NMR and Electronic Relaxation in Paramagnetic Dicopper(II) Compounds

Multinuclear copper complexes of pyridylmethylamide ligands

Copper complexes of a family of pyridylmethylamide ligands HL(Ph), HL(Me3) and HL(Ph3) were synthesized and characterized [HL(Ph) = 2-phenyl-N-(2-pyridylmethyl)acetamide; HL(Me3) = 2,2-dimethyl-N-(2-pyridylmethyl)propionamide; HL(Ph3) = 2,2,2-triphenyl-N-(2-pyridylmethyl)acetamide]. The reaction of copper(II) salts with the HL family and triethylamine in methanol yields copper(II) complexes [Cu4(L(Ph))4(OH)2](ClO4)2 (1), [Cu2(HL(Me3))2(OMe)2(MeOH)2](OTf)2 (2) and [Cu2(HL(Ph3))2(OMe)2(MeOH)2](OTf)2 (3). The complexes have different nuclearity owing to varying steric properties of the ligands used. Complex 1 self-assembles in the presence of excess base to form a tetranuclear complex. Complexes 2 and 3 are binuclear and are bridged by a pair of methoxide ligands. Steric encumbrance of the ligands in 2 and 3 prevent cluster formation.

Proton NMR Spectroscopy and Magnetic Properties of a Solution-Stable Dicopper(II) Complex Bearing a Single μ-Hydroxo Bridge

The reaction of copper(II) perchlorate with the macrocyclic ligand [22]py4pz in the presence of base leads to formation of a dinuclear complex [Cu(2)([22]py4pz)(mu-OH)](ClO(4))(3)xH(2)O, in which two copper ions are bridged by a single mu-hydroxo bridge. Each copper ion is further surrounded by four nitrogen atoms of the ligand. The mu-hydroxo bridge mediates a strong antiferromagnetic coupling (2J = -691(35) cm(-1)) between the metal centers, leading to relatively sharp and well-resolved resonances in the (1)H NMR spectrum of the complex in solution. We herein report the crystal structure, the magnetic properties, and the full assignment of the hyperfine-shifted resonances in the NMR spectrum of the complex, as well as the determination of the exchange coupling constant in solution through temperature-dependent NMR studies.

Self-Assembly and Interconversion of Tetranuclear Copper(II) Complexes

The ligand 1,2-bis(benzimidazol-2-yl)-1,2-ethanediol (H2bzimed, 1) and its N-methylated analogue (H2mbzimed, 2) form a variety of polynuclear complexes with copper(II), all of which contain a planar Cu2O2 lozenge as a central element and in which the bridging oxygen belongs to an alkoxo group of the ligand. Syntheses are reported for dinuclear [Cu2(Hmbzimed)2](ClO4)2 x 1.5H2O, Cu(2)2(2), and the tetranuclear species [Cu4(Hbzimed)4(ClO4)2](NO3)2 x 4H2O, Cu(4)1(4), [Cu4(Hmbzimed)2(mbzimed)Cl2](ClO4)2 x 2H2O x C2H5OH, Cu(4)2(3), and rac-[Cu4(H2bzimed)4(bzimed)(ClO4)2](ClO4)4 x 1.5H2O x 3.5C2H5OH, Cu(4)1(5). Crystal structures are reported for the tetranuclear species. Cu(4)1(4) shows a cubane structure, Cu(4)2(3) a stepped cubane structure, and rac-Cu(4)1(5) a novel structure in which one doubly deprotonated ligand lies between the two Cu2O2 units. Magnetic susceptibility measurements indicate that all complexes show antiferromagnetic coupling in the solid state. Studies in solution (ESI-MS, CD, NMR) show that Cu(2)2(2) and Cu(4)2(3) persist in solution but that Cu(4)1(4) dissociates partially and rac-Cu(4)1(5) completely. The six coordination modes of the ligands are discussed together with the effect of the N-methylation on the ligand conformation.

