Iron(II) spin-crossover complexes in ultrathin films: electronic structure and spin-state switching by visible and vacuum-UV light.

The electronic structure of the iron(II) spin crossover complex [Fe(H2bpz)2(phen)] deposited as an ultrathin film on Au(111) is determined by means of UV-photoelectron spectroscopy (UPS) in the high-spin and in the low-spin state. This also allows monitoring the thermal as well as photoinduced spin transition in this system. Moreover, the complex is excited to the metastable high-spin state by irradiation with vacuum-UV light. Relaxation rates after photoexcitation are determined as a function of temperature. They exhibit a transition from thermally activated to tunneling behavior and are two orders of magnitude higher than in the bulk material.

Self-assembled monolayers of benzylmercaptan and para-cyanobenzylmercaptan on gold: surface infrared spectroscopic characterization.

Self-assembled monolayers (SAMs) on gold substrates were prepared from benzylmercaptan (BM) and para-cyanobenzylmercaptan (pCBM), and the resulting surfaces were investigated using conventional infrared reflection-absorption spectroscopy (IRRAS) as well as polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS). IRRAS data are analyzed by comparison with transmission IR spectra and theoretical (DFT) simulations. The spectroscopic results indicate the presence of well-ordered monolayers of BM and pCBM with an orientation perpendicular to the surface. IRRAS and PM-IRRAS data are compared to each other and the respective merits of both methods are discussed.

Tyrosinase/catecholoxidase activity of hemocyanins: structural basis and molecular mechanism.

The enzymes tyrosinase, catecholoxidase and hemocyanin all share similar active sites, although their physiological functions differ. Hemocyanins serve as oxygen carrier proteins, and tyrosinases and catecholoxidases (commonly referred to as phenoloxidases in arthropods) catalyze the hydroxylation of monophenols or the oxidation of o-diphenols to o-quinones, or both. Tyrosinases are activated in vivo by limited proteolytic cleavage, which might open up substrate access to the catalytic site. It has recently been demonstrated that if hemocyanins are subjected to similar proteolytic treatments (in vitro) they also exhibit at least catecholoxidase reactivity. On the basis of their molecular structures, hemocyanins are used as model systems to understand the substrate-active-site interaction between catecholoxidases and tyrosinases.

Magnetic bistability of molecules in homogeneous solution at room temperature.

Magnetic bistability, as manifested in the magnetization of ferromagnetic materials or spin crossover in transition metal complexes, has essentially been restricted to either bulk materials or to very low temperatures. We now present a molecular spin switch that is bistable at room temperature in homogeneous solution. Irradiation of a carefully designed nickel complex with blue-green light (500 nanometers) induces coordination of a tethered pyridine ligand and concomitant electronic rearrangement from a diamagnetic to a paramagnetic state in up to 75% of the ensemble. The process is fully reversible on irradiation with violet-blue light (435 nanometers). No fatigue or degradation is observed after several thousand cycles at room temperature under air. Preliminary data show promise for applications in magnetic resonance imaging.

Self-assembled monolayers of benzylmercaptan and p-cyanobenzylmercaptan on Au(111) surfaces: structural and spectroscopic characterization.

The formation of self-assembled monolayers of benzylmercaptan (BM) and p-cyanobenzylmercaptan (pCBM) on Au(111) surfaces is investigated by a combination of X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), and scanning tunneling microscopy (STM). The NEXAFS results of pCBM are supported by ab initio calculations. It is found that BM as well as pCBM form well-ordered monolayers with the molecules oriented almost perpendicular to the surface. BM forms a ( radical 3 x radical 3)R30 degrees structure whereas pCBM forms a slightly different c(7 x 7) hexagonal structure. No phase separation is detected for the adsorption of a 1:1 mixture of the two molecules. The implications of the results for the covalent attachment of transition-metal complexes to thiol-functionalized surfaces are discussed.

Reversible dioxygen binding and phenol oxygenation in a tyrosinase model system.

The complex [Cu2(L-66)]2+ (L-66 = a,a'-bis¿bis[2-(1'-methyl-2'-benzimidazolyl)ethyl]amino¿-m-xylene) undergoes fully reversible oxygenation at low temperature in acetone. The optical [lambda(max) = 362 (epsilon 15000), 455 (epsilon 2000), and 550 nm (epsilon 900M(-1)cm(-1))] and resonance Raman features (760 cm(-1), shifted to 719cm(-1)(-1) with 18O2) of the dioxygen adduct [Cu2(L-66)(O2)]2+ indicate that it is a mu-eta2:eta2-peroxodicopper(II) complex. The kinetics of dioxygen binding, studied at - 78 degrees C, gave the rate constant k1 = 1.1M(-1) 5(-1) for adduct formation, and k(-1) =7.8 x 10(-5)s(-1), for dioxygen release from the Cu2O2 complex. From these values, the O2 binding constant K= 1.4 x 10(4)M(-1) at -78 degrees C could be determined. The [Cu2(L-66)(O2)]2+ complex performs the regiospecific ortho-hydroxylation of 4-carbomethoxyphenolate to the corresponding catecholate and the oxidation of 3,5-di-tert-butylcatechol to the quinone at -60 degrees C. Therefore, [Cu2(L-66)]2+ is the first synthetic complex to form a stable dioxygen adduct and exhibit true tyrosinase-like activity on exogenous phenolic compounds.

Purification and spectroscopic studies on catechol oxidases from Lycopus europaeus and Populus nigra: evidence for a dinuclear copper center of type 3 and spectroscopic similarities to tyrosinase and hemocyanin.

We purified two catechol oxidases from Lycopus europaeus and Populus nigra which only catalyze the oxidation of catechols to quinones without hydroxylating tyrosine. The molecular mass of the Lycopus enzyme was determined to 39,800 Da and the mass of the Populus enzyme was determined to 56,050 Da. Both catechol oxidases are inhibited by thiourea, N-phenylthiourea, dithiocarbamate, and cyanide, but show different pH behavior using catechol as substrate. Atomic absorption spectrosopic analysis found 1.5 copper atoms per protein molecule. Using EPR spectroscopy we determined 1.8 Cu per molecule catechol oxidase. Furthermore, EPR spectroscopy demonstrated that catechol oxidase is a copper enzyme of type 3. The lack of an EPR signal is due to strong antiferromagnetic coupling that requires a bridging ligand between the two copper ions in the met preparation. Addition to H2O2 to both enzymes leads to oxy catechol oxidase. In the UV/Vis spectrum two new absorption bands occur at 345 nm and 580 nm. In accordance with the oxy forms of hemocyanin and tyrosinase the absorption band at 345 nm is due to an O2(2-) (pi sigma *)-->Cu(II) (dx2 - y2) charge transfer (CT) transition. The absorption band at 580 nm corresponds to the second O2(2)- (pi v*)-->Cu(II) (dx2 - y2) CT transition. The UV/Vis bands in combination with the resonance Raman spectra of oxy catechol oxidase indicate a mu-eta 2:eta 2 binding mode for dioxygen. The intense resonance Raman peak at 277 cm-1, belonging to a Cu-N (axial His) stretching mode, suggests that catechol oxidase has six terminal His ligands, as known for molluscan and arthropodan hemocyanin.