Protonation and Muoniation Regiochemistry of [FeFe]-Hydrogenase Subsite Analogues

Jamie N.T. Peck, Joseph A. Wright, Stephen Cottrell, Christopher J. Pickett and Upali A.

Jayasooriya.

The mechanisms by which the hydrogenase enzymes catalyse the reversible reduction of

protons to dihydrogen are of intrinsic interest in the context of understanding the chemistry of

fascinating metallosulfur enzymes, and more widely in the context of a developing hydrogen

technology for energy transduction. Protonation of almost any diiron dithiolate complex

yields a bridging hydride as the thermodynamic product. Transient terminal hydrides have

also been observed, and result from the protonation of the vacant site offered by the rotated

bridging carbonyl. Protonation of the Fe2(pdt)(CO)4(PMe3)2 active site was monitored using

stopped flow IR, offering insights into the reaction from ~0.1 seconds onwards, indicating

that protonation proceeds to the Fe ̶ Fe bond.1 The reaction mechanism was exhaustively

modelled using DFT. This suggested the formation of a terminally bound hydride was

kinetically favoured.2 Single electron reduction of the protonated product gave a paramagnetic

species that retained its structure, as indicated by a combined IR, EPR and DFT study.3

In a complementary study using µSR spectroscopy and DFT, the addition of Mu to two di-

iron systems was investigated. This concerted addition of the muon and electron is analogous

to the two-step process of the protonation and reduction of the di-iron system. µSR

spectroscopy offers a unique time window, allowing observation of the reaction during the

first few nano-seconds. We find evidence to suggest Mu binds to the subsite terminally, which

is in agreement with the DFT mechanism.

[1] Wright, J. A., Pickett, C. J. Chem. Commun., 2009, 45, 5719 ̶ 5721.

[2] Lui, Caiping., Peck, J. N. T., Wright, J. A., Pickett, C. J., Hall, M. N., Eur. J. Inorg. Chem., 2011, 1080 ̶

1093.

[3] Jablonskyte, A., Wright, J. A., Fairhurst, S. A., Peck, J. N. T., Ibrahim, S. K., Oganesyan, V. S., Pickett, C. J.

J. Am. Chem. Soc., 2011, 18606 ̶ 18609.

Determining the regiochemistry of

mixed valent [Fe(I)Fe(II)]-hydrogenase

model complexes

Jamie N. T. Peck

University of East Anglia/STFC

Outline

• Introduction to the hydrogenase enzyme

• The Fe2(µ-pdt)(CO)4(PMe3)2 active site mimic and the current understanding of

the protonation mechanism

• Application of µSR can complement the existing data to help understand this

mechanism

Understanding the protonation mechanism of [FeFe]-hydrogenase

Understanding protonation mechanism is crucial for development of more efficient mimics

Ribbon representation of Clostridium

pasteurianum (Cpl) [FeFe]-

hydrogenase Shepard et al. PNAS

2010; 107, 10448-10453

The [FeFe]-hydrogenase active site

in native enzyme

H2 as an alternative energy vector

The [Fe2(µ-pdt)(CO)4(PMe3)2] model complex

IR of starting material

C. Liu, J. N. T. Peck, J. A. Wright, C. J. Pickett and M. B. Hall, Eur. J. Inorg. Chem, 2011, 1080-1093.

Understanding the protonation mechanism of the [Fe2(µ-pdt)(CO)4(PMe3)2]

model complex

J. A. Wright, C. J. Pickett, Chem. Comm, 2009, 5719-5721.

A 2 step reaction was proposed:

0.7s to 160s (0.2s intervals)

No direct evidence for terminal protonation

Protonation reaction monitored by stopped-

flow IR

DFT Calculations of the [FeFe]-Hydrogenase Model complexes

C. Liu, J. N. T. Peck, J. A. Wright, C. J. Pickett and M. B. Hall, Eur. J. Inorg. Chem, 2011, 1080-1093.

Initial coordination of the

acid onto CO in both

pathways

Pathway for terminal

protonation ~18 kcal/mol

lower than for protonation

at the Fe-Fe bond

Bridging pathway Terminal pathway

DFT Calculations of the [FeFe]-Hydrogenase Model complexes

C. Liu, J. N. T. Peck, J. A. Wright, C. J. Pickett and M. B. Hall, Eur. J. Inorg. Chem, 2011, 1080-1093.

Energy barrier for isomerisation from

kinetic product to thermodynamic

product is predicted to be ~8 kcal/mol

lower than for direct protonation at

the Fe-Fe bond

Mixed Valent [Fe(I)Fe(II)]-Hydrogenase Model Complexes

A. Jablonskyte, J. A. Wright, J. N. T. Peck, S. A. Fairhurst, S. K. Ibrahim, V. S. Oganesyan and C. J. Pickett. J. Am. Chem. Soc., 2011, 133, 18606–

18609.

g factor

Experiment 2.0066

DFT 2.0062

H

D

Difference IR spectrum of the reduction of the hydride species Isotropic EPR spectrum

DFT and Muon Spectroscopy of Mixed Valent [Fe(I)Fe(II)]-Hydrogenase Model

complexes

+ Mu

DFT and Muon Spectroscopy of Mixed Valent [Fe(I)Fe(II)]-Hydrogenase Model

complexes

+ Mu

DFT and Muon Spectroscopy of Mixed Valent [Fe(I)Fe(II)]-Hydrogenase Model

complexes

+ Mu

DFT and Muon Spectroscopy of Mixed Valent [Fe(I)Fe(II)]-Hydrogenase Model

complexes

Summary

• Stopped-flow IR suggests protonation to the Fe-Fe bond followed by

rearrangement to thermodynamic product

• IR and EPR confirm the basal/basal-transoid bridging hydride as the

thermodynamic product

• Mechanistic study of the reaction using DFT suggests an intermediate

is first formed where the proton is weakly bound to the CO, before

migrating to a terminal position on the Fe

• Simulated hyperfine constants associated with muonium bound to the

CO give best agreement with preliminary µSR data

Acknowledgements

Dr Upali Jayasooriya, UEA

Prof Chris Pickett, UEA

Dr Vasily Oganesyan, UEA

Dr Joseph Wright, UEA

Prof Michael Hall, Texas A&M

Dr Stephen Cottrell, ISIS