Multifrequency pulsed EPR studies of biologically relevant manganese(II) complexes

Abstract

Electron paramagnetic resonance studies at multiple frequencies (MF EPR) can provide detailed electronic structure descriptions of unpaired electrons in organic radicals, inorganic complexes, and metalloenzymes. Analysis of these properties aids in the assignment of the chemical environment surrounding the paramagnet and provides mechanistic insight into the chemical reactions in which these systems take part. Herein, we present results from pulsed EPR studies performed at three different frequencies (9, 31, and 130 GHz) on [Mn(II)(H 2 O) 6 ] 2+ , Mn(II) adducts with the nucleotides ATP and GMP, and the Mn(II)-bound form of the hammerhead ribozyme (MnHH). Through line shape analysis and interpretation of the zero-field splitting values derived from successful simulations of the corresponding continuous-wave and field-swept echodetected spectra, these data are used to exemplify the ability of the MF EPR approach in distinguishing the nature of the first ligand sphere. A survey of recent results from pulsed EPR, as well as pulsed electron-nuclear double resonance and electron spin echo envelope modulation spectroscopic studies applied to Mn(II)-dependent systems, is also presented. Mn-Containing Biological SystemsManganese operates as a cofactor in numerous proteins, serving both catalytic and structural roles [1][2][3][4]. Many Mn-dependent enzymes take advantage o f the rich redox chemistry available to the metal, accessing the +2, +3, +4, and perhaps even the +5 oxidation states during their turnover. For example, Mn-superoxide dismutase (MnSOD), which detoxifies the cell of the superoxide radical , cycles between the Mn(II) and Mn(III) oxidation states via the ping-pong type mechanism shown below [5][6][7][8][9].(1a) (1b) Other examples of such mononuclear redox-active enzymes include the manganese peroxidase responsible for lignin degradation by white-rot fungus [10][11][12]; a unique Mndependent form of lipoxygenase [13][14][15][16]; oxalate decarboxylase [17,18]; as well as an extradiol catechol dioxygenase [19][20][21]. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author ManuscriptIn order to help minimize kinetic and thermodynamic penalties associated with electron transfer events and the execution of chemical reactions, two or more metal centers can be coupled together to provide active sites capable of conducting multiple electrons while still operating within a physiologically accessible range of reduction potentials [22]. Only three such examples of redox-active polynuclear Mn enzymes are known: Mn-catalase that disproportionates hydrogen peroxide [23][24][25][26]; a Mn form of ribonucleotide reductase (Mn-RNR) [27,28]; and, arguably the most recognized Mn-dependent enzyme, the photosynthetic core of green plants, algae, and certain cyanobacteria termed photosystem II (PSII) [29,30]. Through a series of photoinitiated electron transfer events, oxidizing equivalents are stored on the tetranuclear Mn core of PSII -the oxygen-evolving complex (OEC) -which then extracts four electr...

Probing in vivo Mn2+speciation and oxidative stress resistance in yeast cells with electron-nuclear double resonance spectroscopy

“…Continuous Wave EPR spectra of all the yeast strains, taken at X (9 GHz), Q (35 GHz), and W (95 GHz) bands and electron-spin echo EPR spectra at 35 GHz all show only intense signals from low molecular-weight Mn 2þ (S ¼ 5∕2) complexes, with no evidence of the broader features associated with Mn 2þ enzymes (28)(29)(30). However, we find that EPR spectroscopy is not sensitive to the speciation of Mn 2þ complexed with biologically available ligands (Fig.…”

Pulse Electron Paramagnetic Resonance Studies of the Interaction of Methanol with the S2 State of the Mn4O5Ca Cluster of Photosystem II

“…60,61 Artificial electron acceptor phenyl- p -benzoquinone (PPBQ) was added from a 250 mM stock in DMSO to a final concentration of 1 mM. 13 C-labeled or natural abundance methanol was added to the final resuspension buffer to either 0.5 or 5% (v/v).…”

Characterization of Monomeric MnII/III/IV–Hydroxo Complexes from X- and Q-Band Dual Mode Electron Paramagnetic Resonance (EPR) Spectroscopy

“…This causes mixing of spin states and multiple overlapping transitions, and consequently, the interpretation of spectra has been qualitative. High-field EPR studies on monomeric Mn II and Mn II have been reported in literature, which can give more accurate determinations of g - and D -values 16–20 . We have developed a methodology for quantitative interpretation based on computer simulation of spectra, and here we demonstrate its use in the interpretation of X- and Q-band spectra for both parallel ( B 1 ‖ B ) and perpendicular ( B 1 ⊥ B ) orientations of the oscillating microwave field ( B 1 ) relative to the static field ( B ) 21 .…”