Abstract
We have used
57
Fe nuclear resonance vibrational spectroscopy (NRVS) to study the iron site in the iron-sulfur-cluster-free hydrogenase Hmd from the methanogenic archaeon
Methanothermobacter marburgensis
. The spectra have been interpreted by comparison with a
cis
-(CO)
2
-ligated Fe model compound, Fe(S
2
C
2
H
4
)(CO)
2
(PMe
3
)
2
, as well as by normal mode simulations of plausible active site structures. For this model complex, normal mode analyses both from an optimized Urey-Bradley force field and from complementary density functional theory (DFT) calculations produced consistent results.
Previous IR spectroscopic studies found strong CO stretching modes at 1944 and 2011 cm
−1
, interpreted as evidence for
cis
-Fe(CO)
2
ligation. The NRVS data provide further insight into the dynamics of the Fe site, revealing Fe-CO stretch and Fe-CO bend modes at 494, 562, 590, and 648 cm
−1
, consistent with the proposed
cis
-Fe(CO)
2
ligation. The NRVS also reveals a band assigned to Fe-S stretching motion at ~311 cm
−1
, and another reproducible feature at ~380 cm
−1
. The
57
Fe partial vibrational densities of states (PVDOS) for Hmd can be reasonably well simulated by a normal mode analysis based on a Urey-Bradley force field for a 5-coordinate
cis
-(CO)
2
-ligated Fe site with additional cysteine, water, and pyridone cofactor ligands. A final interpretation of the Hmd NRVS data, including DFT analysis, awaits a 3-dimensional structure for the active site.