J. Phys. Chem. B 2008, 112, 10023–10032
10023
Structure and Vibrational Dynamics of Model Compounds of the [FeFe]-Hydrogenase Enzyme System via Ultrafast Two-Dimensional Infrared Spectroscopy A. I. Stewart,† I. P. Clark,‡ M. Towrie,‡ S. K. Ibrahim,§ A. W. Parker,‡ C. J. Pickett,§ and N. T. Hunt*,† Department of Physics, UniVersity of Strathclyde, SUPA, 107 Rottenrow East, Glasgow G4 0NG, U.K., Central Laser Facility, Central Laser Facility, Science & Technology Research Council, Rutherford Council, Rutherford Appleton Laboratory, Harwell Science and InnoVation Campus, Didcot, Oxfordshire, OX11 OQX, U.K. and School of Chemical Sciences and Pharmacy, UniVersity of East Anglia, Norwich NR4 7TJ, U.K. ReceiVed: April 15, 2008; ReVised Manuscript ReceiVed: June 2, 2008
Ultrafast two-dimensional infrared (2D) spectroscopy has been applied to study the structure and vibrational dynamics of (µ-S(CH2)3S)Fe2(CO)6, a model compound of the active site of the [FeFe]-hydrogenase enzyme system. Comparison of 2D-IR spectra of (µ-S(CH2)3S)Fe2(CO)6 with density functional theory calculations has determined that the solution-phase structure of this molecule is similar to that observed in the crystalline phase and in good agreement with gas-phase simulations. In addition, vibrational coupling and rapid (30 cm-1 in the case of the 2035 cm-1 pump experiment, suggesting that another mechanism may be responsible. When the weaker 1992 cm-1 mode was pumped, the bleach again shows a biexponential decay profile with time scales similar to those above, but in this case two of the three bleaches lying to the blue side of this mode showed a risetime of around 5 ps followed by a ∼145 ps decay. The diagonal transient absorption associated with V ) 1-2 transition of this pumped mode shows a biexponential (albeit poorly defined due to the signal-to-noise ratio for this transition) decay, and the modes
lying to the blue side that produced good quality fits to the data again show a 5 ( 1 ps risetime followed by a 130 ( 10 ps decay, in keeping with the major patterns above. The data observed would appear to be consistent with a model in which the carbonyl stretching modes of (µS(CH2)3S)Fe2(CO)6 are coupled, as each of the off-diagonal peaks is apparently nonzero at zero pump-probe time delay (Figure 5a). This observation is somewhat complicated here by effects arising from coincident pump and probe pulses on the sample at time zero7 but can be reasonably asserted based upon prior 2D-IR studies of metal carbonyl derivatives.1,7 In the case of the bleaches, the off-diagonal bleaches then recover with a single-exponential relaxation time of around 120 ps, which would appear to indicate the vibrational lifetime of these modes in heptane. Such a figure is not inconsistent with previous observations of similar molecules.55,56 Further support for this assignment arises from the fact that all modes, whatever the ultrafast dynamics near time zero, ultimately relax with a similar time constant. In the case of the bleach corresponding to the pumped mode, these consistently show a biexponential decay that is also seen in the decay of the associated transient absorption corresponding to the V ) 1-2 transition of the pumped mode. This biexponential features a fast decay of around 5 ps followed by the longer 120 ps component common to all bleaches. This would appear to be consistent with a quantity of fast vibrational population transfer among the carbonyl stretching modes occurring immediately after the pumping of one of the modes. Such an effect has been observed previously in smaller carbonyl systems when employing narrow-band pump, broad-band probe spectroscopy, with similar time scales reported.55,56 The concept of population transfer is also consistent with the rise times observed for the transient absorption signals, the higher frequency modes each showing a 5-10 ps risetime as population transfer occurs, followed by the universal 100-120 ps relaxation dynamics. Similarly, the observation of a risetime for the bleaches when the 1992 cm-1 mode was pumped can be explained using this model. It should be considered that the signals in this region of the spectrum are smaller than those measured with pump frequencies of 2006 and 2035 cm-1. As such, it is feasible that population transfer leads to increased stimulated emission from the V ) 1 state leading to the observed risetime. This would be less obvious in the presence of stronger bleach signals. The lower signal-to-noise ratios also account for the observed slightly longer decay constant. The observation of only two (within experimental error) time constants would also suggest a relatively straightforward vibrational decay mechanism despite the complex spectroscopy of this system. Such vibrational energy transfer pathways arise through the actions of the solvent bath upon the excited molecule. The long vibrational lifetime of this species arises from the fact that interactions between the carbonyl stretching modes of the solute and the solvent are weak. Furthermore, there are no solvent absorptions close in energy to the excited modes of the solute to facilitate rapid vibrational energy transfer into the surrounding solvent bath. This must therefore occur through an overtone or combination band modes of the solvent. With this in mind, however, it is apparent that the vibrational lifetime of the carbonyl modes of (µ-S(CH2)3S)Fe2(CO)6 is somewhat shorter than that observed in other simpler carbonyl systems.55–57 This may arise from the greater number of carbonyl stretches of similar frequency, allowing a greater flexibility in terms of vibrational relaxation pathway.