Solution structure of human beta-parvalbumin and structural comparison with its paralog alpha-parvalbumin and with their rat orthologs

The aim of this research was to determine the structure of human beta-parvalbumin (109 amino acids) and to compare it with its paralog and ortholog proteins. The structure was determined in solution using multinuclear and multidimensional NMR methods and refined using substitution of the EF-hand Ca(2+) ion with a paramagnetic lanthanide. The resulting family of structures had a backbone rmsd of 0.50 A. Comparison with rat oncomodulin (X-ray, 1.3 A resolution) as well as with human (NMR, backbone rmsd of 0.49 A) and rat (X-ray, 2.0 A resolution) parvalbumins reveals small but reliable local differences, often but not always related to amino acid variability. The analysis of these structures has led us to propose an explanation for the different affinity for Ca(2+) between alpha- and beta-parvalbumins and between parvalbumins and calmodulins.

Binuclear Cu2+ complex mediated discrimination between l-glutamate and l-aspartate in water

L-glutamate and L-aspartate selectivity is achieved by the action of two Cu2+ metal ions rightly disposed in a cyclophane-type macrocyclic framework; electrochemical sensing of glutamate has been achieved by adsorption of the copper complexes on graphite electrodes.

Interaction between the Type-3 Copper Protein Tyrosinase and the Substrate Analogue p-Nitrophenol Studied by NMR

The interaction of the monooxygenating type-3 copper enzyme Tyrosinase (Ty) from Streptomyces antibioticus with its inhibitor p-nitrophenol (pnp) was studied by paramagnetic NMR methods. The pnp binds to oxidized Ty (Ty(met)) and its halide (F(-), Cl(-)) bound derivatives with a dissociation constant in the mM range. The Cu(2) bridging halide ion is not displaced upon the binding of pnp showing that the pnp does not occupy the Cu(2) bridging position. The binding of pnp to Ty(met) or Ty(met)Cl leads to localized changes in the type-3 (Cu-His(3))(2) coordination geometry reflecting a change in the coordination of a single His residue that, still, remains coordinated to Cu. The binding of pnp to Ty(met)Cl causes a decrease in the Cu(2) magnetic exchange parameter -2J from 200 cm(-)(1) in the absence to 150 +/- 10 cm(-)(1) in the presence of pnp. From the (1)H and (2)D NMR relaxation parameters of pnp bound to Ty(met), a structural model of pnp coordination to the Ty type-3 center could be derived. The model explains the absence of hydroxylase activity in the closely related type-3 copper protein catechol oxidase. The relevance of the experimental findings toward the Ty catalytic mechanism is discussed.

Intra and Intermolecular Magnetic Interactions in a Series of Dinuclear Cu(II)/hxta Complexes {H5hxta =N,N‘-(2-hydroxy-1,3-xylylene)-bis-(N-carboxymethylglycine)}: Correlation of Magnetic Properties with Geometry

Nine dinuclear copper(II) complexes with hxta5- ligands [H5hxta = N,N'-(2-hydroxy-1,3-xylylene)-bis-(N-carboxymethylglycine)]: [Cu2(MeO-hxtaH)(H2O)2] x 4H2O (1), [Na(micro-H2O)2(H2O)6][Cu2(Cl-hxta)(H2O)3]2 x 6H2O (2), [Cu(H2O)6][Cu2(Me-hxta)(H2O)2](NO3) x 2H2O (3), [Cu2(R-hxtaH)(H2O)3] x 3H2O [R = Cl (4), CH3 (5), and MeO (6)], [Cu2(MeO-hxtaH2)(micro-X)(CH3OH)] x 3CH3OH [X = Cl (7), Br (8)] and K5Na(micro-H2O)10[Cu2(micro-CO3)(Me-hxta)]2 x 4H2O (9), have been synthesized and structurally characterized. In complexes 4-7, the dinuclear units are linked via novel pairwise supramolecular interactions involving the ligand carboxylate groups. The intra- and intermolecular magnetic interactions have been quantified, and the coupling constants have been related to the structural geometries.

