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Quantum mechanics/molecular mechanics calculation of the Raman spectra of the phycocyanobilin chromophore in alpha-c-phycocyanin
Citation key ISI:000249013900007
Author Mroginski, Maria Andrea and Mark, Franz and Thiel, Walter and Hildebrandt, Peter
Pages 1885-1894
Year 2007
ISSN 0006-3495
DOI 10.1529/biophysj.107.108878
Address 9650 ROCKVILLE PIKE, BETHESDA, MD 20814-3998 USA
Journal Biophys. J.
Volume 93
Number 6
Month SEP
Publisher BIOPHYSICAL SOC
Abstract We have established a quantum mechanics (QM)/molecular mechanics (MM) hybrid method for calculating the Raman spectra of protein-bound cofactors using the a-subunit of C-phycocyanin containing a phycocyanobilin (PCB) chromophore as a test case. The PCB cofactor was described with density functional theory, whereas the protein matrix was treated with the CHARMM force field. The Hessian matrix of the QM region was built by taking into account bonded and nonbonded interactions with the protein environment and projected onto the internal coordinate space. Force constants were scaled with a global set of scaling factors, and the Raman intensities were computed using a finite-field method combined with a fourth-order differentiation algorithm for the calculation of the polarizability derivatives. In general, the QM/MM results provided a substantially improved description of the experimental resonance Raman (RR) spectra of the protein-bound cofactor compared to QM calculations of isolated PCB models in vacuo. The results allow the assessment of the effect of the protein-cofactor interactions on the RR spectra and reveal the potential and limitations of QM calculations on isolated tetrapyrroles for determining the chromophore structures in the various species and states of phytochromes for which three-dimensional structures are not available.
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