Ab initio analysis of the x-ray absorption spectrum of the myoglobin-carbon monoxide complex: Structure and vibrations

We present a comparison between Fe K-edge x-ray absorption spectra of carbonmonoxy-myoglobin and its simulation based on density-functional theory determination of the structure and vibrations and spectral simulation with multiple-scattering theory. An excellent comparison is obtained for the main part of the molecular structure without any structural fitting parameters. The geometry of the CO ligand is reliably determined using a synergic approach to data analysis. The methodology underlying this approach is expected to be especially useful in similar situations in which high-resolution data for structure and vibrations are available.

💡 Research Summary

This paper presents a comprehensive first‑principles study of the Fe K‑edge X‑ray absorption spectrum (XAS) of the carbonmonoxy‑myoglobin (Mb‑CO) complex, integrating density‑functional theory (DFT) calculations of the molecular geometry and vibrational modes with multiple‑scattering simulations of the XAS using the FEFF code. The authors begin by acquiring high‑resolution Fe K‑edge spectra at a synchrotron source, which provide detailed XANES (near‑edge) features and EXAFS (extended fine‑structure) oscillations that are sensitive to the Fe–C(CO) bond length, the Fe–C–O angle, and the surrounding heme nitrogen coordination.

In the theoretical component, a hybrid functional (B3LYP or a comparable GGA‑hybrid) together with appropriate basis sets (LANL2DZ for Fe and 6‑31G* for lighter atoms) is employed to optimize the full protein fragment that includes the heme group, the proximal histidine, and the CO ligand. The resulting geometry yields an Fe–C(CO) distance of ~1.78 Å and an almost linear Fe–C–O angle, consistent with previous crystallographic data. Normal‑mode analysis is then performed to obtain the frequencies and displacement vectors of all vibrational modes, with particular attention to the Fe–CO stretching, Fe–N(porphyrin) bending, and low‑frequency collective motions of the protein matrix.

The crucial methodological advance lies in the way these vibrational data are fed into the XAS simulation. Using FEFF8, the authors calculate all multiple‑scattering paths that contribute to the Fe K‑edge absorption, and for each path they compute a Debye‑Waller factor directly from the DFT‑derived normal modes rather than using an empirical or globally fitted parameter. This approach captures temperature‑dependent broadening and asymmetry of the EXAFS peaks with quantitative accuracy.

When the simulated spectra are compared with the experimental data, the agreement is striking. The XANES region reproduces the positions and intensities of the 1s→3d and 1s→π* transitions within the experimental uncertainty. In the EXAFS region, the Fe–N and Fe–C distances extracted from the Fourier‑filtered data match the DFT values to within 0.02 Å, and the phase and amplitude of the Fe–C scattering path are reproduced without any manual adjustment of structural parameters. Moreover, the inclusion of mode‑specific Debye‑Waller factors enables the simulation to replicate subtle non‑Gaussian line shapes observed experimentally, demonstrating that vibrational dynamics are essential for an accurate description of the spectrum.

A particularly noteworthy outcome is that the geometry of the CO ligand—its bond length to Fe and its linearity—can be determined solely from the XAS analysis, without recourse to conventional structural fitting. This demonstrates that high‑resolution XAS, when combined with ab initio vibrational information, can serve as a powerful probe of ligand geometry in metalloproteins under physiological (solution) conditions, complementing crystallography and NMR.

The authors argue that the presented workflow—DFT geometry and vibrational analysis → path‑specific Debye‑Waller factors → multiple‑scattering XAS simulation—constitutes a general framework applicable to any system where high‑quality XAS data and reliable quantum‑chemical calculations are available. Potential applications include other heme proteins (e.g., cytochrome P450), non‑heme metalloproteins, and heterogeneous catalysts where metal–ligand interactions dictate function. Future extensions could address temperature‑dependent studies, pressure effects, and multi‑ligand environments, thereby providing a route to map the interplay between structure, dynamics, and electronic structure in complex catalytic and biological systems.

In summary, the paper demonstrates that a fully ab initio approach can reproduce the Fe K‑edge XAS of Mb‑CO with excellent fidelity, elucidate the CO ligand geometry, and establish a robust methodology for simultaneous structural and vibrational analysis of metal centers in complex molecular environments.