Compounds of the [FeFe]-Hydrogenase Enzyme System The population transfer process, especially to modes lying to higher energy than the pumped mode, has also been studied previously.58 This is thought to arise from interactions with the low-frequency density of states of the surrounding solvent bath. Most liquids possess a broad band of inter- and intramolecular vibrational modes in the range of 0-150 cm-1, though this can be broader in more strongly interacting liquids.59–62 Thus, population transfer to higher and lower frequency modes within 150 cm-1 of the pumped mode is facilitated by scattering mechanisms involving phonon modes of the solvent bath allowing rapid equilibration of the excited vibrational population among the carbonyl stretching modes.58 The observation of such vibrational relaxation mechanisms in model compounds of the hydrogenase enzyme system is of significance for the full enzyme. In particular, it is interesting to note that the active site of the enzyme exists within a pocket provided by the protein scaffold into which no aqueous solvent can penetrate. This implies that any such vibrational relaxation of the enzyme would have to be provided by the surrounding scaffolding. As such, future comparisons with similar measurements involving the enzyme would be instructive. In addition to obtaining information relating to the vibrational dynamics and energy transfer pathways of this molecule, the 2D-IR dynamical studies above also provide the opportunity to establish the molecular structure of the carbonyl ligands in solution. While the crystalline structure of (µ-S(CH2)3S)Fe2(CO)6 is known,63 the solution-phase structure remains unclear. With the use of the anisotropy of the off-diagonal peaks in the 2D-IR spectra, it is possible to compare the observed angular relationships between the transition dipole moments of the carbonyl modes with those calculated using DFT. Good agreement with the simulations would confirm that the calculated gas-phase structure, which is very similar to that observed from X-ray crystallography,63 and the solution-phase structure are similar and also confirm the ordering of the modes in the simulated FTIR spectrum and thus their assignment. Furthermore, the time-dependence of the anisotropy allows the determination of the molecular rotation times of (µ-S(CH2)3S)Fe2(CO)6 in solution. The data in Figure 5b shows the anisotropy for each of the observed bands when a pump frequency of 2006 cm-1 is used. In this case, it can be seen from the anisotropy values at time zero that the pumped mode (and the associated V ) 1-2 transition), in red, possess transition dipole moments that are parallel to each other, as would be expected, and furthermore, that it lies perpendicular to those of the other three most intense carbonyl stretching modes, as indicated by anisotropy values of 0.4 and -0.2, respectively. Repeating this operation for each of the three pumped modes at 2035, 2006, and 1992 cm-1 reveals the transition dipole angular relationships for each of the four main carbonyl stretches. These are all consistent with the calculated values in Table 2, confirming that the point group in the solution phase is indeed cs and the validity of the mode assignments calculated by DFT. Fitting of the temporal decay of the anisotropy parameters in Figure 5b shows a single-exponential relationship with a decay time constant of around 17 ps. This indicates the rotational time scale of the molecule in heptane. This value is somewhat smaller than may be expected for such a large molecule; however, it must be taken into account that the molecule is approximately spherical in overall shape and that the viscosity of the solvent is very low (0.387 mPa · s at 298 K).64 An additional factor that must also be taken into consideration is the limited interaction with the surrounding solvent, which will
J. Phys. Chem. B, Vol. 112, No. 32, 2008 10029
Figure 6. FTIR (top) and 2D-IR (bottom) spectra of (µS(CH2)3S)Fe2(CO)6 dissolved in CH3CN. The pump-probe time delay was 5 ps, and the pump-probe polarization geometry was parallel when recording the 2D-IR spectrum. Dashed lines are used to guide the eye. Small deviations of the line positions from the diagonal are caused by calibration errors of the pump frequency, which are exacerbated by the broader lineshapes observed in acetonitrile.
