe3+ ions in the B-sites of the perovskite lattice structure of PMN and 0.9 PMN/0.1 PT materials with cubic and rhombic symmetries, respectively. The sharp EPR signal observed at g = 1.94 is assigned to Ti3+ ion for PT and 0.9 PMN/0.1 PT powders. In addition, a broader EPR signal at g = 2.28−2.30 for PMN obtained by the molten salt method is assigned to axial Pb3+ ion sites in this PMN material. EPR results obtained here for the Fe3+ ions in the B-sites of the PMN materials, in particular, suggest that both cubic and rhombic symmetry sites corresponding to a range of Nb(OMg)x(ONb)6-x site configurations exist in the PMN. These EPR results indicate that PMN likely exists with partial B-site cation (Mg/Nb) ordering in the perovskite lattice structure.

X-band, Q-band EPR and EHT-MO calculations on [Re II(NO)Cl 2(OPPh 3) 3] [ReO 4

X-band and Q-band EPR spectra of the cationic 5d 5 ‘low-spin’ ( S= 1 2 ) nitrosylrhenium(II) complex [Re II(NO)Cl 2(OPPh 3) 3] [ReO 4] show a well resolved 185' 187Re hyperfine pattern characterized by slightly rhombic g and A Re tensors. The hyperfine parameters are discussed in terms of spin-density distribution and compared with the result obtained from extended Hückel theory molecular orbital (EHT-MO) calculations.

The Radical Pair State in Photosystem I Single Crystals: Orientation Dependence of the Transient Spin-Polarized EPR Spectra

The light-induced, charge-separated state in single crystals of Photosystem I (PSI) from the cyanobacterium Synechococcus elongatus is investigated with transient, direct-detection EPR spectroscopy. The orientation of the phylloquinone head group of A1 within the PSI reaction center is determined from the orientation dependence of the spin-polarized X-band EPR spectrum of the radical pair made up of the primary donor, P700, and the acceptor, A1. From the angular dependence of the overall spin-polarization pattern an upper limit ∠(c, zd) ≤ 30° is evaluated for the angle between the crystallographic c-axis, collinear with the membrane normal, and the dipolar axis, zd, connecting the electron spin density centers of and . A partially resolved hyperfine coupling (hfc) is assigned to the hfc tensor of the 2-methyl group of A1. Its A∥ principal axis encloses an angle of β = 35°−55° with c. Simulations of the rotation patterns support a lower limit for the angle ∠(c, zd) ≥ 25° with a larger error than for the upper limit. zd is confirmed to be parallel to both the gxx principal axis of g( ) and to the CO carbonyl bonds within ±5°. With respect to rotation around the gxx axis, the angle between the (c, zd) plane and the quinone plane of the head group can only be specified within an upper limit of 60°. Together with independent knowledge about the location of A1 within the PSI reaction center, a nearly complete structural model for the head group of the functional A1 cofactor is achieved.

Anion-dependence of the polymeric structures of manganese(II) complexes of N,N′-butylenebis(imidazole)

The preparations and X-band EPR spectra are reported of a series of manganese(II) complexes with N,N′-butylenebis(imidazole) (=BBI) of the types Mn(BBI) 2X 2 (X = Cl, Br, NO 3, NCS) and [Mn(BBI) 3]X 2 (X = ClO 4, I). An X-ray crystallographic study on Mn(BBI) 2Cl 2 (1) shows that each Mn atom is coordinated to four imidazole nitrogen atoms and two chloride ions in a trans-octahedral arrangement and that the BBI ligands link the Mn centres into sheets of contiguous 44-membered rings. X-Ray studies on [Mn(BBI) 3](ClO 4) 2, and on analogous compounds containing combinations of ClO 4 −\\PF 6 − or ClO 4 −\\AsF 6 − as counter anions, reveal the formation of a family of interpenetrating network structures with anion-dependent cavity sizes.

