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Absorption spectra of biological systems from TDDFT

Authors: M.A.L. Marques

Ref.: Mini-workshop on TDDFT, organized by F. Nogueira. Departamento de Física da Universidade de Coimbra, Portugal, 6/5/2005 (2005)

Abstract: An extensive study of the optical absorption spectra of the green fluorescent protein (GFP) and its blue mutant (BFP) is presented. We investigate different protonation states of the chromophore and analyze the role of the protein environment and of thermal fluctuations. The role of the environment is twofold: i)~it induces structural modifications of the gas-phase chromophore, the most important being the torsion of the imida-rings; and ii)~it makes a local-field modification of the external electromagnetic field. It turns out that the torsion of the imida-rings shifts significantly the gas-phase spectra to lower energies, whereas the consistent inclusion of the closest residues field produces only minor modifications on the spectra. The calculated spectra turn out to be in very good agreement with experimental and allow the extraction of relevant chemical information from the absorption spectra. These results demonstrate the power of combining time-dependent density functional calculations and optical absorption measurements to discern the relevant chemical information on the nature and state of chromopeptides.

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