Soft x-ray absorption spectroscopy of metalloproteins and high-valent metal-complexes at room temperature using free-electron lasers

Abstract

X-ray absorption spectroscopy at the L-edge of 3d transition metals provides unique information on the local metal charge and spin states by directly probing 3d-derived molecular orbitals through 2p-3d transitions. However, this soft x-ray technique has been rarely used at synchrotron facilities for mechanistic studies of metalloenzymes due to the difficulties of x-ray-induced sample damage and strong background signals from light elements that can dominate the low metal signal. Here, we combine femtosecond soft x-ray pulses from a free-electron laser with a novel x-ray fluorescence-yield spectrometer to overcome these difficulties. We present L-edge absorption spectra of inorganic high-valent Mn complexes (Mn ∼ 6–15 mmol/l) with no visible effects of radiation damage. We also present the first L-edge absorption spectra of the oxygen evolving complex (Mn4CaO5) in Photosystem II (Mn < 1 mmol/l) at room temperature, measured under similar conditions. Our approach opens new ways to study metalloenzymes under functional conditions.

Local electronic structure of the peptide bond probed by resonant inelastic soft X-ray scattering

“…Soft X-ray spectroscopic techniques, namely X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy (XES), and resonant inelastic X-ray scattering (RIXS) have proven to be very powerful tools to probe the electronic structure of such molecules. XAS, in particular, has been used to study amino acids, [1][2][3][4][5][6][7][8][9][10][11][12][13] small peptides, [1][2][3][4]8,[13][14][15][16][17][18][19] and proteins 8,16,17,[20][21][22][23][24] -as a solid, in the gas phase, as well as in aqueous solution. A spectral database of the XAS spectra of all proteinogenic amino acids is available.…”

X-ray-induced sample damage at the Mn L-edge: a case study for soft X-ray spectroscopy of transition metal complexes in solution

“…20 , 21 We are not aware of any comparable work with soft X-rays on transition-metal complexes in solution and at room temperature, relevant for studying metalloproteins under ambient conditions and catalytic processes in solution without the influence of dose-dependent sample damage. 22 Quantifying these dose limits and unraveling the underlying damage mechanism is expected to provide important information for the design and interpretation of the respective experimental approaches.…”

Thermodynamics of Proton and Electron Transfer in Tetranuclear Clusters with Mn–OH2/OH Motifs Relevant to H2O Activation by the Oxygen Evolving Complex in Photosystem II

“…3 PSII utilizes a nearby tyrosine radical (Y z• ) as a mediator to transfer electrons/protons away from the OEC during turnover; because of the nature of the tyrosine radical, proton-coupled electron transfer (PCET) of the H 2 O-derived ligands bound to the OEC is considered to play an important role in the Kok cycle. 3–4 Due to the wealth of information available in X-ray crystallographic 1b–d, 5 , EPR 6 , and X-ray absorption 2b, 6d, 7 spectroscopic techniques, much is known about the Mn oxidation states and electronic environment of the OEC in the S 0 through S 3 states of the Kok cycle. More challenging has been understanding the precise protonation state of H 2 O ligands and relevant neighboring amino acid residues of any S-state; computational studies of the OEC have considered a variety of possible protonation states.…”