Pulsed EPR and NMR spectroscopy of paramagnetic iron porphyrinates and related iron macrocycles: how to understand patterns of spin delocalization and recognize macrocycle radicals

Pulsed EPR spectroscopic techniques, including ESEEM (electron spin echo envelope modulation) and pulsed ENDOR (electron-nuclear double resonance), are extremely useful for determining the magnitudes of the hyperfine couplings of macrocycle and axial ligand nuclei to the unpaired electron(s) on the metal as a function of magnetic field orientation relative to the complex. These data can frequently be used to determine the orientation of the g-tensor and the distribution of spin density over the macrocycle, and to determine the metal orbital(s) containing unpaired electrons and the macrocycle orbital(s) involved in spin delocalization. However, these studies cannot be carried out on metal complexes that do not have resolved EPR signals, as in the case of paramagnetic even-electron metal complexes. In addition, the signs of the hyperfine couplings, which are not determined directly in either ESEEM or pulsed ENDOR experiments, are often needed in order to translate hyperfine couplings into spin densities. In these cases, NMR isotropic (hyperfine) shifts are extremely useful in determining the amount and sign of the spin density at each nucleus probed. For metal complexes of aromatic macrocycles such as porphyrins, chlorins, or corroles, simple rules allow prediction of whether spin delocalization occurs through sigma or pi bonds, and whether spin density on the ligands is of the same or opposite sign as that on the metal. In cases where the amount of spin density on the macrocycle and axial ligands is found to be too large for simple metal-ligand spin delocalization, a macrocycle radical may be suspected. Large spin density on the macrocycle that is of the same sign as that on the metal provides clear evidence of either no coupling or weak ferromagnetic coupling of a macrocycle radical to the unpaired electron(s) on the metal, while large spin density on the macrocycle that is of opposite sign to that on the metal provides clear evidence of antiferromagnetic coupling. The latter is found in a few iron porphyrinates and in most iron corrolates that have been reported thus far. It is now clear that iron corrolates are remarkably noninnocent complexes, with both negative and positive spin density on the macrocycle: for all chloroiron corrolates reported thus far, the balance of positive and negative spin density yields -0.65 to -0.79 spin on the macrocycle. On the other hand, for phenyliron corrolates, the balance of spin density on the macrocycle is zero, to within the accuracy of the calculations (Zakharieva, O.; Schünemann, V.; Gerdan, M.; Licoccia, S.; Cai, S.; Walker, F. A.; Trautwein, A. X. J. Am. Chem. Soc. 2002, 124, 6636-6648), although both negative and positive spin densities are found on the individual atoms. DFT calculations are invaluable in providing calculated spin densities at positions that can be probed by (1)H NMR spectroscopy, and the good agreement between calculated spin densities and measured hyperfine shifts at these positions leads to increased confidence in the calculated spin densities at positions that cannot be directly probed by (1)H NMR spectroscopy. (13)C NMR spectroscopic investigations of these complexes should be carried out to probe experimentally the nonprotonated carbon spin densities.     

Models of the low-spin iron(III) hydroperoxide intermediate of heme oxygenase: magnetic resonance evidence for thermodynamic stabilization of the d(xy) electronic state at ambient temperatures

The (13)C pulsed ENDOR and NMR study of [meso-(13)C-TPPFe(OCH(3))(OO(t)Bu)](-) performed in this work shows that although the unpaired electron in low-spin ferrihemes containing a ROO(-) ligand resides in a d(pi) orbital at 8 K, the d(xy) electron configuration is favored at physiological temperatures. The variable temperature NMR spectra indicate a dynamic situation in which a heme with a d(pi) electron configuration and planar porphyrinate ring is in equilibrium with a d(xy) electron configuration that has a ruffled porphyrin ring. Because of the similarity in the EPR spectra of the hydroperoxide complexes of heme oxygenase, cytochrome P450, and the model heme complex reported herein, it is possible that these two electron configurations and ring conformations may also exist in equilibrium in the enzymatic systems. The ruffled porphyrinate ring would aid the attack of the terminal oxygen of the hydroperoxide intermediate of heme oxygenase (HO) on the meso-carbon, and the large spin density at the meso-carbons of a d(xy) electron configuration heme suggests the possibility of a radical mechanism for HO. The dynamic equilibrium between the ruffled (d(xy)) and planar (d(pi)) conformers observed in the model complexes also suggests that a flexible heme binding cavity may be an important structural motif for heme oxygenase activity.     

Modified molecular charge similarity indices for choosing molecular analogues

Toward the goal of defining a molecular charge similarity idex that best quantifies the concept of molecular similarity as it relates to biological activity, we have evaluated a variety of definitions of the molecular charge distribution function, ρ, for use in the charge similarity index formalism. Spatially distributed nuclear charges are incorporated into electron distribution functions to approximately account for the screening of core electronic charge and to model the net effect of the total charge distribution in a manner that better reflects the inherent relation to the molecular electrostatic potential. The resulting charge similarity indices are evaluated based on their sensitivity to relative molecule displacement and their ability to meaningfully group or order a simple set of molecular structures: CH₃CH₂CH₃, CH₃OCH₃, and CH₃SCH₃.     

Matrix-dependent instability of selenium(IV) stored in teflon containers

Solutions of selenium(IV) standards with different acid matrices were stored in containers constructed of boosilicate glass, conventional polyethylene, and fluorinated ethylenepropylene (teflon FEP). After 50 days of storage in FEP, there were highly significant losses of Se(IV) from standards in either 5% HCl/5% H₂SO₄, or 5% H₂SO₄. Increasing the hydrochloric acid concentration, e.g., 15% HCl/5% H₂SO₄, greatly reduced this loss. Addition of selenium-75 (selenate-free) indicated that the losses did not result from physical adsorption onto container surfaces. It is shown that the losses were caused by oxidation of Se(IV) to Se(VI).     