Stabilization of the T-state of ferrous human adult and fetal hemoglobin by Ln(III) complexes: A thermodynamic study

The effect of the lanthanide(III) complexes [Gd(1,4,7,10-tetraazacyclododecane- N, N′, N″, N′′′-tetrakis(methylenephosphonate))] 5− (Gd-DOTP) and La-DOTP on the oxygen binding and spectroscopic properties of human adult and fetal hemoglobin (HbA and HbF, respectively) has been investigated. The affinity of Gd-DOTP and La-DOTP for oxygenated HbA (HbAO 2; K HbAO 2 =2.6 × 10 −3 M) is closely similar to that observed for Ln(III) complexes association to nitrosylated HbA (HbANO K HbANO=1.8 × 10 −3 M) and to aquo-met HbA (met-HbA; K met-HbA=1.9 × 10 −3 M), being lower than that determined for Gd-DOTP and La-DOTP binding to the deoxygenated form of the tetramer (HbAd; K HbAd=3.0 × 10 −4 M). The affinity of Gd-DOTP for deoxygenated HbF (HbFd; K HbFd=9.5 × 10 −4 M) and oxygenated HbF (HbFO 2; K HbFO 2 =3.7 × 10 −3 M) is lower than that observed for Ln(III) complexes association to HbAd and HbAO 2, respectively. Gd-DOTP and La-DOTP bind to HbA and HbF with a 1:1 stoichiometry per tetramer. Increasing Gd-DOTP and La-DOTP concentration, oxygen affinity for HbA decreases (i.e. P 50 increases), this effect being minor for HbF. Upon binding of Ln(III) complexes to HbANO, the X-band EPR spectrum and the absorption spectrum in the Soret region display the characteristics which have been attributed to the T-state of the ligated tetramer. These results represent a clear cut evidence for the specific binding of Gd-DOTP and La-DOTP to the 2,3- D-glycerate bisphosphate (BPG) pocket (i.e. at the dyad axis, in between the β-chains) of HbA and HbF. The effect of Ln(III) complexes on the ligand binding and spectroscopic properties of HbA and HbF is reminiscent that of BPG, the physiological modulator of human Hb action.

Monomeric Cu (II) complexes with tetradentate bis (benzimidazole) ligand ; synthesis, EPR, spectral and electrochemical studies

A series of 1 : 1 copper (II) complexes with the tetradentate ligand 1,2 bis (2 benzimidazolyloxamethyl) benzene (L) have been prepared with stoichiometry of Cu (L) X 2 · nH 2O (X = Cl −, NO − 3, OAc − and HCOO −) . X-band EPR spectra of the above complexes examined as a frozen DMSO\\DMF solution indicate g ‖ > g↑ > 2.0 and show four or less than four g ‖ lines and a broadening of the g↑ component suggesting a lowered site symmetry for the copper (II) ion. The g↑ component of all the complexes show nitrogen superhyperfine structure. A clear five line pattern is found in the g↑ region with B N sime; 15G±1 for Cu (L) X 2 · nH 2O complexes indicating that two nitrogen from the benzimidazole ring are coordinated to the central Cu II in the equatorial basal plane while the other two positions in the sphere are occupied by the anionic ligand (X) . Interestingly with ClO − 4 as the anion, a 1 : 2 complex results with stoichiometry of [Cu (L) 2] (ClO 4) 2. The X-band EPR spectra of [Cu (L) 2] (ClO 4) 2 complex shows a clear nine line SHF structure on the g↑ component indicating the presence of four nitrogen atoms coordinated to the central Cu II atom ; this confirms the retention of two ligands around Cu II in solution. Molecular orbital coefficients β 1 and β 1′ have been evaluated from the expression used by Manoharan and Rogers. The trends of β 1 and β 1′ indicate increasing electron delocalization on nitrogen atom as we change the exogenous ligand from formate\\acetate-chloride to nitrate. The covalency parameter, α 2, has also been evaluated and reflects the decreasing trend of covalency in the order, nitrate > chloride ≥ acetate > formate for complexes containing these ligands. This trend of order is in conformity with that obtained from MO coefficients. All the complexes exhibit a visible band with λ max in the 625–690 nm region, in DMF ; suggesting a distorted geometry for copper (II) ion in the present series of complexes. Cyclic voltammetric studies of copper (II) complexes reveal that the E 1\\2 for the Cu II\\Cu I couple shifts to most negative value with HCOO − and OAc − anionic ligands implying that these anion stabilize the Cu II state whereas E 1\\2 data for NO − 3 reveals that this anion destabilizes the Cu II state. A plot of visible band energy vs E 1\\2 shows that as the energy of the d x2− y2 orbital decreases, an anodic shift in Cu II → Cu I reduction potential is observed.

