Uno Ferro,a de novo Designed Protein,Binds Transition Metals with High Affinity and Stabilizes Semiquinone Radical Anion

Metalloenzymes often utilize radicals in order to facilitate chemical reactions. Recently, DeGrado and co-workers have discovered that model proteins can efficiently stabilize semiquinone radical anion produced by oxidation of 3,5-di-tert-butylcatechol (DTBC) in the presence of two zinc ions. Here, we show that the number and the nature of metal ions have relatively minor effect on semiquinone stabilization in model proteins, with a single metal ion being sufficient for radical stabilization. The radical is stabilized by both metal ion, hydrophobic sequestration, and interactions with the hydrophilic residues in the protein interior resulting in a remarkable, nearly 500 mV change in the redox potential of the SQ ^{. −}/catechol couple compared to bulk aqueous solution. Moreover, we have created 4G-UFsc, a single metal ion-binding protein with pm affinity for zinc that is higher than any other reported model systems and is on par with many natural zinc-containing proteins. We expect that the robust and easy-to-modify DFsc/UFsc family of proteins will become a versatile tool for mechanistic model studies of metalloenzymes.     

彩色显影剂CD-4质子化常数和半醌摩尔吸光系数的测定

对苯二胺类彩色显影剂(PPD_s)经两步电子转移依次生成半醌自由基(SQDI_s)和醌二亚胺(QDI_s)^[1]。     

Homobinuclear bis-3,6-di-tert-butyl-o-benzosemiquinone and bis-3,6-di-tert-butyl-o-catecholate complexes of PdII and PtII with bridging ligands. Molecular structure of (3,6-Cat)2Pd2L2 (L = dppm and dppy)

Reactions of the binuclear complexes M₂L₂Cl₂ [M = Pt or Pd, L = bis(diphenylphosphino)methane (dppm) or 2-(diphenylphosphino)pyridine (dppy)] with thallium 3,6-ditert-butyl-o-benzosemiquinone (3,6-SQ) in solutions have been studied. The formation of the corresponding bis(semiquinone) binuclear derivatives M₂L₂(SQ)₂ with retention of the metal-metal bond has been established by the ESR method. The average distances between the centers of localization of unpaired electrons, which were determined from the ESR spectral data for the complexes studied, allow conclusions about the parallel arrangement of the semiquinone ligands in skewed conformations in a square-pyramidal coordination sphere of both metal atoms. At room temperature, the biradical complexes are slowly converted to diamagnetic catecholate compounds with cleavage of the M-M bond and with retention of the bridging structure of the dppm and dppy ligands. The structures of catecholate complexes of palladium have been established by X-ray structural analysis.Translated from Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 464–468, February, 1996.     

The reaction of 3,6-di-tert-butyl-1,2-benzoquinone and 3,6-di-tert-butylcatechol withtert-butyl hydroperoxide

Reaction of 3,6-di-tert-butyl-1,2-benzoquinone and 3,6-di-tert-butylcatechol withtert-butyl hydroperoxide in aprotic solvents leads to the generation of semiquinone (SQ^.H), alkylperoxy (ROO^.), and alkyloxy radicals. The reaction of SQ^.H and ROO^. produces 2,5-di-tert-butyl-6-hydroxy-1,4-benzoquinone, 3,6-di-tert-butyl-1-oxacyclohepta-3,5-diene-2,7-dione, and 2,5-di-tert-butyl-3,6-dihydroxy-1,4-benzoquinone. The radical generated from solvent attacks SQ^.H at position 4 with C−C bond formation. 4-Benzyl-2,5-di-tert-butyl-6-hydroxycyclohexa-2,5-diene-1-dione produced in this way is transformed into 4-benzyl-3,6-di-tert-butyl-1,2-benzoquinone under the reaction conditions. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 943–946, May, 1999.     

Linear free‐energy relationships in semiquinone species and their Mn(II) and Cu(II) complexes

Linear free‐energy relationships for a series of functionalized semiquinone ligands and their Mn^II‐ and Cu^IIhydro‐tris(3‐cumenyl‐5‐methylpyrazolyl) borate complexes were examined. Quinone–semiquinone cycle half‐wave reduction potentials and semiquinone hydrogen hyperfine coupling constants (a_H) were determined and their correlation with Hammett σ parameters reported. A new σ parameter, σ_aH, has been proposed. Mn^II and Cu^II metal complex metal–ligand charge transfer and n → π* UV transitions were found to be modulated by substituents. Satisfactory Hammett correlations between UV transitions and various σ values have been determined and compared in a number of instances. Copyright © 2011 John Wiley & Sons, Ltd.     