Catalytic Oxidation of 3,5-Di-tert-butylcatechol by a Series of Mononuclear Manganese Complexes: Synthesis, Structure, and Kinetic Investigation

The manganese compounds [Mn(bpia)(OAc)(OCH(3))](PF(6)) (1), [Mn(bipa)(OAc)(OCH(3))](PF(6)) (2), [Mn(bpia)(Cl)(2)](ClO(4)) (3), [Mn(bipa)(Cl)(2)](ClO(4)) (4), [Mn(Hmimppa)(Cl)(2)] x CH(3)OH (5), and [Mn(mimppa)(TCC)] x 2CHCl(3) (6) (bpia = bis(picolyl)(N-methylimidazole-2-yl)amine; bipa = bis(N-methylimidazole-2-yl)(picolyl)amine; Hmimppa = ((1-methylimidazole-2-yl)methyl)((2-pyridyl)methyl)(2-hydroxyphenyl)amine; TCC = tetrachlorocatechol) were synthesized and characterized by various techniques such as X-ray crystallography, mass spectrometry, IR, EPR, and UV/vis spectroscopy, cyclic voltammetry, and elemental analysis. 1 and 2 crystallize in the triclinic space group Ponemacr; (No. 2), 4 and 6 crystallize in the monoclinic space group P2(1)/n (No. 14), and 5 crystallizes in the orthorhombic space group Pna2(1). Complexes 1-4 are structurally related to the proposed active site of the manganese-dependent extradiol-cleaving catechol dioxygenase exhibiting an N(4)O(2) donor set (1 and 2) or N(4)Cl(2) donor set (3 and 4). Cyclic voltammetric data show that the substitution of oxygen donor atoms with chloride causes a shift of redox potentials to more positive values. These compounds show high catalytic activity regarding the oxidation of 3,5-di-tert-butylcatechol to 3,5-di-tert-butylquinone exhibiting saturation kinetics at high substrate concentrations. The turnover numbers k(cat) = (86 +/- 7) h(-1) (1), k(cat) = (101 +/- 4) h(-1) (2), k(cat) = (230 +/- 4) h(-1) (3), and k(cat) = (130 +/- 4) h(-1) (4) were determined from the double reciprocal Lineweaver-Burk plot. Compounds 5 and 6 can be regarded as structural and electronic Mn analogues for substituted forms of Fe-containing intradiol-cleaving catechol dioxygenases. To our knowledge 5 is the first mononuclear Mn(II) compound featuring an N(3)OCl(2) donor set.

A strategy for the NMR characterization of type II copper(II) proteins: the case of the copper trafficking protein CopC from Pseudomonas Syringae

CopC from Pseudomonas syringae was found to be a protein capable of binding both Cu(I) and Cu(II) at two different sites. The solution structure of the apo protein is available, and structural information has been obtained on the Cu(I) bound form. We attempt here to set the limits for the determination of the solution structure of a Cu(II) protein, such as the Cu(II) bound form of CopC, in which the Cu(II) ion takes a type II coordination. The electron relaxation time is estimated from NMRD measurements to be 3 ns which leads to a correlation time for the nuclear spin-electron spin dipolar interaction of 2 ns. This information allowed us to tailor the NMR experiments and to fully exploit purely heteronuclear spectroscopy to assign as many signals as possible. In this way, 37 (13)C and 11 (15)N signals that completely escape detection with conventional approaches were assigned. Paramagnetic based structural constraints were obtained by measuring paramagnetic longitudinal relaxation enhancements (rho(para)) which allowed us to precisely locate the copper ion within the protein frame. Pseudocontact shifts (pcs's) were also used as constraints for 83 (1)H and 18 (13)C nuclei. With them, together with other standard structural constraints, a structure is obtained (and submitted to PDB) where information is only missing in a sphere with a 6 A radius from the copper ion. If we borrow information from EXAFS data, which show evidence of two copper coordinated histidines, then His 1 and His 91 are unambiguously identified as copper ligands. EXAFS data indicate two more light donor atoms (O/N) which could be from Asp 27 and Glu 89, whereas the NMRD data indicate the presence of a semicoordinated water molecule at 2.8 A (Cu-O distance) roughly orthogonal to the plane identified by the other four ligands. This represents the most extensively characterized structure of a type II Cu(II) protein obtained employing the most advanced NMR methods and with the aid of EXAFS data. The knowledge of the location of the Cu(II) in the protein is important for the copper transfer mechanism.