Modeling the Oxygen-Evolving Complex of Photosystem II. Synthesis, Redox Properties, and Core Interconversion Studies of Dimanganese Complexes Having {MnIII2(μ-O)(μ-OAc)2}2+, {MnIIIMnIV(μ-O)2(μ-OAc)}2+, and {MnIV2(μ-O)2(μ-OAc)}3+Cores with MeL as a Terminal Ligand: A New Asymmetric Mixed-Valence Core,1

The synthesis of [MnIIIMnIV(μ-O)2(μ-OAc)(MeL)2][ClO4]2·H2O (1a), [MnIIIMnIV(μ-O)2(μ-OAc)(MeL)2][BF4]2·2MeCN (1b), [MnIII2(μ-O)(μ-OAc)2(MeL)2][PF6]2·H2O (2a), [MnIII2(μ-O)(μ-OAc)2(MeL)2][ClO4]2·H2O (2b), and [MnIV2(μ-O)2(μ-OAc)(MeL)2][ClO4]3·H2O (3), containing a common tridentate facially capping ligand with chelate ring asymmetry, (2-pyridylmethyl)(2-pyridylethyl)methylamine (MeL), is described. X-ray crystallographic analysis of complex 1b revealed that it has an asymmetric MnIIIMnIV(μ-O)2(μ-OAc)}2+ core, as a result of different binding modes adopted by MeL at the Mn(III) versus the Mn(IV) site. The present group of complexes have both kinds of dimanganese core structure as that in the proposed tetranuclear structural model for the OEC in PSII. Temperature-dependent magnetic properties reveal the following results: 2J = −288, −244, and +2 cm-1 for 1a, 3, and 2a, respectively. Complex 1a has a doublet ground state and its X-band EPR spectrum at 77 K exhibits a 16-line pattern at g = 2. Owing to their s...

EPR Study of the Dynamic Jahn-Teller Effect of Cu2+ in Cd(HCOO)22H2O Single Crystals

The X-band EPR spectra of Cu+ doped into cadmium formate dihydrate single crystal was studied at 113 °K. The powder spectrum of the same material was studied over the temperature range from 113 to 300 °K The observed values of the g and A hyperfine coupling constants and linewidths were found to be temperature dependent. The temperature dependence of Cu2+ EPR spectrum values were discussed in terms of dynamic Jahn-Teller effect. The tensor elements of g and A were calculated from the polycrystalline samples at various temperature and it is found that the spectrum at 113 °K has given fallowings, g xx=2.108±0.002, g yy=2.085±0.002, g zz=2.431±0.002 and A xx=48±1, A yy=17±1, A zz=11±1 Gauss.

Synthesis, Molecular Structure, and Reactivity of Dinuclear Copper(II) Complexes with Carboxylate-Rich Coordination Environments.

