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81.
Zhi-Yong Yang Emilio Jimenez-Vicente Hayden Kallas Dmitriy A. Lukoyanov Hao Yang Julia S. Martin del Campo Dennis R. Dean Brian M. Hoffman Lance C. Seefeldt 《Chemical science》2021,12(20):6913
The electronic structure of the active-site metal cofactor (FeV-cofactor) of resting-state V-dependent nitrogenase has been an open question, with earlier studies indicating that it exhibits a broad S = 3/2 EPR signal (Kramers state) having g values of ∼4.3 and 3.8, along with suggestions that it contains metal-ions with valencies [1V3+, 3Fe3+, 4Fe2+]. In the present work, genetic, biochemical, and spectroscopic approaches were combined to reveal that the EPR signals previously assigned to FeV-cofactor do not correlate with active VFe-protein, and thus cannot arise from the resting-state of catalytically relevant FeV-cofactor. It, instead, appears resting-state FeV-cofactor is either diamagnetic, S = 0, or non-Kramers, integer-spin (S = 1, 2 etc.). When VFe-protein is freeze-trapped during high-flux turnover with its natural electron-donating partner Fe protein, conditions which populate reduced states of the FeV-cofactor, a new rhombic S = 1/2 EPR signal from such a reduced state is observed, with g = [2.18, 2.12, 2.09] and showing well-defined 51V (I = 7/2) hyperfine splitting, aiso = 110 MHz. These findings indicate a different assignment for the electronic structure of the resting state of FeV-cofactor: S = 0 (or integer-spin non-Kramers state) with metal-ion valencies, [1V3+, 4Fe3+, 3Fe2+]. Our findings suggest that the V3+ does not change valency throughout the catalytic cycle.Active site FeV-cofactor of the V-nitrogenase and the EPR spectrum of the reduced cofactor showing 51V-hyperfine coupling. 相似文献
82.
A systematic study on the effect of additives on the isomerizability of 1-alkoxycarbonyl alkenylcopper(I) intermediates revealed that the lithium chelator 12-crown-4 is significantly more efficient than HMPA at retarding isomerization. On the other hand, 12-crown-4 alone is unable to promote effective coupling with alkyl halides. The combination of 12-crown-4 and a reduced amount of HMPA, however, led to an improved procedure for the preparation of useful, isomerically pure tetrasubstituted alkenes such as allylation reagents and skipped dienes. 相似文献
83.
The effective vibronic Hamiltonian for a linear tetra-atomic molecule in a Pi state has been investigated. In addition to the usual vibrational and Renner-Teller coupling terms, the bending mode anharmonicity, spin-orbit coupling, and Fermi resonance interactions have been added to the model. Terms in the Hamiltonian up to the fourth order are given explicitly for molecules of C(infinityupsilon) symmetry and simplifications for symmetric D(infinityh) molecules are discussed. The matrix elements for the HCCS free radical have been obtained and are used to analyze the observed ground-state levels of HCCS and DCCS in a companion paper. The Sears resonance vibronic interaction that couples levels with the selection rules DeltaK=+/-1, DeltaSigma=-/+1, and DeltaP=0 has also been studied and the matrix elements derived. The determinable combinations of signs for the major parameters in the model are discussed. 相似文献
84.
Reaction of optically active ketone complexes (+)-(R)-[(η5-C5H5)Re(NO)-(PPh3)(η1-O=C(R)(CH3)]+ BF4− (R = CH2CH3, CH(CH3)2m C(CH3)3, C6H5) with K(s-C4H9)3BH gives alkoxide complexes (+)-(RS)-(η5-C5H5)Re(NO)(PPh3)-(OCH(R)CH3) (73–90%) in 80–98% de. The alkoxide ligand is then converted to Mosher esters (93–99%) of 79–98% de. 相似文献
85.
