© Copyright 1996 by the American Chemical Society
VOLUME 100, NUMBER 16, APRIL 18, 1996
LETTERS Carbonyl Tilting and Bending Potential Energy Surface of Carbon Monoxyhemes Abhik Ghosh† and David F. Bocian* Department of Chemistry, UniVersity of California, RiVerside, California 92521-0403 ReceiVed: NoVember 30, 1995; In Final Form: January 25, 1996X
The CO tilting (τ) and bending (β) potential energy surface of carbon monoxyheme has been investigated with local density functional calculations. The calculations indicate that τ and β are strongly coupled and that simultaneous, in-phase displacements along these coordinates represent a low-energy pathway across the surface. Calculations on two small model complexes indicate that strong τ-β coupling also occurs in these systems. Accordingly, this feature appears to be a general characteristic of the Fe-CO unit in a squareplanar coordination geometry. In-phase τ-β CO deformations (τ + β) of as much as 25° can take place with the expenditure of relatively small amounts of energy (2 kcal/mol or less). However, very large distortions of 45-60° are energetically demanding and unreasonable. The calculations also rule out the possibility that the coordination geometry of the proximal histidine is the major determinant of the CO orientation. Both a full vibrational analysis on a small model complex and a limited analysis on a tetraatomic model yield calculated frequencies and isotope shifts for the Fe-CO unit in excellent agreement with those observed for carbon monoxyhemes. Collectively, our calculated τ-β potential energy surface provides a plausible explanation for the wide variation in CO orientations reported for carbon monoxymyoglobin and also account for the unusual vibrational characteristics of the Fe-CO unit.
Introduction Carbon monoxymyoglobin (MbCO) is the paradigm for assessing the factors which control ligand binding to heme proteins.1 This understanding has been hampered by the controversy over the issue of deformability of the Fe-CO unit as a result of its interaction with the protein matrix. X-ray and neutron diffraction studies on native sperm whale MbCO indicate that the CO ligand is tilted/bent 45-60° from the normal to the heme plane.2-4 The CO tilt (τ) and bend angles (β) are defined in Figure 1. The large tilt/bend of the CO ligand provides a qualitative rationale for the diminished CO binding affinity of myoglobin relative to protein-free heme (where a † Current address: Department of Chemistry, Institute of Mathematical and Physical Sciences (IMR), University of Tromsø, Breivika, N-9037 Tromsø, Norway. X Abstract published in AdVance ACS Abstracts, April 1, 1996.
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linear structure is preferred5).1 However, the dogma of drastically tilted/bent CO (τ + β ∼45-60°) has been challenged. Early IR photoselection measurements on native MbCO yielded CO orientations (, see Figure 1) of only 15-35°;6,7 a recent study gave < 7°.8 Polarized single-crystal IR experiments indicated values of 5 kcal/mol). The energies associated with selected τ + β displacements on the two potential energy surfaces are compared in Table 2. The large, negative value of kτβ also leads to a new interpretation of certain vibrational features of the Fe-CO unit.
Figure 4. Calculated vibrational frequencies and eigenvectors for selected A′ modes of model complex B. (a) νCO, (b) νFe-C, (c) ip op ip δFeCO , and (d) δFeCO . The frequencies of the A′′ partners of δFeCO , and op -1 δFeCO are 49 and 622 cm , respectively. Note that the calculated δFeCO modes cannot be accurately described as either a “tilt” or a “bend” due to the significant mixing of these motions (see text).