The dinucleating ligand N,N'-(2-hydroxy-5-methyl-1,3-xylylene)bis(N-(carboxymethyl)glycine) (CH(3)HXTA) has been used to synthesize the dinuclear Cu(II) bis(pyridine) complex Na[Cu(2)(CH(3)HXTA)(Py)(2)].1.5(1,4-dioxane) (Na(1)): triclinic space group P&onemacr; (a = 12.550(3) Å, b = 13.413(3) Å, c = 13.540(4) Å, alpha = 117.12(2) degrees, beta = 104.70 (2) degrees, and gamma = 92.13(2) degrees ). The structure shows two distinct distorted square pyramidal Cu(II) centers with each Cu(II) ion bound by two carboxylate oxygen atoms, one amine nitrogen atom, a phenolate oxygen atom, and one pyridine nitrogen atom. The Cu--Cu separation is 3.531 Å, and the Cu1-O1-Cu2 angle is 123.7 degrees. The phenyl ring of the CH(3)HXTA ligand is twisted relative to the Cu1-O1-Cu2 plane, and the resulting dihedral angle is 44.2 degrees. The electronic absorption spectrum of 1 in aqueous solution at pH 3 suggests a shift toward trigonal bipyramidal Cu(II) coordination in solution. Spectral titration of Na[Cu(2)(CH(3)HXTA)(H(2)O)(2)] with L (where L = pyridine or sodium cyanide) results in complexes with terminal L groups. These exogenous ligands appear to bind in a positive cooperative stepwise fashion. Variable-temperature magnetic susceptibility data for 1 indicate that the Cu(II) ions are antiferromagnetically coupled (-2J = 168 cm(-)(1)). X-band EPR spectra of an aqueous solution of 1 shows isotropic signals with g = 2.14, while a powdered sample of 1 provides no EPR spectrum. A DeltaM(s) = 2 transition at g = approximately 4.5, expected for weakly magnetically coupled Cu(II) ions, is not observed for powdered samples but is observed for a methanolic solution sample of 1. On the basis of these data, the two Cu(II) ions are antiferromagnetically coupled in the solid state but due to a coordination geometry change become weakly ferromagnetically or antiferromagnetically coupled in solution. (1)H NMR studies on a methanol solution of 1 are consistent with weak spin-coupling in solution.

EPR study of the impurity paramagnetic centres in (CaO−Ga2O3−GeO2) glasses

The X-band EPR spectra of Cr3+, Mn2+, and Fe3+ impurity ions in glasses of (CaO−Ga2O3−GeO2) system are investigated in the 77÷300 K temperature range. The experimental data analysis yields the following results: (i) Impurity chromium ions are incorporated into the (CaO−Ga2O3−GeO2) glasses network in Cr3+ (3d3,4F3/2) paramagnetic valence state only and occupy the strong distorted oxygen coordinated octahedral sites. (ii) For all activated and non-activated (CaO−Ga2O3−GeO2) glasses the iron impurity is present at concentration roughly 0.01 wt.%. Isotropic EPR signals atgeff=4.29 andgeff=2.00 are assigned to Fe3+ (3d5,6S5/2) ions in the sites with strong rhombic distortion and in the sites with nearly cubic symmetry respectively. (iii) The manganese EPR spectrum in (CaO−Ga2O3−GeO2) glasses is weakly dependent on temperature, doping procedure as well as manganese concentration. EPR spectra of impurity manganese ions in glasses with Ca3Ga2Ge3O12 and Ca3Ga2Ge4O14 compositions are virtually identical and belong to Mn2+ (3d5,6S5/2) ions. Impurity manganese ions are incorporated into the (CaO−Ga2O3−GeO2) glass network as isolated Mn2+ centres and clusters of Mn2+ ions.

Effects of polar carotenoids on the shape of the hydrophobic barrier of phospholipid bilayers