occur mainly through dispersive interactions. As such, it may be expected that slip boundary conditions may be more applicable than stick conditions, leading to a decoupling of the molecular rotational motion from the surrounding solvent. Other Solvents. In an effort to further determine the effects of the solvent bath upon the phenomena described above, the molecule (µ-S(CH2)3S)Fe2(CO)6 was also dissolved in acetonitrile (CH3CN) and hexadecane. The former is significantly more polar than both heptane and hexadecane, while hexadecane offers an almost order of magnitude increase in solvent viscosity. The results obtained in hexadecane were largely in excellent agreement with those obtained for the heptane sample. The 1D and 2D-IR data showed little change, as would be expected for such similar solvents. The major change was observed when studying the dynamical data, i.e., that obtained by measuring the time-dependence of the vibrationally excited states of each of the carbonyl stretching modes as slices through the 2D-IR spectrum. While the vibrational lifetime data as shown in Table 3 was observed to be identical to that of the heptane sample, within experimental error (see Table 3), the anisotropy relaxation time increased to 53 ps. This is as might be expected given a more viscous solvent (viscosity of hexadecane ) 3.03 mPa · s at 298 K64) although it is interesting to note that the increase is not proportional to the change in viscosity, as suggested by the Debye-Stokes-Einstein relation. The latter observation may be due in large part to the lack of interaction between solvent and solute leading to weak coupling between the rotational relaxation time and the macroscopic solvent viscosity as discussed above. The 1D and 2D-IR spectra of (µ-S(CH2)3S)Fe2(CO)6 dissolved in CH3CN are shown in Figure 6. The 2D-IR spectrum shown was obtained using a pump-probe time delay of 5 ps and parallel pump-probe polarization geometry, though as with the heptane and hexadecane samples, no evolution of the 2DIR peak shapes or positions was observed with either pump-probe delay time or polarization geometry.
10030 J. Phys. Chem. B, Vol. 112, No. 32, 2008
Stewart et al. TABLE 4: Results of Fitting Magic Angle Dynamical Data for (µ-S(CH2)3S)Fe2(CO)6 Dissolved in CH3CNa solvent CH3CN
pump υ (cm-1) probe υ (cm-1) B/A 2074
τD2 (ps) 123 107 85 95
2074 2032 1995 2063 2021 1974
B B B A A A
0.0003 0.0008 0.0003 0.0002 0.0002 0.0003
129
2032
2074 2032 1995 2063 2021 1974
B B B A A A
0.0013 0.0054 0.0020 0.0011 0.0011 0.0014
118 102 123 137 89 131
1995
2074 2032 1995 2063 2021 1974
B B B A A A
0.0006 0.0022 0.0012 0.0006 0.0005 0.0007
108 118 120 144 155 124
Figure 7. Vibrational lifetime dynamics for (µ-S(CH2)3S)Fe2(CO)6 dissolved in CH3CN. Data shown is magic angle relaxation of signals observed when the pump wavelength was 2032 cm-1.