Catecholase activity of a series of dicopper(II) complexes with variable Cu-OH(phenol) moieties

The catecholase activity of a series of dicopper(II) complexes containing different numbers of phenol groups coordinated to the metal centers was studied to identify functional as well as structural models for the type III copper enzymes tyrosinase and catechol oxidase. The syntheses and characterization of complexes [Cu(2)(H(2)bbppnol)(mu-OAc)(H(2)O)(2)]Cl(2).2H(2)O (1) and [Cu(2)(Hbtppnol)(mu-OAc)](ClO(4))(2) (2) were previously reported by us (Inorg. Chim. Acta 1998, 281, 111-115; Inorg. Chem. Commun. 1999, 2, 334-337), and complex [Cu(2)(P1-O(-))(OAc(-))](ClO(4))(2) (3) was previously reported by Karlin et al. (J. Am. Chem. Soc. 1997, 119, 2156-2162). The catalytic activity of the complexes 1-3 on the oxidation of 3,5-di-tert-butylcatechol was determined spectrophotometrically by monitoring the increase of the 3,5-di-tert-butyl-o-benzoquinone characteristic absorption band at about 400 nm over time in methanol saturated with O(2)/aqueous buffer pH 8 solutions at 25 degrees C. The complexes were able to oxidize 3,5-di-tert-butylcatechol to the corresponding o-quinone with distinct catalytic activity. A kinetic treatment of the data based on the Michaelis-Mentèn approach was applied. The [Cu(2)(H(2)bbppnol)(mu-OAc)(H(2)O)(2)]Cl(2) small middle dot2H(2)O complex showed the highest catalytic activity of the three complexes as a result of a high turnover rate (k(cat) = 28 h(-1)) combined with a moderate substrate-catalyst binding constant (K(ass) = 1.3 x 10(3) M(-1)). A mechanism for the oxidation reaction is proposed, and reactivity differences, k(cat)/K(M) of the complexes, were found to be dependent on (DeltaE)(1,2), the difference in the driving force for the reduction reactions Cu(II)(2)/Cu(II)Cu(I) and Cu(II)Cu(I)/Cu(I)(2).

Cobalt(II) and copper(II) binding of Bacillus cereus trinuclear phospholipase C: a novel 1H NMR spectrum of a 'Tri-Cu(II)' center in protein

The phosphatidylcholine-preferring phospholipase C from Bacillus cereus (PC-PLC(Bc)) is a tri-Zn enzyme with two 'tight binding' and one 'loose binding' sites. The Zn2+ ions can be replaced with Co2+ and Cu2+ to afford metal-substituted derivatives. Two Cu2+-substituted derivatives are detected by means of 1H NMR spectroscopy, a 'transient' derivative and a 'stable' derivative. The detection of sharp hyperfine-shifted 1H NMR signals in the 'transient' derivative indicates the formation of a magnetically coupled di-Cu2+ center, which concludes that the Zn2+ ions in the dinuclear (Zn1 and Zn3) sites are more easily replaced by Cu2+ than that in the Zn2 site. This might possibly be the case for Co2+ binding. Complete replacement of the three Zn2+ ions can be achieved by extensive dialysis of the enzyme against excess Cu2+ to yield the final 'stable' derivative. This derivative has been determined to have five-coordinated His residues and an overall S'=1/2 spin state with NMR and EPR, consistent with the formation of a tri-Cu2+ center (i.e. a di-Cu2+/mono-Cu2+ center) in this enzyme. The binding of substrate to the inert tri-Cu2+ center to form an enzyme-substrate (ES) complex is clearly seen in the 1H NMR spectrum, which is not obtainable in the case of the native enzyme. The change in the spectral features indicates that the substrate binds directly to the trinuclear metal center. The studies reported here suggest that 1H NMR spectroscopy can be a valuable tool for the characterization of di- and multi-nuclear metalloproteins using the 'NMR friendly' magnetically coupled Cu2+ as a probe.