The value of A z (z-component of the hyperfine interaction tensor) obtained directly from X-band EPR spectra of stearic acid spin labels and tempocholine dipalmitoylphosphatidic acid ester in frozen suspension of phosphatidylcholine (PC) membranes has been used as a hydrophobicity parameter. Using probes with the nitroxide moiety at various depths in the membrane, the shape of the hydrophobic barrier, which is determined by the extent of water penetration into the membrane, has been estimated. Incorporation of 10 mol% polar carotenoids, zeaxanthin, violaxanthin, or lutein into the saturated PC bilayer significantly increases the hydrophobicity of the membrane interior but decreases hydrophobicity (increases water penetration) in the polar headgroup region. Hydrophobicity at the membrane center increases from the level of propanol–pentanol, which have dielectric constants of 10–20, to the level of dipropylamine, with a dielectric constant close to 3. Longer alkyl chains decrease the effect of polar carotenoids in the polar headgroup region, but not in the central hydrophobic region. In an unsaturated egg yolk PC membrane, polar carotenoids were found to increase the hydrophobicity of the membrane interior to a higher level than in saturated PC membranes. At the membrane center hydrophobicity reaches the level close to pure hexane ( ϵ≃2). The above results were confirmed by studying accessibility of Fe(CN) 3− 6 ion dissolved in water into dimyristoyl-PC-lutein membranes at 30°C. Obtained hydrophobicity profiles correlate well with permeability data for water in the literature.

Study of strongly overlapping Rh2+ EPR spectra by high-resolution magnetic resonance techniques

Recent EPR and ENDOR publications demonstrated that several closely related Rh2+ complexes may be simultaneously present in rhodium doped NaCl and AgCl. These complexes only differ with respect to vacancy compensation. They exhibit strongly overlapping X-band EPR spectra with g⊥ ≈ 2.5 and g‖ ≈ 2. In the present study, solution- and melt-grown NaCl single crystals doped with Rh3+ were subjected to several treatments, such as X-irradiation, heating and thermal pulse annealing. While X-band EPR alone has insufficient resolution to resolve the spectra of the various rhodium complexes, a combined X-band ENDOR and W-band EPR study allows a consistent spectrum decomposition and interpretation. The X- and W-band Rh2+ spectra of both solution- and melt-grown NaCl single crystals are discussed in terms of the same [RhCl6]2−·nvac (n = 0, 1, 2) complexes.

Iron(III) and iron(II) complexes of 1-thia-4,7-diazacyclononane ([9]aneN2S) and 1,4-dithia-7-azacyclononane ([9]aneNS2). X-Ray structural analyses, magnetic susceptibility, Mössbauer, EPR and electronic spectroscopy †

The complexes [Fe([9]aneN2S)2][ClO4]2, [Fe([9]aneN2S)2][ClO4]3 and [Fe([9]aneNS2)2][ClO4]2 ([9]aneN2S = 1-thia-4,7-diazacyclononane and [9]aneNS2 = 1,4-dithia-7-azacyclononane) have been prepared and the latter two characterised by X-ray crystallography. The Mossbauer spectra (isomer shift/mm s–1, quadrupole splitting/mm s–1, 4.2 K) for [Fe([9]aneN2S)2][ClO4]2 (0.52, 0.57), [Fe([9]aneN2S)2][ClO4]3 (0.25, 2.72) and [Fe([9]aneNS2)2][ClO4]2 (0.43, 0.28) are typical for iron(II) and iron(III) complexes. Variable-temperature susceptibility measurements for [Fe([9]aneN2S)2][ClO4]2 (2–300 K) revealed temperature-dependent behaviour in both the solid state [2.95 µB (300 K)–0.5 µB (4.2 K)] and solution (ΔH 0 20–22 kJ mol–1, ΔS 0 53–60 J mol–1 K–1). For [Fe([9]aneN2S)2][ClO4]3 in the solid state [2.3 µB (300 K)–1.9 µB (4.2 K)] the magnetic data were fit to a simple model (H = –λL·S + µLz) to give the spin–orbit coupling constant (λ) of –260 ± 10 cm–1. The solid-state X-band EPR spectrum of [Fe([9]aneN2S)2][ClO4]3 revealed axial symmetry (g⊥ = 2.607, g|| = 1.599). Resolution of g⊥ into two components at Q-band frequencies indicated a rhombic distortion. The low-temperature single-crystal absorption spectra of [Fe([9]aneN2S)2][ClO4]2 and [Fe([9]aneNS2)2][ClO4]2 exhibited additional bands which resembled pseudo-tetragonal low-symmetry splitting of the parent octahedral 1A1g → 1T2g and 1A1g → 1T1g transitions. However, the magnitude of these splittings was too large, requiring 10Dq for the thioether donors to be significantly larger than for the amine donors. Instead, these bands were tentatively assigned to weak, low-energy S → FeII charge-transfer transitions. Above 200 K, thermal occupation of the high-spin 5T2g ground state resulted in observation of the 5T2g → 5Eg transition in the crystal spectrum of [Fe([9]aneN2S)2][ClO4]2. From a temperature-dependence study, the separation of the low-spin 1A1g and high-spin 5T2g ground states was approximately 1700 cm–1. The spectrum of the iron(III) complex [Fe([9]aneN2S)2][ClO4]3 is consistent with a low-spin d5 configuration.

