3 research outputs found
Characterization of Monomeric Mn<sup>II/III/IV</sup>âHydroxo Complexes from X- and QâBand Dual Mode Electron Paramagnetic Resonance (EPR) Spectroscopy
Manganeseâhydroxo species
have been implicated in CâH
bond activation performed by metalloenzymes, but the electronic properties
of many of these intermediates are not well characterized. The present
work presents a detailed characterization of three Mn<sup><i>n</i></sup>âOH complexes (where <i>n</i> =
II, III, and IV) of the trisÂ[(<i>N</i>â˛-<i>tert</i>-butylureaylato)-<i>N</i>-ethylene]Âaminato ([H<sub>3</sub>buea]<sup>3â</sup>) ligand using X- and Q-band dual mode electron
paramagnetic resonance (EPR). Quantitative simulations for the [Mn<sup>II</sup>H<sub>3</sub>bueaÂ(OH)]<sup>2â</sup> complex demonstrated
the ability to characterize similar Mn<sup>II</sup> species commonly
present in the resting states of manganese-containing enzymes. The
spin states of the Mn<sup>III</sup> and Mn<sup>IV</sup> complexes
determined from EPR spectroscopy are <i>S</i> = 2 and 3/2,
respectively, as expected for the <i>C</i><sub>3</sub> symmetry
imposed by the [H<sub>3</sub>buea]<sup>3â</sup> ligand. Simulations
of the spectra indicated the constant presence of two Mn<sup>IV</sup> species in solutions of [Mn<sup>IV</sup>H<sub>3</sub>bueaÂ(OH)] complex.
The simulations of perpendicular- and parallel-mode EPR spectra allow
determination of zero-field splitting and hyperfine parameters for
all complexes. For the Mn<sup>III</sup> and Mn<sup>IV</sup> complexes,
density functional theory calculations are used to determine the isotropic
Mn hyperfine values, to compare the excited electronic state energies,
and to give theoretical estimates of the zero-field energy
Synthesis and Reactivity of Tripodal Complexes Containing Pendant Bases
The
synthesis of a new tripodal ligand family that contains tertiary amine
groups in the second-coordination sphere is reported. The ligands
are trisÂ(amido)Âamine derivatives, with the pendant amines attached
via a peptide coupling strategy. They were designed to function as
new molecular catalysts for the oxygen reduction reaction (ORR), in
which the pendant acid/base group could improve the catalyst performance.
Two members of the ligand family were each metalated with cobaltÂ(II)
and zincÂ(II) to afford trigonal-monopyramidal complexes. The reaction
of the cobalt complexes <b>[CoÂ(L)]</b><sup><b>â</b></sup> with dioxygen reversibly generates a small amount of a cobaltÂ(III)
superoxo species, which was characterized by electron paramagnetic
resonance (EPR) spectroscopy. Protonation of the zinc complex ZnÂ[NÂ{CH<sub>2</sub>CH<sub>2</sub>NCÂ(O)ÂCH<sub>2</sub>NÂ(CH<sub>2</sub>Ph)<sub>2</sub>}<sub>3</sub>)]<sup>â</sup> (<b>[ZnÂ(TN</b><sup><b>Bn</b></sup><b>)]</b><sup><b>â</b></sup>) with
1 equiv of acid occurs at a primary-coordination-sphere amide moiety
rather than at a pendant basic site. The addition of excess acid to
any of the complexes <b>[MÂ(L)]</b><sup><b>â</b></sup> results in complete proteolysis and formation of the ligands <b>H</b><sub><b>3</b></sub><b>L</b>. These undesired
reactions limit the use of these complexes as catalysts for the ORR.
An alternative ligand with two pyridyl arms was also prepared but
could not be metalated. These studies highlight the importance of
the stability of the primary-coordination sphere of ORR electrocatalysts
to both oxidative <i>and</i> acidic conditions
LâEdge Xâray Absorption Spectroscopy of Dilute Systems Relevant to Metalloproteins Using an Xâray Free-Electron Laser
L-edge spectroscopy of 3d transition
metals provides important
electronic structure information and has been used in many fields.
However, the use of this method for studying dilute aqueous systems,
such as metalloenzymes, has not been prevalent because of severe radiation
damage and the lack of suitable detection systems. Here we present
spectra from a dilute Mn aqueous solution using a high-transmission
zone-plate spectrometer at the Linac Coherent Light Source (LCLS).
The spectrometer has been optimized for discriminating the Mn L-edge
signal from the overwhelming O K-edge background that arises from
water and protein itself, and the ultrashort LCLS X-ray pulses can
outrun X-ray induced damage. We show that the deviations of the partial-fluorescence
yield-detected spectra from the true absorption can be well modeled
using the state-dependence of the fluorescence yield, and discuss
implications for the application of our concept to biological samples