Speciation of the catalytic oxygen evolution system: [MnIII/IV2(mu-O)2(terpy)2(H2O)2](NO3)3 + HSO5-

[MnIII/IV2(-O)2(terpy)2(OH2)2](NO3)3 (1, where terpy = 2,2':6'2' '-terpyridine) + oxone (2KHSO5 x KHSO4 x K2SO4) provides a functional model system for the oxygen-evolving complex of photosystem II that is based on a structurally relevant Mn-(-O)2-Mn moiety (Limburg, J.; et al. J. Am. Chem. Soc. 2001, 123, 423-430). In this study, electron paramagnetic resonance, ultraviolet-visible spectroscopy, electrospray ionization mass spectrometry, X-ray absorption spectroscopy, and gas-phase stable isotope ratio mass spectrometry were utilized to identify the title compounds in the catalytic solution. We find that (a) O2 evolution does not proceed through heterogeneous catalysis by MnO2 or other decomposition products, that (b) O atoms from solvent water are incorporated into the evolved O2 to a significant extent but not into oxone, that (c) the MnIII/IV2 title compound 1 is an active precatalyst in the catalytic cycle of O2 evolution with oxone, while the MnIV/IV2 oxidation state is not, and that (d) the isotope label incorporation in the evolved O2, together with points a-c above, is consistent with a mechanism involving competing reactions of oxone and water with a "MnV=O" intermediate in the O-O bond-forming step.     

Secondary coordination sphere interactions facilitate the insertion step in an iridium(III) CO2 reduction catalyst

There is considerable interest in both catalysts for CO(2) conversion and understanding how CO(2) reacts with transition metal complexes. Here we develop a simple model for predicting the thermodynamic favorability of CO(2) insertion into Ir(III) hydrides. In general this reaction is unfavorable; however, we demonstrate that with a hydrogen bond donor in the secondary coordination sphere it is possible to isolate a formate product from this reaction. Furthermore, our CO(2) inserted product is one of the most active water-soluble catalysts reported to date for CO(2) hydrogenation.     

Nuclear spin dependence of the reaction of H(3)+ with H2. II. Experimental measurements

The nuclear spin dependence of the chemical reaction H(3)(+)+ H(2) → H(2)?+ H(3)(+) has been studied in a hollow cathode plasma cell. Multipass infrared direct absorption spectroscopy has been employed to monitor the populations of several low-energy rotational levels of ortho- and para-H(3)(+) (o-H(3)(+) and p-H(3)(+)) in hydrogenic plasmas of varying para-H(2) (p-H(2)) enrichment. The ratio of the rates of the proton hop (k(H)) and hydrogen exchange (k(E)) reactions α ≡ k(H)/k(E) is inferred from the observed p-H(3)(+) fraction as a function of p-H(2) fraction using steady-state chemical models. Measurements have been performed both in uncooled (T(kin) ～ 350 K) and in liquid-nitrogen-cooled (T(kin) ～ 135 K) plasmas, marking the first time this reaction has been studied at low temperature. The value of α has been found to decrease from 1.6 ± 0.1 at 350 K to 0.5 ± 0.1 at 135 K.     

Nuclear spin dependence of the reaction of H(3)+ with H2. I. Kinetics and modeling

The chemical reaction H(3)(+) + H(2) → H(2) + H(3)(+) is the simplest bimolecular reaction involving a polyatomic, yet is complex enough that exact quantum mechanical calculations to adequately model its dynamics are still unfeasible. In particular, the branching fractions for the "identity," "proton hop," and "hydrogen exchange" reaction pathways are unknown, and to date, experimental measurements of this process have been limited. In this work, the nuclear-spin-dependent steady-state kinetics of the H(3)(+) + H(2) reaction is examined in detail, and employed to generate models of the ortho:para ratio of H(3)(+) formed in plasmas of varying ortho:para H(2) ratios. One model is based entirely on nuclear spin statistics, and is appropriate for temperatures high enough to populate a large number of H(3)(+) rotational states. Efforts are made to include the influence of three-body collisions in this model by deriving nuclear spin product branching fractions for the H(5)(+) + H(2) reaction. Another model, based on rate coefficients calculated using a microcanonical statistical approach, is appropriate for lower-temperature plasmas in which energetic considerations begin to compete with the nuclear spin branching fractions. These models serve as a theoretical framework for interpreting the results of laboratory studies on the reaction of H(3)(+) with H(2).     

C-H oxidation by platinum group metal oxo or peroxo species

While C-H oxidation by ruthenium oxo compounds has been broadly applied in organic synthesis, examples of C-H oxidation by metal oxo complexes from the rest of the platinum group are still rare. We survey the preparation and reactivity of these late-transition metal oxo and peroxo complexes in this tutorial review.     