EPR investigations on europium(II)-doped aluminates

The europium(II)-doped aluminates, SrAl2O4 and CaAl2O4with β-tridymite structure and BaMgAl10O17 with β-Al2O4 structure, have been investigated by means of EPR spectroscopy. From the complicated X-band EPR spectra observed for SrAl2O4:Eu,Dy and CaAl2O4:Eu,Nd it is deduced that they have large zero-field splittings in contrast to BaMgAl10O17:Eu. High frequency EPR spectroscopy at 180 GHz shows symmetrical weak satellites on both sides of the strongest resonance centred around at 6.45 T. Spectral simulation with second order perturbation has made it clear that for SrAl2O4:Eu,Dy the europium(II) ions occupy three different sites, while for the CaAl2O4 they occupy a single site with trigonal anti-prism symmetry, despite the fact that there are two other sites associated with octahedral symmetry.

Synthetic Models of the Inactive Copper(II)−Tyrosinate and Active Copper(II)−Tyrosyl Radical Forms of Galactose and Glyoxal Oxidases

A series of CuII and ZnII complexes with new ligands having either one or two substituted phenolates appended to the 1,4,7-triazacyclononane frame were prepared and characterized by optical absorption, EPR, NMR, and/or resonance Raman spectroscopy, cyclic voltammetry, and, in eight cases, X-ray crystallography. Features of the active site geometries of the CuII−tyrosinate forms of galactose and glyoxal oxidases (GAO and GLO) were modeled by these complexes, including the binding of a redox-active phenolate and an exogenous ligand (Cl-, CH3CO2-, or CH3CN) in a cis-equatorial position of a square pyramidal metal ion. The role of the unique ortho S−C covalent bond between a cysteine (C228) and the equatorial tyrosinate (Y272) in the proteins was probed through an examination of the optical absorption and electrochemical properties of sets of similar complexes comprised of phenolate ligands with differing ortho substituents, including thioether groups. The o-alkylthio unit influences the PhO- → CuII LMCT transition and the MII−phenolate/MII−phenoxyl radical redox potential, but to a relatively small degree. Electrochemical and chemical one-electron oxidations of the CuII and ZnII complexes of ligands having tert-butyl protecting groups on the phenolates yielded new species that were identified as novel MII−phenoxyl radical compounds analogous to the active CuII−tyrosyl radical forms of GAO and GLO. The MII−phenoxyl radical species were characterized by optical absorption, EPR, and resonance Raman spectroscopy, as well as by their stoichiometry of formation and chemical reduction. Notable features of the CuII−phenoxyl radical compounds that are similar to their protein counterparts include EPR silence indicative of magnetic coupling between the CuII ion and the bound radical, a band with λmax ≈ 410 nm (ε ≈ 3900 M-1 cm-1) in UV−vis spectra diagnostic for the phenoxyl radical, and a feature attributable to the phenoxyl radical C−O vibration (ν7a) in resonance Raman spectra. Similar Raman spectra and electrochemical behavior for the ZnII analogs, as well as an isotropic signal at g = 2.00 in their X-band EPR spectra, further corroborate the formulations of the MII−phenoxyl radical species.

