Robert H. Beer

Fast Fenton footprinting: a laboratory-based method for the time-resolved analysis of DNA, RNA and proteins

‘Footprinting’ describes assays in which ligand binding or structure formation protects polymers such as nucleic acids and proteins from either cleavage or modification; footprinting allows the accessibility of individual residues to be mapped in solution. Equilibrium and time-dependent footprinting links site-specific structural information with thermodynamic and kinetic transitions. The hydroxyl radical (·OH) is a particularly valuable footprinting probe by virtue of it being among the most reactive of chemical oxidants; it reports the solvent accessibility of reactive sites on macromolecules with as fine as a single residue resolution. A novel method of millisecond time-resolved ·OH footprinting has been developed based on the Fenton reaction, Fe(II) + H2O2 → Fe(III) + ·OH + OH−. This method can be implemented in laboratories using widely available three-syringe quench flow mixers and inexpensive reagents to study local changes in the solvent accessibility of DNA, RNA and proteins associated with their biological function.

The interconversion of dichlorobis(N-n-propylsalicylaldimine)zinc(II) and bis(N-n-propylsalicylaldiminato)zinc(II)

The interconversion of the zinc(II) complex of the neutral ligand adduct of N-n-propylsalicylaldimine Zn(L p r H) 2 Cl 2 and its salicylaldiminato counterpart Zn(L p r ) 2 is investigated. The compound Zn(L p r H) 2 Cl 2 is prepared by the reaction of anhydrous ZnCl 2 with 2 equiv. of N-n-propylsalicylaldimine (L p r H) in benzene. A crystallographic study of the distorted tetrahedral Zn(L p r H) 2 Cl 2 adduct reveals that the oxygen atom of the ligand is deprotonated and bound to the zinc atom while the nitrogen is protonated and non-coordinating. An infrared spectrum of Zn(L p r H) 2 Cl 2 exhibits a C=N stretch at a higher energy (1658 cm - 1 ) than the free ligand (1632 cm - 1 ) consistent with the presence of the iminium moiety. In contrast, the deprotonated ligand of the crystallographically characterized salicylaldiminato complex Zn(L p r ) 2 coordinates to zinc in its prototypical bidentate monoanionic coordination mode. Deprotonation of Zn(L p r H) 2 Cl 2 with Et3N or NaOH forms Zn(L p r ) 2 . The reverse reaction, protonation of Zn(L p r ) 2 with anhydrous HCl, produces Zn(L p r H) 2 Cl 2 . These reactions demonstrate the interrelationship between the zinc salicylaldimine adduct and its corresponding salicylaldiminato complex.

A comprehensive series of Group 14 derivatives of the plenary mixed-metal polyoxometalate [PNbW11O40]4−

Abstract The Group 14 derivatized polyoxometalate series [LNbOEMe3]3− (L=α-{PW11O39}7−; E=Si, Ge, Sn, Pb) can be prepared from the n-Bu4N+ salt of the niobium substituted Keggin ion [LNbO]4− and Me3EOTf (OTf=O3SCF3) (53–76% yield). The t-butyl species [LNbOCMe3]3− was obtained by ligand exchange between [LNbOSiMe3]3− and excess t-butanol. Multinuclear NMR spectroscopic data of the series and a crystallographic study of [LNbOGeMe3]3− indicate that the {Me3E}+ group binds selectively to the terminal ONb oxygen atom. The carbon and silicon derivatives are susceptible to hydrolysis, however, the germanium, tin and lead species are hydrolytically stable. Facile cleavage of the NbO–E bond with acetate or halide (F, Cl, Br, I) restores [LNbO]4−.