Structural characterization of a lanthanum bistriflimide complex, La(N(SO2CF3)2)3(H2O)3, and an investigation of La, Sm, and Eu electrochemistry in a room-temperature ionic liquid, [Me3N(n)Bu][N(SO2CF3)2

The reduction of selected lanthanide cations to the zerovalent state in the room-temperature ionic liquid [Me3N(n)Bu][TFSI] is reported (where TFSI = bistriflimide, [N(SO2CF3)2]-). The lanthanide cations were introduced to the melt as the TFSI hydrate complexes [Ln(TFSI)3(H2O)3] (where Ln = La(III), Sm(III) or Eu(III)). The lanthanum compound [La(TFSI)3(H2O)3] has been crystallographically characterized, revealing the first structurally characterized f-element TFSI complex. The lanthanide in all three complexes was shown to be reducible to the metallic state in [Me3N(n)Bu][TFSI]. For both the Eu and Sm complexes, reduction to the metallic state was achieved via divalent species, and there was an additional observation of the electrodeposition of Eu metal.     

Use of Dimethylsilyl Ethers for Characterizing Primary Aliphatic Alcohols: A comparison of mass spectrometric fragmentation of di- and trimethylsilyl derivatives

Mass spectral fragmentation patterns of dimethylsilyl (DMS) ethers of primary unbranched, branched, and secondary unbranched aliphatic alcohols in the C₅ to C₁₀ range are compared with those of the corresponding trimethylsilyl (TMS) derivatives. Unlike their TMS analogues, DMS ethers of primary alcohols exhibit pronounced rupture of the C? C bond adjacent to the oxygen atom within the alkyl moiety (loss of an alkyl radical R) in marked preference to cleavage within the silyl substituent (loss of CH₃). Within this class of compounds, complementary preparation of DMS derivatives can therefore be used to establish or to confirm the site, and thus the primary nature of the hydroxyl group, whereas preparation of TMS ethers may be of advantage in deducing molecular size. For the derivatives of secondary alcohols this diagnostically useful difference in fragmentation behaviour is not observed.     

INTERACTIONS AMONG PHOTOSYNTHETIC ANTENNA EXCITED STATES

Abstract. The data of Kung and DeVault (1978) showing high-order fluorescence from chromatophores of photosynthetic bacteria are analyzed in relation to other data on first-order fluorescence of photosynthetic systems, particularly that of Monger and Parson (1977). The wavelengths of emission observed (down to 445 nm) require energy equivalent to two lowest singlet-excited states. The dependence on excitation intensity is best explained by any of the following third-order processes: (a) 3 S ₁→3 S ₀; (b) 2 S ₁+ T , → 2 S ₀+ T ₁; (c) S ₁+ 2 T ₁→ 3 S ₀. However, (c) is ruled out because it predicts heavy T ₁-destruction which is not observed. Contribution from the second order process: 2 S ₁→ S ₀ is probable, but even the data of Monger and Parson show that it is insufficient by itself. Two-photon absorption: S ₀+ hv ₁→ S ₁; S ₁+ hv ₁→ S _n; S _n S ₀+ hv ₂ could also account for the high-order fluorescence and its dependence on excitation intensity. [ S ₀, S ₁ S _n are ground, first excited and a higher excited singlet states, respectively, of antenna bacteriochlorophyll, T _t is the lowest triplet state, c/v , is the exciting wavelength (694 or 868 nm) and c/v ₂ the wavelength of the high-order fluorescence (445, 535. or 600 nm), where c = velocity of light.] Maximum values are estimated for some of the rate constants.     

Strontium-89- und Strontium-90-Bestimmungen bei der Entwicklung Graphitmoderierter Hochtemperaturreaktoren

A summary is given about hitherto studied and applied versions of sample pretreatment and separation techniques of⁸⁹Sr and⁹⁰Sr from fission and activation products in plate-out experiments for security studies of graphite moderated gas cooled high temperature pebble bed reactors. The latest and most succesfully applied version with a combined double-column system is described and its performance and results are discussed in detail.     

Subpicosecond pulse generation in synchronously pumped and hybrid ring dye lasers

Subpicosecond pulse generation has been examined in synchronously pumped mode-locked ring dye laser systems. These include hybrid and composite absorber/gain media arrangements as well as a simple synchronous cavity. The shortest pulses recorded were 0.3 ps for the hybrid system, and this has been shown to be critically dependent on the positioning of the absorber jet in the centre of the cavity to better than 50 m. Stable operation for subpicosecond pulse generation has been achieved in the ring configuration with greater wavelength tunability and higher average power conversion efficiency than with conventional cavity arrangements.     

Detection of norbolethone,an anabolic steroid never marketed,in athletes' urine

Norbolethone (13-ethyl-17-hydroxy-18,19-dinor-17alpha-pregn-4-en-3-one) is a 19-nor anabolic steroid first synthesized in 1966. During the 1960s it was administered to humans in efficacy studies concerned with short stature and underweight conditions. It has never been reported by doping control laboratories. Norbolethone was identified in two urine samples from one athlete by matching the mass spectra and chromatographic retention times with those of a reference standard. The samples also contained at least one likely metabolite. The samples were also unusual because the concentrations of endogenous steroids were exceptionally low. Since norbolethone is not known to be marketed by any pharmaceutical company, a clandestine source of norbolethone may exist.