The effects of the polar solvent are clearly apparent upon the FTIR spectrum of (µ-S(CH2)3S)Fe2(CO)6, the line broadening effect leading to a dramatic loss of spectral resolution. However, it is interesting to note that this does not manifest itself as inhomogeneous broadenening of the 2D-IR lineshapes. Such broadening would be expected to lead to a diagonal elongation of the peaks in the 2D-IR spectrum, but this is not the case.18 A slight elongation along the pump frequency axis direction is observed, though this is likely to be due to the combination of increased line width as compared with the heptane and hexadecane samples with the finite bandwidth of the pump pulse leading to this apparent broadening effect. A similar phenomenon has been previously observed when studying broad lineshapes with the double-resonance 2D-IR method.7 One possible explanation for the lack of diagonal elongation of the lineshapes is that the time scale of any inhomogeneous broadening is faster than the experimental time resolution. This would lead to complete spectral diffusion within the pump-probe time delay period and hence circular lineshapes at all pump-probe time delays, as have been observed. Indeed, diagonally elongated lineshapes are most often observed in hydrogen-bonded systems such as aqueous peptide solutions and indeed in water itself.65,66 As no hydrogen bonding is present in the CH3CN/(µS(CH2)3S)Fe2(CO)6 solution, it is possible that any interactions are weaker and thus more transient than hydrogen-bonding processes. It is also instructive to consider the vibrational lifetime data obtained for the CH3CN solution. As in the case of hexadecane and heptane, the temporal dependence of the magic angle dynamics and anisotropy of each of the modes has been studied at pump frequencies corresponding to each of the three bands observed in the 1D infrared spectrum at 2074, 2032, and 1995 cm-1. In the case of the anisotropy decay, a value of 21 ps was observed in all cases, which corresponds well with the value of 17 ps in heptane given the similar viscosity values for the two solvents (CH3CN viscosity ) 0.369 mPa · s at 298 K64). It should be noted that anisotropy parameters were obtained only for the two higher frequency carbonyl stretching modes of (µS(CH2)3S)Fe2(CO)6 in CH3CN as the broadened linewidths will lead to convolution effects where the lower frequency absorptions overlap. The dynamics observed for each of the bands observed in CH3CN when pumped at 2032 cm-1 are displayed in Figure 7. This data corresponds to the magic angle polarization geometry and as such relates to the vibrational lifetime of the transitions studied. As in the case of heptane above, these data were quantified by fitting to exponential functions. In contrast to the heptane and hexadecane samples, however, a single-exponential decay profile sufficed in each case, providing good agreement
a1
b
a
A single-exponential function was used in each case. Note that B/A correspond to a bleach or transient absorption signal, respectively. b Undefined due to poor S/N ratio.
with the observed decay profiles. The results of this fitting process are shown in Table 4. In each case, a decay time scale of 110 ( 20 ps was observed. It is interesting to note that the observed biexponential decays exhibited by the pumped transitions are no longer observed, similarly that the rise times shown by the transient absorptions lying to the blue side of the pumped bands are also no longer evident. The red traces in Figure 7, corresponding to the pumped transition and its V ) 1-2 transient absorption partner, do appear to show a biexponential character, but this appears to be extremely fast and was not reliably resolvable during the fitting process. These observations imply that the solvent-solute interactions are somewhat stronger in CH3CN, as might be anticipated from the broader lineshapes and more polar nature of the solvent. This apparently speeds up the process of vibrational population transfer between the carbonyl modes, hence the fact that the 5 ps processes observed in the alkane solvents are no longer observed. In light of this, it is interesting to note that the overall vibrational relaxation time of the carbonyl modes is similar to that obtained in the alkane solvents. It might be expected that this process would be faster in a more polar solvent; however, metal carbonyl transitions are well separated from most solvent bands, even