Synthesis and X-Band Epr Spectra of Binary and Ternary Complexes of Copper(II) with Salicylideneanthranilic Acid and Imidazoles

The synthesis and EPR spectra of some binary and ternary complexes of copper(II) with a terdentate ligand, salicylideneanthranilic acid (H2SAA) and some monodentate imidazoles viz. imidazole (im) and 2-ethylimidazole (e-im), are described. The binary complex with copper(II) and salicylideneanthranilic acid shows a subnormal magnetic moment and EPR spectra for spin-spin interaction. The zero-field splitting parameter D has been evaluated as 190 G and the copper-copper distance as 5.227 Å. The ternary complexes show magnetic moments and EPR spectra which suggest a distorted tetrahedral geometry around copper(II). Referee I: N. K. Kaushik Referee II: M. Athar

Influence of iron-removal procedures on sequential electron transfer in photosynthetic bacterial reaction centers studied by transient EPR spectroscopy.

Electron spin polarized electron paramagentic resonance (ESP EPR) spectra were obtained with deuterated iron-removed photosynthetic bacterial reaction centers (RCs) to specifically investigate the effect of the rate of primary charge separation, metal-site occupancy, and H-subunit content on the observed P865+QA- charge-separated state. Fe-removed and Zn-substituted RCs from Rb. sphaeroides R-26 were prepared by refined procedures, and specific electron transfer rates (kQ) from the intermediate acceptor H- to the primary acceptor QA of (200 ps)-1 vs (3-6 ns)-1 were observed. Correlation of the transient EPR and optical results shows that the observed slow kQ rate in Fe-removed RCs is H-subunit-independent, and, in some cases, independent of Fe-site occupancy as Zn2+ substitution does not ensure retention of the native kQ. In addition, shifts in the optical spectrum of P865 and differences in the high-field region of the Q-band ESP spectrum for Fe-removed RCs with slow kQ indicate possible structural changes near P865. The experimental X-band and Q-band spin-polarized EPR spectra for deuterated Fe-removed RCs where kQ is at least 15-fold slower at room temperature than the (200 ps)-1 rate observed for native Fe-containing RCs have different relative amplitudes and small g-value shifts compared to the spectra of Zn-RCs which have a kQ unchanged from native RCs. These differences reflect the trends in polarization predicted from the sequential electron transfer polarization (SETP) model [Morris et al. (1995) J. Phys. Chem. 99, 3854-3866; Tang et al. (1996) Chem. Phys. Lett. 253, 293-298]. Thus, SETP modeling of these highly resolved ESP spectra obtained with well-characterized proteins will provide definitive information about any light-induced structural changes of P865, H, and QA that occur upon formation of the P865+QA- charge-separated state.

Where Are the Sugar Radicals in Irradiated DNA?

Numerous sugar radicals have been observed by EPR and ENDOR spectroscopy in X-irradiated nucleosides and nucleotides. However, no sugar radicals have been observed in irradiated DNA. One possibility exists that sugar radicals may be present in relatively small abundance and therefore have so far escaped detection. Another possibility is that each of the five carbon-centered H-abstraction sugar radicals may exist in a wide range of conformations. Adding together various groups of radicals with different hyperfine couplings and anisotropic g factors may result in very broad EPR lines which would be difficult to detect. Using the crystal structure of a B-DNA dodecamer, the conformations of the five H-abstraction sugar radicals have been computed at all sites. The X-band EPR spectra for each radical were then simulated using typical alpha-proton and beta-proton couplings computed from the various radical conformations. The results suggest considerable broadenings of the EPR lines for the C2', C3' H-abstraction radicals. Composite EPR spectra were simulated by adding 15% of an H-abstraction radical to the DNA spectrum. The results indicate that the outer lines of the C1' radical are visible and should be easy to identify. The broad spectra of the C2' and C3' radicals are barely visible. The simulated spectra of the C4' and C5' radicals are basically doublets and are obscured by the DNA signal.