Interaction between hypocrellin and aliphatic amines

The interaction of hypocrellin, including hypocrellin A (HA) and hypocrellin B (HB), with aliphatic amines in deaerated solutions has been studied by ESR and nanosecond transient absorption spectra. In polar solvents, the acid-base interaction between hypocrellin and amines was observed without irradiation. The signals of semiquinone radical anions of hypocrellm and the spin-trapping adduct of α-phenyl-N-tertbutyl-ratrone (PNB) with the aminoalkyl radicals have been detected in photoinduced ESR studies. The transient absorption of excited triplet state of HA and semiquinone radical anion of HA have been observed in laser flash photolysis studies.     

O-H bond dissociation energies in hydroquinones and 4-hydroxyphenoxyl radicals and effect of solvation on the kinetics of reactions involving hydroquinones and semiquinone radicals

The O-H bond dissociation energies (D _OH) in the molecules of 2,5-dimethylhydroquinone (1) and 2,5-di-tert-butylhydroquinone (2) and in the corresponding semiquinone radicals (5 and 8, respectively) were estimated by the method of intersecting parabolas (IP) from experimental data on the rate constants for the reactions of these compounds with N-phenyl-1,4-benzoquinonemonoimine (3) and using the density functional B3LYP/6-31+G* quantum chemical calculations. When calculating the D _OH values by the IP method, solvation of reactants and transition states should be taken into account. The energies of solvation of quinones, semiquinone radicals, and hydroquinones were evaluated by the PCM method. The results of quantum chemical calculations obtained with inclusion of the effects of solvation and the D _OH estimates obtained by the IP method are in good agreement, being equal to 337.9±1.6, 242.5±1.4, and 242.7±3.4 kJ mol⁻¹ for molecule 1 and radicals 5 and 8, respectively. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2244–2251, October, 2005.     

竹红菌素在DMF-H2O混合体系中的电化学研究

本文研究了竹红菌素HA,HB以及13-SO₃Na-DDHA在DMF-H₂O(体积分数φ=1)混合体系中的电化学氧化还原性质。结果表明,它们都为两步单电子还原过程,反应机理为:HA,HB:HA(HB)+e+H⁺⇔HAH^·(HBH^·)+e⇔HAH^·油溶性。水溶性13-SO₃Na-DDHA:(第一步)13-SO₃Na-DDHA+e⇔13-SO₃Na-DDHA^-·(质子化)13-SO₃Na-DDHA^-·+H⁺⇔13-SO₃Na-DDHAH^·;(第二步)13-SO₃Na-DDHAH^·+e⇔13-SO₃Na-DDHAH^-·.并且,HA,HB的单电子还原半醒自由基非常稳定,而13-SO₃Na-DDHA单电子还原半醌自由基存在歧化反应,其速率常数k_f=0.408.     

几种醌类化合物与三乙胺电子转移反应过程的ESR研究

用ESR研究了菲醌(PQ)、四氟对寒醌(TClQ)、2,3-二氯-5,6-二氰苯醌(DDQ)和苯醌(BQ)与三乙胺、(Et₃bN)的电子转移反应过程。实验结果表明,醌上取代基吸电子能力越强,越易与Et₃N反应,但所形成半醌负离子自由基的稳定性,则并未有此规律,而是由自由基终止机理所决定。由实验得到了DDQ与Et₃N的表变曲线。本文讨论了DDQ与Et₃N反应的机理,并得其反应的微分方程,用实验拟合曲线确定速率常数。然后,对该方程求解,与实验曲线比较初步确定了该反应的历程。     

Practical Synthesis of p‐ and o‐Amino‐ and Methoxyphenolic Anthraquinones

New versatile and selective methods for the syntheses of substituted amino‐ and methoxyphenolic anthraquinones ( I–IV ) based on fusion of cyanophthalides ( V ) and semiquinone aminals ( VI , VII ) under basic conditions are described.