Use of chemical shifts in macromolecular structure determination.

Electrochemical conversion of CO2 into liquid fuels, powered by renewable electricity, offers one means to address the need for the storage of intermittent renewable energy. Here we present a cooperative catalyst design of molecule–metal catalyst interfaces with the goal of producing a reaction-intermediate-rich local environment, which improves the electrosynthesis of ethanol from CO2 and H2O. We implement the strategy by functionalizing the copper surface with a family of porphyrin-based metallic complexes that catalyse CO2 to CO. Using density functional theory calculations, and in situ Raman and operando X-ray absorption spectroscopies, we find that the high concentration of local CO facilitates carbon–carbon coupling and steers the reaction pathway towards ethanol. We report a CO2-to-ethanol Faradaic efficiency of 41% and a partial current density of 124 mA cm−2 at −0.82 V versus the reversible hydrogen electrode. We integrate the catalyst into a membrane electrode assembly-based system and achieve an overall energy efficiency of 13%. Electrochemical conversion of CO2 into liquid fuels, powered by renewable electricity, offers one means to address the need for the storage of intermittent renewable energy. Now, Sargent and co-workers present a cooperative catalyst design of molecule–metal interfaces to improve the electrosynthesis of ethanol from CO2 by producing a reaction-intermediate-rich local environment.

Cooperative CO2-to-ethanol conversion via enriched intermediates at molecule–metal catalyst interfaces

The paramagnetism of lanthanide ions offers outstanding opportunities for fast determinations of the three-dimensional (3D) structures of protein-ligand complexes by nuclear magnetic resonance (NMR) spectroscopy. It is shown how the combination of pseudocontact shifts (PCSs) induced by a site-specifically bound lanthanide ion and prior knowledge of the 3D structure of the lanthanide-labeled protein can be used to achieve (i) rapid assignments of NMR spectra, (ii) structure determinations of protein-protein complexes, and (iii) identification of the binding mode of low-molecular weight compounds in complexes with proteins. Strategies for site-specific incorporation of lanthanide ions into proteins are summarized.

NMR structure determination of protein - ligand complexes by lanthanide labeling

Vibrational spectra of CO, NO, and O2 adducts of heme proteins contain information on interactions of the heme and its bound ligands with the surrounding protein matrix that may help in elucidating the mechanism of small-molecule activation. Whereas the heme−CO system is well studied and a framework exists for the interpretation of such interactions, heme−NO and −O2 complexes have not been systematically investigated. Here we examine resonance Raman spectra of all three classes of adducts, combining literature values with new data for FeIINO porphyrins having both electron-donating and electron-withdrawing substituents. Negative linear correlations are observed for all three adducts between their Fe−XO and X−O stretching frequencies. The slopes of these correlation lines are −0.4 for five-coordinate FeCO and FeNO porphyrins and −0.8 for five-coordinate FeO2 adducts. Thus, Fe−NO and Fe−O2 bonds are equally or even more sensitive than Fe−CO bonds to electronic influences that affect metal-to-ligand π back-b...

Determinants of the FeXO (X = C, N, O) Vibrational Frequencies in Heme Adducts from Experiment and Density Functional Theory

The full series of lanthanide ions (except the radioactive promethium and the S-state gadolinium) has been incorporated into the C-terminal calcium binding site of the dicalcium protein calbindin D9k. A fairly constant coordination environment is maintained throughout the series. At variance with several lanthanide complexes with small chelating ligands investigated in the past, the large protein moiety provides a large number of NMR signals whose hyperfine shifts can be exclusively ascribed to pseudocontact shifts (PCS). The chemical shifts of 1H and 15N backbone and side chain amide NH groups were accurately measured through HSQC experiments. 1097 PCS were estimated from these by subtracting the diamagnetic contributions measured on HSQC spectra of either the 4f0 lanthanum(III) or the 4f14 lutetium(III) derivatives and used to define a quality factor for the structure. The differences in diamagnetic chemical shifts between the two diamagnetic blanks were relatively small, although some were not negligib...

Magnetic susceptibility tensor anisotropies for a lanthanide ion series in a fixed protein matrix.

We report resonance Raman studies of the Fe-C-O distortion in sterically hindered heme-CO complexes. The steric hindrance is provided by a hydrocarbon chain strapped across one face of the heme. Increasing the steric hindrance (by decreasing the chain length), which reduces the CO binding affinity, is found to increase the Fe-CO stretching frequencies: heme 5 (unstrapped), 495 cm-1; FeSP-15, 509 cm-1; FeSP-14, 512 cm-1; FeSP-13, 514 cm-1. This is interpreted in terms of a decrease in the CO effective mass and increased interactions between the C atom of CO and the N atom(s) of the pyrrole ring(s). Resonance Raman enhancement of the Fe-C-O bending mode upon Soret excitation may be correlated with the overlap between the porphyrin (pi*) and CO (pi*) orbitals when the CO ligand is tilted. Its intensity relative to that of the Fe-CO stretching mode increases with increasing steric hindrance in these "strapped hemes". In addition, we have estimated the Fe-C-O angles from isotope data in various heme-CO complexes. It is inferred that the angles are 167 +/- 5 degrees (FeSP-15) and 175 +/- 5 degrees (FeSP-14, FeSP-13, Mb X CO, and Hb X CO).

Resonance Raman detection of Fe-CO stretching and Fe-C-O bending vibrations in sterically hindered carbonmonoxy "strapped hemes". A structural probe of Fe-C-O distortion.

The stretching frequency of the iron-carbon bond, v(Fe-CO), is a direct measure of the iron-carbon bond strength when there is no change in the Fe-C-O geometry. Here we report resonance Raman detection of v(Fe-CO) frequencies in the CO complexes of iron (II) alpha, alpha, alpha, alpha-mesotetrakis(o-pivalamidophenyl)porphyrin, FeII(TpivPP), with trans ligands of varying strength: N-methylimidazole (N-MeIm), 1,2-dimethylimidazole (1,2-Me2Im), pyridine (py), and tetrahydrofuran (THF). It was found that the weaker the iron-trans ligand bond, the stronger the iron-carbon bond. Comparisons of sterically hindered (1,2-Me2Im) and unhindered (N-MeIm) bases are of particular interest because of their implication in the phenomenon of hemoglobin cooperativity and the mechanisms of protein control of heme reactivity. While the CO binding affinity of FeII(TpivPP)(1,2-MeIm) is approximately 400 times lower than that of FeII(TpivPP)(N-MeIm), the v(Fe-CO) frequency for the former (at 496 cm-1) is higher than that for the latter (at 489 cm-1). This example shows that the CO binding affinity cannot be directly correlated with the strength of the iron-carbon bond. Comparison of the CO binding to FeII(TpivPP)(THF) and FeII(TpivPP)(N-MeIm) reveals a similar relationship; the v(Fe-CO) frequency (at 527 cm-1) in FeII(TpivPP)(THF)(CO) is 38 cm-1 higher than that in FeII(TpivPP)(N-MeIm)(CO), but the CO binding affinity is lower for the THF complex.

Resonance Raman studies of carbon monoxide binding to iron "picket fence" porphyrin with unhindered and hindered axial bases. An inverse relationship between binding affinity and the strength of iron-carbon bond.

Ellen A. Kerr

Papers

Resonance Raman detection of Fe-CO stretching and Fe-C-O bending vibrations in sterically hindered carbonmonoxy "strapped hemes". A structural probe of Fe-C-O distortion.

Resonance Raman studies of carbon monoxide binding to iron "picket fence" porphyrin with unhindered and hindered axial bases. An inverse relationship between binding affinity and the strength of iron-carbon bond.