Paramagnetic NMR investigations of high-spin nickel(II) complexes. Controlled synthesis, structural, electronic, and magnetic properties of dinuclear vs. mononuclear species

New dissymmetric tertiary amines (N(3)SR) with varying N/S donor sets have been synthesized to provide mono- and dinuclear complexes. Acetate ions are used to complete the octahedral coordination sphere around nickel(II) atom(s). The facile conversion of mononuclear to dinuclear systems can be controlled to produce either mono- or dinuclear complexes from the same ligand. The dinuclear complex a(BPh(4))(2) ([Ni(2)(N(3)SSN(3))(OAc)(2)](BPh(4))(2)) has been characterized in the solid state by X-ray diffraction techniques as solvate: a(BPh(4))(2).(1/2)[5(CH(3)OH).(CH(3)CN).(CH(3)CH(2)OH)]. The two Ni atoms are six-coordinated and bridged by a disulfide group and two bidentate acetates. Magnetic susceptibility reveals a weak ferromagnetic exchange interaction between the two Ni atoms with J = 2.5(7) cm(-1). UV-vis studies suggest that the six-coordinated structure persists in solution. The (1)H NMR spectrum of a(BPh(4))(2) exhibits sharp significantly hyperfine shifted ligand signals. A complete assignment of resonances is accomplished by a combination of methods: 2D-COSY experiments, selective chemical substitution, and analysis of proton relaxation data. Proton isotropic hyperfine shifts are shown to originate mainly from contact interactions and to intrinsically contain a small J-magnetic coupling and/or zero-field splitting contribution. A temperature dependence study of longitudinal relaxation times indicates that a very unusual paramagnetic Curie dipolar mechanism is the dominant relaxation pathway in these weakly ferromagnetically spin-coupled dinickel(II) centers. The mononuclear nickel(II) analogue exhibits extremely broader (1)H NMR signals and only partial analysis could be performed. These data are consistent with a shortening of electronic relaxation times in homodinuclear compounds with respect to the corresponding mononuclear species.

Synthesis, Structure, Magnetic Properties, and (1)H NMR Studies of a Moderately Antiferromagnetically Coupled Binuclear Copper(II) Complex

A binuclear Cu(II) complex of [(Cu(2)(HAP)(2)IPA)(OH)(H(2)O)](ClO(4))(2).H(2)O (HAP = 3-amino-1-propanol; IPA = 2-hydroxy-5-methylisophthalaldehyde) has been synthesized and characterized by X-ray crystallography, by solid state magnetic susceptibility, and in solution by (1)H NMR studies. The binuclear copper(II) complex crystallizes in the orthorhombic system, space group Pbcn, a = 27.9972(9) Å, b = 8.8713(3) Å, c = 19.5939(6) Å, and Z = 8. The two copper(II) atoms in this binuclear Cu(II) complex are bridged by the oxygen atoms of the phenolate and hydroxy groups. The axial position at one Cu atom is occupied by a water molecule while another Cu has weak interaction with a perchlorate group. The coordination geometries around the two Cu atoms are distorted square pyramid and square planar. The solid state magnetic susceptibility measurement reveals a moderate antiferromagnetic exchange interaction between the two Cu atoms with a -2J value of 113 +/- 9 cm(-)(1). The variable-temperature (1)H NMR studies in CD(3)CN solution show that the observed relatively sharp hyperfine shifted signals follow a Curie behavior. The exchange coupling constant (-2J) obtained in solution by using chemical shift as a function of temperature also reveals a moderate antiferromagnetic exchange interaction between two Cu(II) ions. An analysis of the relaxation data shows that the reorientational correlation time (tau(c)) is dominated probably by a combination of electronic relaxation time tau(s) and rotational correlation time (tau(r)) due to an exchange-modulated dipolar mechanism for this moderately antiferromagnetically coupled binuclear copper(II) system.