The regioselectivity of the metal-catalyzed ring opening of unsymmetrical 1,2-dioxines to cis-gamma-hydroxyenones was investigated using two different Co(II) salen complexes. Regioselectivity was determined by direct examination of the enone ratios and by derivitization with a stabilized phosphorus ylide. The steric influence of the substituents on the 1,2-dioxine was the primary influence on regioselectivity. Temperature played little role; however, solvent and selection of Co(II) complex could be used to mildly influence the outcome of the rearrangement for selected substrates. The origins of the selectivity for the reaction are discussed. 相似文献
86.
The first examples of manipulating stereocontrol solely by reaction topography in radical cyclizations starting from acyclic precursors are reported. The kinetic model for acyclic compound stereoselection is verified experimentally by conducting a series of radical cyclizations of 1,3-dihalo-2-(1-phenyl-3-butynyl)propanes with triphenyltin hydride and measuring the ratios of the products. Monohalide intermediates are observed for the first time, and evidence that bromide- and iodide-substituted radicals have different cyclization rate constants is provided. 相似文献
87.
van de Waterbeemd H Smith DA Jones BC 《Journal of computer-aided molecular design》2001,15(3):273-286
Lipophilicity, often expressed as distribution coefficients (log D) in octanol/water, is an important physicochemical parameter influencing processes such as oral absorption, brain uptake and various pharmacokinetic (PK) properties. Increasing log D values increases oral absorption, plasma protein binding and volume of distribution. However, more lipophilic compounds also become more vulnerable to P450 metabolism, leading to higher clearance. Molecular size and hydrogen bonding capacity are two other properties often considered as important for membrane permeation and pharmacokinetics. Interrelationships among these physicochemical properties are discussed. Increasing size (molecular weight) often gives higher potency, but inevitably also leads to either higher lipophilicity, and hence poorer dissolution/solubility, or to more hydrogen bonding capacity, which limits oral absorption. Differences in optimal properties between gastrointestinal absorption and uptake into the brain are addressed. Special attention is given to the desired lipophilicity of CNS drugs. In examples using -blockers, Ca channel antagonists and peptidic renin inhibitors we will demonstrate how potency and pharmacokinetic properties need to be balanced. 相似文献
88.
Kanai M Porfyrakis K Briggs GA Dennis TJ 《Chemical communications (Cambridge, England)》2004,(2):210-211
We report the purification of the nitrogen-containing incar-fullerenes iNC(60) and iNC(70), and their characterisation by UV-Vis absorption spectroscopy. 相似文献
89.
The 444 nm 2 0 (1) bands of the A 2A'-X 2A" transition of the jet-cooled HP 35Cl and HP 37Cl radicals have been studied at high resolution using the pulsed electric discharge technique with a precursor mixture of PCl3 and H2. Spectra recorded with linewidths of approximately 360 MHz revealed resolved hyperfine structure in both isotopomers arising from the excited state Fermi contact interaction of the unpaired electron with the magnetic moment of the 31P nucleus, with aF'=0.0641(10) cm(-1) and 0.0636(31) cm(-1) for HP 35Cl and HP 37Cl, respectively. No contribution from the ground state, or excited state contributions from the hydrogen or chlorine nuclei were resolved, confirming ab initio predictions that HPCl is a p pi radical in the X state, and an s sigma radical with a substantial contribution from the phosphorus 3s atomic orbital in the A state. The free atom comparison method has been used to estimate that the singly occupied molecular orbital in the excited state has 14% phosphorus 3s character. 相似文献
90.
Addition of stabilized Horner-Wadsworth-Emmons (HWE) phosphonates to substituted 1,2-dioxines leads to diastereomerically pure di- and trisubstituted cyclopropanes in high yields and represents a viable alternative to ylides in the cyclopropanation reaction involving 1,2-dioxines. While yields are comparable, reaction times with these stabilized phosphonates were accelerated and the diastereoselectivity for this cyclopropanation reaction was significantly greater than for the previously reported examples employing ylides. 相似文献