High turnover remote catalytic oxygenation of alkyl groups: how steric exclusion of unbound substrate contributes to high molecular recognition selectivity

H-bonding mediated molecular recognition between substrate and ligand -COOH groups orients the substrate so that remote, catalyzed oxygenation of an alkyl C-H bond by a Mn-oxo active site can occur with very high (>98%) regio- and stereoselectivity. This paper identifies steric exclusion-exclusion of non H-bonded substrate molecules from the active site-as one requirement for high selectivity, along with the entropic advantage of intramolecularity. If unbound substrate molecules were able to reach the active site, they would react unselectively, degrading the observed selectivity. Both of the faces of the catalyst are blocked by two ligand molecules each with a -COOH group. The acid p-(t)BuC6H4COOH binds to the ligand -COOH recognition site but is not oxidized and merely blocks approach of the substrate therefore acting as an effective inhibitor for ibuprofen oxidation in both free acid and ibuprofen ester form. Dixon plots show that inhibition is competitive for the free acid ibuprofen substrate, no doubt because this substrate can compete with the inhibitor for binding to the recognition site. In contrast, inhibition is uncompetitive for the ibuprofen-ester substrate, consistent with this ester substrate no longer being able to bind to the recognition site. Inhibition can be reversed with MeCOOH, an acid that can competitively bind to the recognition site but, being sterically small, no longer blocks access to the active site.     

Molecular recognition in homogeneous transition metal catalysis: a biomimetic strategy for high selectivity

Traditional methods for selectivity control in homogeneous transition metal catalysis either employ steric effects in a binding pocket or chelate control. In a supramolecular strategy, encapsulation of the substrate can provide useful shape and size selectivity. A fully developed molecular recognition strategy involving hydrogen bonding or solvophobic forces has given almost completely regioselective functionalization of remote, unactivated C-H bonds.     

Rapamycin analogs with differential binding specificity permit orthogonal control of protein activity

Controlling protein dimerization with small molecules has broad application to the study of protein function. Rapamycin has two binding surfaces: one that binds to FKBP12 and the other to the Frb domain of mTor/FRAP, directing their dimerization. Rapamycin is a potent cell growth inhibitor, but chemical modification of the surface contacting Frb alleviates this effect. Productive interactions with Frb-fused proteins can be restored by mutation of Frb to accommodate the rapamycin analog (a rapalog). We have quantitatively assessed the interaction between rapalogs functionalized at C16 and C20 and a panel of Frb mutants. Several drug-Frb mutant combinations have different and nonoverlapping specificities. These Frb-rapalog partners permit the selective control of different Frb fusion proteins without crossreaction. The orthogonal control of multiple target proteins broadens the capabilities of chemical induction of dimerization to regulate biologic processes.     

Determination of mu-oxo exchange rates in di-mu-oxo dimanganese complexes by electrospray ionization mass spectrometry

A time-resolved mass spectrometric technique has been used for the determination of rates of exchange of mu-O atoms with water for the complexes [(mes-terpy)2Mn2(III/IV)(mu-O)2(H2O)2](NO3)3 (1, mes-terpy = 4'-mesityl-2,2':6',2' '-terpyridine), [(bpy)4Mn2(III/IV)(mu-O)2](ClO4)3 (2, bpy = 2,2'-bipyridine), [(phen)4Mn2(III/IV)(mu-O)2](ClO4)3 (3, phen = 1,10-phenanthroline), [(bpea)2Mn2(III/IV)(mu-O)2(mu-OAc)](ClO4)2 (4, bpea = bis(2-pyridyl)ethylamine), [(bpea)2Mn2(IV/IV)(mu-O)2(mu-OAc)](ClO4)3 (4ox), [(terpy)4Mn4(IV/IV/IV/IV)(mu-O)5(H2O)2](ClO4)6 (5, terpy = 2,2':6',2'-terpyridine), and [(tacn)4Mn4(IV/IV/IV/IV)(mu-O)6]Br(3.5)(OH)0.5.6H2O (6, tacn = 1,4,7-triazacyclononane). The rate of exchange of mu-OAc bridges with free acetate in solution has been measured for complexes 4 and 4ox. These are the first measurements of rates of ligand exchange on biologically relevant high-valent Mn complexes. The data analysis method developed here is of general utility in the quantitation of isotope exchange processes by mass spectrometry. We find that the presence of labile coordination sites on Mn increases mu-O exchange rates, and that all-Mn(IV) states are more inert toward exchange than mixed Mn(III)-Mn(IV) states. The rates of mu-O exchange obtained in this work for a di-mu-oxo Mn2(III/IV) dimer with labile coordination sites are compared with the oxygen isotope incorporation rates from substrate water to evolved dioxygen measured in different S states of the oxygen evolving complex (OEC) of photosystem II (PSII). On the basis of this comparison, we propose that both substrate waters are not bound as mu-O bridges between Mn atoms in the S2 and S3 states of the OEC.     

Synthesis of 2-acetamido-1,2-dideoxy-d-galacto-nojirimycin [DGJNAc] from d-glucuronolactone: the first sub-micromolar inhibitor of α-N-acetylgalactosaminidases

2-Acetamido-1,2-dideoxy-d-galacto-nojirimycin [DGJNAc], prepared in 20% overall yield from d-glucuronolactone, is the first potent competitive sub-micromolar inhibitor of α-N-acetyl-galactosaminidases (K_i 0.081 μM from chicken liver, K_i 0.136 μM from Charonia lampas). DGJNAc is a good competitive—whereas the enantiomer l-DGJNAc is a very weak but non-competitive—inhibitor of β-hexosaminidases.