Ultrafast IR spectroscopy of [CpFe(CN) 2 (CO)]K as a probe of structure, dynamics & solvation of the [NiFe] hydrogenase active site Barbara Procacci 1 , Amy Farmer 1 , Solomon L.D. Wrathall, 1 Daniel J. Shaw 1 , Greg Greetham 3 , Marius Horch 2 , Yvonne Rippers 2 , Jason M Lynam 1 , Neil T Hunt 1 1 Department of Chemistry and York Biomedical Research Institute, University of York, UK 2 Department of Physics, Freie Universität Berlin, Germany, 3 STFC Central Laser Facility, Rutherford Appleton Laboratory, UK Ultrafast two-dimensional infrared (2D-IR) spectroscopy of the Fe(CN) 2 (CO) component of the active site of the Hyd-1 [NiFe] hydrogenase enzyme from Escherichia coli ( Ec Hyd-1) revealed dynamic and spectroscopic properties that deviated significantly from aqueous solution, suggesting an important role for the protein in controlling the catalytic molecular environment.[1] In this work, we extend our study of the influence of the hydrogenase protein scaffold via a series of 2D-IR and IR pump -IR probe spectroscopy measurements on the model compound [CpFe(CN) 2 (CO)]K ( M1 ) [2] in a number of solvents to provide more detailed insight into the nature of the hydrogenase active site environment. The symmetric and asymmetric ν CN modes of M1 in solution were found to be strongly coupled and, in common with Ec Hyd-1, exhibited diagonal anharmonicities which differed substantially. While this suggests an intrinsic spectroscopic property of the Fe(CN) 2 (CO) unit related to the Darling-Dennison resonance [3], it was observed that the anharmonicity of the high frequency ν CN mode of M1 was uniquely sensitive to the identity of the solvent. By this measure, the value derived for Ec Hyd-1 showed closest similarity with H 2 O. The vibrational lifetimes of the ν CO mode of M1 showed relatively little variation (16 - 24 ps) across the range of solvents investigated, with the exception of H 2 O (5 ± 1 ps). In contrast, the lifetimes of the ν CN modes were found to be very sensitive to the nature of the solvent, ranging from 6 ± 1 ps (H 2 O) to 147 ± 14 ps in DMSO. Mixtures of water/DMSO were also used to study the role of H-bonding in dynamics and solvation of the ligands. Comparisons with data for Ec Hyd-1 (ν CO : 16-25 ps; ν CN : 29-37 ps, both depending on redox state) suggest a strong similarity with organic protic solvents, indicating an important role for H-bonding in the observed dynamics, but confirming that bulk-like H 2 O does not interact with the enzyme active site. In stark contrast to observation on Ec Hyd-1, the lineshapes of the peaks of M1 in 2D-IR spectra were all found to exhibit spectral diffusion. The associated timescales ranged from rapid (H 2 O: 1-2 ps) to 10s of ps in organic solvents. The lack of agreement with the enzyme suggests a very rigid environment that does not mimic the solution phase to any substantial degree. Taking these finding together, we find that the protein scaffold in Ec Hyd-1 creates an environment most similar to a protic organic solvent, where H-bonding to CN ligands is an important factor in controlling the local dynamic
environment. References
1. S. L. D. Wrathall, B. Procacci et al., Phys. Chem. Chem. Phys. 24 , 24767 (2022). 2. D. J. Darensbourg, J. H. Reibenspies et al, J. Am. Chem. Soc. 119 , 7903 (1997). 3. Y. Rippers, B. Procacci, N.T.Hunt, M. Horch, Catalysts 12 , 988 (2022)
P46
© The Author(s), 2023
Made with FlippingBook Learn more on our blog