Structural and 1H NMR spectroscopic characterization of bis(N-isopropylsalicylaldiminato)iron(II)

Abstract Coordinatively unsaturated iron(II) species form reactive organometallic and coordination complexes and are integral to the reactivity of non-heme iron proteins and their synthetic analogues. Iron(II) Schiff base complexes have proven to be easily prepared and useful starting materials for such compounds. Here we report a detailed preparative procedure and the solid-state structure of the iron(II) complex of N-isopropylsalicylaldimine (LiprH) based on the compounds first synthesized by Larkworthy (J. Chem. Soc., A (1968) 1048). The title compound is prepared by adding 2 equiv. of salicylaldehyde (salH) to Fe(O2CCH3)2 in KOH–CH3OH to produce a precursor formulated as Fe(sal)2 which is reacted subsequently with isopropylamine in THF to form Fe(Lipr)2 in 34% isolated yield. A single crystal X-ray crystallographic study of Fe(Lipr)2 reveals a mononuclear complex with two bidentate salicylaldiminate ligands bound to an iron(II) atom in a tetrahedral coordination geometry. The 1H NMR spectrum of Fe(Lipr)2 in benzene-d6 exhibits six paramagnetically shifted ligand resonances ranging from +195 to −31 ppm that are consistent with a mononuclear high spin (S=2) iron(II) complex in solution. Upon exposure to air, Fe(Lipr)2 forms the oxo-bridged dinuclear iron(III) complex [(Lipr)2Fe]2O as shown by 1H NMR spectroscopy.

A tetranuclear manganese oxo complex with terminal and bridging trifluoroacetate ligands: synthesis and structure of [Mn 4 O 2 (O 2 CCF 3 ) 8 (bpy) 2 ]

Abstract Efforts to prepare tetranuclear manganese oxo complexes that model the water oxidizing center in Photosystem II have focused largely on using unsubstituted acetate and benzoate ligands. We report the reaction of Mn(O2CCF3)2, (n-Bu4N)[MnO4], and bipyridine in trifluoroacetic acid yields the compound [Mn4O2(O2CCF3)8(bpy)2]. X-ray crystallographic studies of this complex show a non-planar {Mn4O2}8+ center with an arrangement of bipyridine and bridging trifluoroacetate ligands that is very similar to the acetate complex [Mn4O2(O2CCH3)7(bpy)2]+ [Christou, et al. J. Am Chem. Soc. 11 (1989) 2086]. The structure differs, however, in that the coordination sites occupied previously by a bridging bidentate acetate ligand are occupied by two terminal monodentate trifluoroacetate ligands in a cis arrangement. One of the terminal trifluoroacetate ligands is hydrogen-bonded to a co-crystallized trifluoroacetic acid molecule.

An iron(III)trichloride adduct of N-isopropylsalicylaldimine: preparation, X-ray structure and NMR spectroscopic characterization

Abstract The reaction of anhydrous iron(III)trichloride and N-isopropylsalicylaldimine (LiprH) in benzene yields the adduct Fe(LiprH)Cl3. The X-ray crystal structure of the compound shows a mononuclear tetrahedral iron(III) center with only the oxygen atom of the potentially bidentate Schiff base ligand coordinated to iron(III)trichloride. The imine nitrogen of the coordinated ligand is protonated and non-coordinating with LiprH binding to iron in its phenolate-iminium (zwitterion) form with a { Fe  O- - -H  N  C } intraligand hydrogen bond. The solid-state infrared spectrum of Fe(LiprH)Cl3 exhibits an increase in the energy of the CN stretch from 1631 cm −1 in the free ligand to 1649 cm −1 consistent with the protonation of the imine upon coordination. The solution 1H NMR spectrum of Fe(LiprH)Cl3 and the solution 2H NMR spectrum of its deuterated derivative Fe(LiprD)Cl3 also support this coordination mode displaying isotropically shifted iminium C  1 H and N2H resonances at 21.8 and −43.7 ppm, respectively. A well-resolved solid-state 15N magic angle spinning NMR resonance with a centerband at 970 ppm can be observed at 25 °C with the 15N enriched derivative of Fe(LiprH)Cl3.