1H NMR studies on the CuA center of nitrous oxide reductase from Pseudomonas stutzeri

1H NMR spectra of the CuA center of N2OR from Pseudomonas stutzeri, and a mutant enzyme that contains only CuA, were recorded in both H2O- and D2O-buffered solution at pH 7.5. Several sharp, well-resolved hyperfine-shifted 1H NMR signals were observed in the 60 to -10 ppm chemical shift range. Comparison of the native and mutant N2OR spectra recorded in H2O-buffered solutions indicated that several additional signals are present in the native protein spectrum. These signals are attributed to a dinuclear copperII center. At least two of the observed hyperfine-shifted signals associated with the dinuclear center, those at 23.0 and 13.2 ppm, are lost upon replacement of H2O buffer with D2O buffer. These data indicate that at least two histidine residues are ligands of a dinuclear CuII center. Comparison of the mutant N2OR 1H NMR spectra recorded in H2O and D2O indicates that three signals, c (27.5 ppm), e (23.6 ppm), and i (12.4 ppm), are solvent exchangeable. The two most strongly downfield-shifted signals (c and e) are assigned to the two N epsilon 2H (N-H) protons of the coordinated histidine residues, while the remaining exchangeable signal is assigned to a backbone N-H proton in close proximity to the CuA cluster. Signal e was found to decrease in intensity as the temperature was increased, indicating that proton e resides on a more solvent-exposed histidine residue. One-dimensional nOe studies at pH 7.5 allowed the histidine ring protons to be definitively assigned, while the remaining signals were assigned by comparison to previously reported spectra from CuA centers. The temperature dependence of the observed hyperfine-shifted 1H NMR signals of mutant N2OR were recorded over the temperature range of 276-315 K. Both Curie and anti-Curie temperature dependencies are observed for sets of hyperfine-shifted protons. Signals a and h (cysteine protons) follow anti-Curie behavior (contact shift increases with increasing temperatures), while signals b-g, i, and j (histidine protons) follow Curie behavior (contact shift decreases with increasing temperatures). Fits of the temperature dependence of the observed hyperfine-shifted signals provided the energy separation (Delta EL) between the ground (2B3u) and excited (2B2u) states. The temperature data obtained for all of the observed hyperfine-shifted histidine ligand protons provided a Delta EL value of 62 +/- 35 cm-1. The temperature dependence of the observed cysteine C beta H and C alpha H protons (a and h) were fit in a separate experiment providing a Delta EL value of 585 +/- 125 cm-1. The differences between the Delta EL values determined by 1H NMR spectroscopy and those determined by EPR or MCD likely arise from coupling between relatively low-frequency vibrational states and the ground and excited electronic states.

Synthesis, Structure, and Solution NMR Studies of Cyanide-Copper(II) and Cyanide-Bridged Iron(III)-Copper(II) Complexes