in the case of CH3CN, which possesses a CN stretching mode at 2250 cm-1. The reason for this is that, although closer than any alkane solvent vibrational modes, this band is still some 176 cm-1 above the highest frequency carbonyl stretching mode of (µ-S(CH2)3S)Fe2(CO)6. Thus, given that the involvement of the low-frequency density of states of the solvent would be required in order to utilize this mode as a relaxation pathway, the fact that the CH3CN density of states does not have significant amplitude at 176 cm-1 may well be prohibitive.58 IV. Conclusions Ultrafast 2D-IR spectroscopy has been applied for the first time to study the solution-phase structure and vibrational dynamics of a model compound specifically relating to the
Compounds of the [FeFe]-Hydrogenase Enzyme System hydrogenase enzyme system. 2D-IR spectra in conjunction with DFT calculations have shown that the structure of (µS(CH2)3S)Fe2(CO)6 in solution is very similar to that predicted in the gas phase and measured using X-ray crystallography. Ultrafast dynamics obtained from 2D-IR spectroscopy reveal that vibrational relaxation following excitation of individual carbonyl modes occurs on two separate time scales in alkane solvents: Vibrational population transfer among the carbonyl modes occurs on a time scale of around 5 ps followed by 100-120 ps vibrational relaxation to the ground state. While both these effects might be expected to be solvent-mediated, the population transfer rate appeared to be most strongly influenced by solvent polarity. Data recorded for (µS(CH2)3S)Fe2(CO)6 in CH3CN showed only a 100-120 ps decay time scale, suggesting that the population transfer rate had become too fast to be reliably resolved. This is supported by the observation of 2D-IR lineshapes apparently indicating non-inhomogeneously broadened transitions. Conversely, little effect upon the vibrational relaxation time was observed. The reason for this discrepancy arises from the greater interactions with the polar solvent through the low-frequency density of states of CH3CN, allowing more effective population transfer among closely spaced carbonyl modes as compared with that of alkane solvents. However, the increased width of the density of states of CH3CN in comparison with that of alkane solvents is insufficient to bridge the gap between the carbonyl modes and high-frequency vibrational absorptions of CH3CN, which would facilitate a significantly shorter vibrational lifetime. It is anticipated that the results obtained herein will be of benefit as comparison points for future studies of the structure and vibrational dynamics of the full enzyme system, in which the ability to separate the active site and protein scaffold will provide new insights into the role of the protein pocket. Furthermore, they will contribute to the knowledge base surrounding applications of hydrogenase enzyme-based technologically relevant systems for hydrogen production and utilization. Acknowledgment. The authors would like to acknowledge the Engineering and Physical Sciences Research Council of the U.K. (EPSRC) for an Advanced Research Fellowship (N.T.H.) and postgraduate studentship (A.I.S.). Funding for this work has also been provided by the U.K. Science and Technology Facilities Council (STFC) for work carried out at the Central Laser Facility, Rutherford Appleton Laboratory. CJP and SKI wish to thank the BBSRC and EPSRC (Supergen V) for supporting this work. References and Notes (1) Khalil, M.; Demirdoven, N.; Tokmakoff, A. J. Phys. Chem. A 2003, 107, 5258. (2) Finkelstein, I. J.; Zheng, J. R.; Ishikawa, H.; Kim, S.; Kwak, K.; Fayer, M. D. Phys. Chem. Chem. Phys. 2007, 9, 1533. (3) Bredenbeck, J.; Helbing, J.; Kolano, C.; Hamm, P. ChemPhysChem 2007, 8, 1747. (4) Asplund, M. C.; Zanni, M. T.; Hochstrasser, R. M. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 8219. (5) Hahn, E. L. Phys. ReV. 1950, 80, xxx. (6) Hamm, P.; Lim, M.; De Grado, W. F.; Hochstrasser, R. M. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 2036. (7) Cervetto, V.; Helbing, J.; Bredenbeck, J.; Hamm, P. J. Chem. Phys. 2004, 121, 5935. (8) Demirdoven, N.; Cheatum, C. M.; Chung, H. S.; Khalil, M.; Knoester, J.; Tokmakoff, A. J. Am. Chem. Soc. 2004, 126, 7981. (9) Cheatum, C. M.; Tokmakoff, A.; Knoester, J. J. Chem. Phys. 2004, 120, 8201. (10) Chung, H. S.; Khalil, M.; Tokmakoff, A. Biophys. J. 2004, 86, 526A.
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