This is illustrated in Figure 4, which shows the calculated vibrational frequencies and eigenvectors for selected A' modes of complex B. These modes include the CO stretch, νCO, the Fe-C stretch, νFe-C, and the in-phase and out-of-phase τ-β ip op and δFeCO , respectively. The deformations, designated δFeCO δFeCO modes cannot be described as either a “tilt” or a “bend” due to the significant mixing of these motions. This mixing has not been previously considered in the analysis of the vibrational spectra of the carbon monoxyhemes. The calculated op frequencies of νCO (1985 cm-1), νFe-C (524 cm-1), and δFeCO -1 (612 cm ) are in remarkably good agreement with those observed for MbCO (1944, 512, and 577 cm-1, respectively).24 ip A band due to δFeCO has not been identified in the vibrational spectra of carbon monoxyhemes; however, our calculations indicate the frequency is very low (84 cm-1).25 It should also be noted that a calculation using an L-shaped tetraatomic model (NpFeCO) and the τ-β force constants for PFe(Im)(CO) gave op ip ∼ 601 cm-1 and δFeCO ∼ 136 cm-1. The calculated δFeCO 13CO and C18O isotope shifts of δop -1 FeCO are ∼-19 and ∼0 cm , respectively, both of which are in excellent agreement with those observed for MbCO (∼-17 and ∼-1 cm-1)24,26 and carbon monoxyhemes in general.14b This agreement lends further credibility to the quality of the calculated τ-β potential energy surface. The 577-cm-1 band of MbCO (and other carbon monoxybend mode14,24,27 hemes) has generally been attributed to the δFeCO op rather than the δFeCO deformation indicated by our calculabend has tions. The assignment of the 577-cm-1 band to δFeCO provoked controversy14b,27-29 because this frequency is higher than that of the 512-cm-1 νFe-C mode. As a consequence, several groups have proposed that the 577-cm-1 band is not a fundamental vibration. Tsuboi has proposed that this band is bend 28 . Kitagawa and co-workers have the overtone of δFeCO suggested that the 577-cm-1 band is a combination of an unobserved porphyrin vibration and a ∼365-cm-1 mode asbend fundamental.29 Our calculations clearly signed to the δFeCO support the assignment of the 577-cm-1 band to a fundamental vibration. Its high frequency is a consequence of the strong op above νFe-C and simultatilt-bend mixing which raises δFeCO ip neously pushes δFeCO to very low frequency (Figure 4). The
Letters ∼365-cm-1 band observed by Kitagawa and co-workers cannot ip because the observed 13CO isotope shift be attributed to δFeCO -1 29 (∼-12 cm ) is inconsistent with that calculated for this type of motion (∼-1 cm-1). The softness of the potential surface along the in-phase tiltbend coordinate has significant implications for the structure of the Fe-CO unit in MbCO and other heme proteins. In particular, distal-pocket interactions which destabilize the FeCO unit by only 2 kcal/mol could result in changes in the CO orientation of 25° (Table 2). This range of CO orientations encompasses those measured in most of the spectroscopic and crystallographic experiments on MbCO (Table 1).6-11 Accordingly, the measured differences in the CO orientation may in fact be real and reflect the effects of variations in the medium (crystal versus solution, type of crystal), pH, temperature, or other factors on the exact structure of the protein. This represents a significant departure from the currently favored view that the Fe-CO unit is relatively undeformable. Support for the picture of a relatively undeformable Fe-CO unit has been derived from crystallographic studies of sterically