A range of small molecules, such as cyanide, are known to bind and/or inhibit the active site of the heme-copper oxidase enzymes. As such, model studies are aimed at elucidating ligand binding modes and their subsequent impact on spectroscopic properties of derived complexes. We describe here the isolation and characterization of two compounds containing the Fe-CN-Cu moiety, [(py)(F(8)-TPP)Fe(III)-CN-Cu(II)(TMPA)](2+) (5) and [(F(8)-TPP)Fe(III)-(CN)(2)-{Cu(II)(TMPA)}(2)](3+) (6) [py = pyridine, TMPA = tris(2-pyridylmethyl)amine, and (F(8)-TPP) = tetrakis(2,6-difluorophenyl)porphyrinate(2-)]. [Cu(II)(TMPA)(CH(3)CN)](ClO(4))(2) and [(py)(F(8)-TPP)Fe(III)(CN)] (3) react to yield 5, while 6 is formed by combination of [Cu(II)(TMPA)(CN)]PF(6) (2-(PF(6))) and [(F(8)-TPP)Fe(III)(PF(6))] (4). Complex 2-(PF(6)) crystallizes in the orthorhombic space group Iba2 with a = 17.2269(5) Å, b = 17.3143(4) Å, and c = 14.4971(4) Å, Z = 8, complex (5-(Sb/P)F(6))(1.5)(ClO(4))(0.5) was obtained in the orthorhombic space group P222 with a = 17.9541(2) Å, b = 20.5359(1) Å, and c = 21.2023(2) Å, Z = 4, and 6-(PF(6))(3) crystallized in the monoclinic space group P2(1)/c with a = 15.318(4) Å, b = 33.921(2) Å, and c = 19.649(6) Å, beta = 109.69(2) degrees, and Z = 4. Compound 5 possesses a low-spin iron(III) center, bridged via cyanide to copper. The iron-cyanide vector deviates slightly from linearity (174.6(5) degrees ). The copper(II) ion is five-coordinated by the TMPA N-donor atoms and the cyanide carbon atom. The Cu(TMPA) moiety is bent with an angle of 163.8(5) degrees around the cyanide-copper vector. Compound 6 possesses a low-spin iron(III) atom axially coordinated by two cyanide ligands capped on either side by trigonally coordinated [Cu(TMPA)] moieties. The [Cu(1)(TMPA)] unit is twisted somewhat ( angleCu1-N&tbd1;C = 168 degrees ), whereas the [Cu(2)(TMPA)] unit is coordinated in a nearly linear fashion with respect to the cyanide-iron vector ( angleCu2-N&tbd1;C4 = 175 degrees ). (1)H and (2)H NMR spectroscopy on 5 and 6 confirmed the low-spin nature of these iron complexes (pyrrole resonance found at -11.1 and -8 ppm, respectively). The NMR data as well as observed solution magnetic moment (&mgr;(B) = 2.7 for 5; &mgr;(B) = 3.4 for 6) suggest ferromagnetic coupling between the paramagnetic metal ions. This gives rise to an enhancement of the electronic relaxation rate for Cu(II) in both 5 and 6 allowing for the observation of the sharp and downfield shifted TMPA ligand proton signals.

1H NMR spectroscopy of the binuclear Cu(II) active site of Streptomyces antibioticus tyrosinase

The 600 MHz 1H NMR spectrum of tyrosinase (31 kDa) of Streptomyces antibioticus in the oxidized, chloride-bound form is reported. The downfield part of the spectrum (15-55 ppm) exhibits a large number of paramagnetically shifted signals. The paramagnetism is ascribed to a thermally populated triplet state. The signals derive from six histidines binding to the metals through their Nepsilon atoms. There is no evidence for endogenous bridges. The exchange coupling, -2J, amounts to 298 cm(-1). In the absence of chloride the peaks broaden. This is ascribed to a slowing down of the electronic relaxation. The exchange coupling decreases to -2J=103 cm(-1).