hindered carbon monoxyhemes which reveal moderate τ + β angles of 12-15°.12,13 Distortions of this size cost less than 0.7 kcal/mol on our potential energy surface, consistent with the relatively large nearest contacts (∼3-4 Å) between the strap/ cap and the CO oxygen. This energy is also well within the range which could be imparted by steric and electrostatic interactions in the distal heme pocket.14b Accordingly, the τ + β deformations of 12-15° observed for sterically hindered MbCO model complexes should not be regarded as the upper limit of CO deformation in heme proteins. A trend worth pointing out and common to all crystallographic results on MbCO (Table 1) and model complexes is that β > τ. This is qualitatively consistent with our finding that kββ < kττ (Figure 3). Despite of our finding of a soft τ-β deformation pathway, our potential energy surface cannot accommodate some of the very large τ + β angles predicted in certain diffraction studies of MbCO.2-4 However, Spiro and co-workers have pointed out that uncertainties in the atom positions in these data lead to uncertainties in the CO orientation of ∼25°.14b This large uncertainty, along with the fact that τ + β angles of 25° are calculated to be energetically quite accessible, could account for the extremely large τ + β angles measured in certain diffraction studies.2-4 Finally, we speculate on the possible influence of the shape of the τ-β potential energy surface on the dynamics of ligand binding/photodissociation. Crystallographic and spectroscopic studies of photodissociated MbCO have shown that the CO molecule resides in the center of the distal pocket with its molecular axis approximately orthogonal to the heme normal.3,8,11 Accordingly, the CO molecule must rotate as it leaves the heme group. A trajectory in which CO tilts and bends inphase as it dissociates from (or approaches) the heme provides a low-energy pathway which could assist in this rotation. Acknowledgment. This work was supported by grant GM36234 (D.F.B) from the National Institute of General Medical Sciences. References and Notes (1) For a recent comprehensive review see: Springer, B. A.; Sligar, S. G.; Olson, J. S.; Phillips, G. N., Jr. Chem. ReV. 1995, 94, 699. (2) Kuriyan, J.; Wilz, S.; Karplus, M.; Petsko, G. A. J. Mol. Biol. 1986, 192, 133. (3) Teng, T.-Y.; Sˇ rajer, V.; Moffat, K. Nat. Struct. Biol. 1994, 1, 701. (4) Cheng, X.; Schoenborn, B. P. J. Mol. Biol. 1991, 220, 381.
J. Phys. Chem., Vol. 100, No. 16, 1996 6367 (5) Peng, S.-M.; Ibers, J. A. J. Am. Chem. Soc. 1976, 98, 8032. (6) Moore, J. N.; Hansen, P. A.; Hochstrasser, R. M. Proc. Natl. Acad. Sci. U.S.A. 1988, 85, 5062. (7) Ormos, P.; Brauenstein, D.; Frauenfelder, H.; Hong, M. K.; Lim, S.-L.; Sauke, T. B.; Young, R. D. Proc. Natl. Acad. Sci. U.S.A. 1988, 85, 8492. (8) Lim, M.; Jackson, T. A.; Anfinrud, P. A. Science 1995, 269, 962. (9) Ivanov, D.; Sage, J. T.; Keim, M.; Powell, J. R.; Asher, S. A.; Champion, P. M. J. Am. Chem. Soc. 1994, 116, 4139. (10) Quillan, M. L.; Arduini, R. M.; Olson, J. S.; Phillips, G. N., Jr. J. Mol. Biol. 1993, 234, 140. (11) Schlichting, I.; Berendzen, J.; Phillips, G. N., Jr.; Sweet, R. M. Nature 1994, 371, 808. (12) (a) Momenteau, M.; Scheidt, W. R.; Eigenbrot, C. W.; Reed, C. A. J. Am. Chem. Soc. 1988, 110, 1207. (b) Tetreau, C.; Lavalette, D.; Momenteau, M.; Fischer, J.; Weiss, R. J. Am. Chem. Soc. 1994, 116, 11840. (13) (a) Kim, K.; Fettinger, J.; Sessler, J. L.; Cyr, M.; Hugdahl, J.; Collman, J. P.; Ibers, J. A. J. Am. Chem. Soc. 1989, 111, 403. (b) Kim, K.; Ibers, J. A. J. Am. Chem. Soc. 1991, 113, 6077. (14) (a) Li, X.