Molybdopterin radical in bacterial aldehyde dehydrogenases

The EPR spectra of three different molybdoprotein aldehyde dehydrogenases, one purified from Comamonas testosteroni and two purified from Amycolatopsis methanolica, showed in their oxidized state a novel type of signal. These three enzymes contain two different [2Fe-2S] centers, one flavin and one molybdopterin cytosine dinucleotide, as cofactors all of which are expected to be EPR silent in the oxidized state. The new EPR signal is isotropic with g = 2.004 both at X-band and Q-band frequencies, consists of six partially resolved lines, and shows Curie temperature behavior suggesting that the signal is due to an organic radical with S = 1/2. The EPR spectra of Comamonas testosteroni aldehyde dehydrogenase obtained after cultivation in media containing 15NH4Cl and/or after substitution of H2O for D2O show the presence of both nitrogen and proton hyperfine interactions. Simulations of the spectra of the four possible isotope combinations yield a single set of hyperfine coupling constants. The electron spin shows hyperfine interaction with a single I = 1 (0.9 mT) ascribed to a N nucleus, with a single I = 1/2 (1.5 mT) ascribed to one nonexchangeable H nucleus, and with two, exchangeable, identical I = 1/2 spins (0.6 mT) ascribed to two identical exchangeable protons. Taken together, the observations and simulations rule out amino acid residues or flavin as the origin of the radical. The values of the various hyperfine coupling constants are consistent with the properties expected for a molybdenum(VI)-trihydropterin radical in which the N5 atom is engaged in two hydrogen-bonding interactions with the protein. The majority of the electron (spin) density of the radical is located at and around the N5 atom and at the proton bound to the C6 atom of the pterin ring. The EPR spectrum of the molybdopterin radical broadens above 65 K and is no longer detectable above 168 K, indicating that it is not magnetically isolated. The line broadening is ascribed to cross-relaxation with a nearby, rapidly relaxing, oxidized [2Fe-2S] center involving its magnetic S = 1 excited state in this process. The amount of radical was apparently not changed by addition of aldehydes or oxidants, but it disappeared upon reduction by sodium dithionite. Therefore, whether the molybdenum(VI) trihydropterin radical as detected here is a functional intermediate in catalysis remains to be investigated further.

Determination of the magnetic axes of cobalt(II) and nickel(II) azurins from 1H NMR data: influence of the metal and axial ligands on the origin of magnetic anisotropy in blue copper proteins

The orientation and the axial, Deltachiax, and rhombic, Deltachirh, components of the magnetic susceptibility tensor anisotropy for the cobalt(II) and nickel(II) derivatives of azurin from Pseudomonas aeruginosa have been determined from 1H NMR data. For both derivatives, the axial geometry of the system determines the orientation of the chi-tensor, whose z-axis forms an angle of 18.6 and 20.1 degrees with the Cu-OGly45 axial bond in the cobalt(II) and nickel(II) derivatives, respectively. For protons close to this axis, large negative pseudocontact shifts are observed, while those close to the NNS plane of the equatorial ligands experience lower and positive pseudocontact shifts for the same distance. Dipolar shifts are larger in the cobalt derivative, not only because of the larger spin number but also due to its intrinsically higher anisotropy. The contact contribution to the hyperfine shifts for the coordinated residues has been evaluated and analyzed in terms of unpaired spin delocalization mechanisms and geometry considerations. The results are extended to other blue copper proteins whose cobalt derivatives have been studied by 1H NMR. The electronic structure and its implications in the redox properties of the native copper proteins are also commented.

A Trapped Intermediate in the Copper(II)-Mediated Template Synthesis of an Amino Acid-Containing Ligand

A transition metal ion-templating reaction that has been widely exploited for the synthesis of nitrogen macrocycles, sepulchrates, and linear tetradentate ligands is the template-directed Mannich condensation of carbon acid and aldehyde equivalents with amines. In the course of investigating the copper(II) template-directed synthesis of linear tetradentate amino acid-containing ligands for the design and synthesis of double-strand DNA cleavage agents, we have trapped and characterized an intermediate in the templating reaction. Condensation of bis(phenylalaninato)copper(II) with formaldehyde and nitroethane led to a nitro-substituent-bearing precursor complex that upon reduction is crystallographically characterized as ((2S)-5-amino-2-benzyl-6-hydroxy-5-methyl-3-azahexanoate)copper(II) (space group P2(1)2(1)2(1), a = 9.0066(5) Å, b = 11.1040(6) Å, c = 16.009(2) Å, alpha = beta = gamma = 90.0 degrees, Z = 4, R(1) = 0.0265, wR(2) = 0.0612). NMR investigation of the precursor complex and the precursor and product ligands shows that the templated ligands each contain only a single phenylalanine unit, not the two phenylalanine units expected on the basis of similar synthetic procedures. The structure of the reduced product confirms this conclusion and provides structural characterization of a template intermediate trapped during ligand assembly.