-Y.; Spiro, T. G. J. Am. Chem. Soc. 1988, 110, 6024. (b) Ray, G. B.; Li, X.-Y.; Ibers, J. A.; Sessler, J. L.; Spiro, T. G. J. Am. Chem. Soc. 1994, 116, 162. (15) (a) Almlo¨f, J.; Fischer, T. H.; Gassman, P. G.; Ghosh, A.; Ha¨ser, M. J. Phys. Chem. 1993, 97, 10964. (b) Mercha´n, M.; Ortı´, E.; Roos, B. Chem. Phys. Lett. 1994, 221, 136. (16) (a) Jewsbury, P.; Yamamoto, S.; Minato, T.; Saito, M.; Kitagawa, T. J. Am. Chem. Soc. 1994, 116, 11587. (b) Jewsbury, P.; Yamamoto, S.; Minato, T.; Saito, M.; Kitagawa, T. J. Phys. Chem. 1995, 99, 12677. (17) For expositions on DFT, see: (a) Labanowski, J. W.; Andzelm, J. Density Functional Methods in Chemistry; Springer: New York, 1991. (b) Parr, R. G.; Yang, W. Density Functional Theory of Atoms and Molecules; Oxford Univeristy Press: New York, 1989. (18) For chemical applications of DFT, see: (a) Ziegler, T. Chem. ReV. 1991, 91, 651. (b) Andzelm, J.; Wimmer, E. J. Chem. Phys. 1992, 96, 1280. (c) Johnson, B. G.; Gill, P. M. W.; Pople, J. A. J. Chem. Phys. 1993, 98, 5612. (19) For selected applications of DFT to porphyrins, see: (a) Ghosh, A.; Almlo¨f, J. Chem. Phys. Lett. 1993, 213, 519. (b) Jones, D. H.; Hinman, A. S.; Ziegler, T. Inorg. Chem. 1993, 32, 2092. (c) Ghosh, A. J. Am. Chem. Soc. 1995, 117, 4691. (d) Kalsbeck, W. A.; Ghosh, A.; Pandey, R. K.; Smith, K. M.; Bocian, D. F. J. Am. Chem. Soc. 1995, 117, 10959. (e) Matsuzawa, N.; Ata, M.; Dixon, D. A. J. Phys. Chem. 1995, 99, 7698. (20) For a description of the DMol program, see: (a) Delley, B. J. Chem. Phys. 1990, 92, 508. (b) Delley, B. J. Chem. Phys. 1991, 94, 7245. (c) DMol User Guide; Biosym Technolgies, Inc.: 9685 Scranton Road, San Diego, CA 92121. (21) von Barth, U.; Hedin, L. J. Phys. C 1972, 5, 1629. (22) For previous reports of geometry optimizations of porphyrins, see refs 15 and: (a) Ghosh, A.; Almlo¨f, J. J. Phys. Chem. 1995, 99, 1073. (b) Ghosh, A. Angew. Chem., Int. Ed. Engl. 1995, 34, 1028; Angew. Chem. 1995, 107, 1117. (c) Reimers, J. R.; Lu¨, T. X.; Crossley, M. J.; Hush, N. S. J. Am. Chem. Soc. 1995, 117, 2855. (23) We are currently investigating the origin of the discrepancy between our result and that of Jewsbury et al.16 Possible reasons for this discrepancy include inadequacies of the geometry optimization procedure for soft internal coordinates, MP2 theory, and/or the HF reference wave function. (24) Tsubaki, M.; Srivastava, R. B.; Yu, N.-T. Biochemistry 1982, 21, 1132. (25) Transition-metal carbonyl compounds exhibit vibrational bands in the 60-100-cm-1 region which have been identified as CMC bending modes (Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed.; Wiley: New York, 1986; pp 291-308). These modes ip ; however, the exact forms of the normal are qualitatively similar to δFeCO coordinates are different due to the high symmetry of the compounds. In addition, normal-coordinate calculations on Fe(CO)5 (Jones, L. H.; McDowell, R. S.; Goldblatt, M.; Swanson, B. I. J. Chem. Phys. 1972, 57, 2050) include a negative CFeC-FeCO bend-bend interaction constant but this constant is small (-0.12 mdyn Å/rad2) compared with that calculated here (-0.40 mdyn Å/rad2). (26) Ling, J.; Li, T.; Olson, J. S.; Bocian, D. F. Biochim. Biophys. Acta 1994, 1188, 417. (27) Hu, S.; Vogel, K. M.; Spiro, T. G. J. Am. Chem. Soc. 1994, 116, 11187. (28) Tsuboi, M. Ind. J. Pure Appl. Phys. 1988, 26, 188. (29) Hirota, S.; Ogura, T.; Shinzawa-Itoh, K.; Yoshikawa, S.; Nagai, M.; Kitagawa, T. J. Phys. Chem. 1994, 98, 6652.
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