We consider continuum dielectric models as minimal models to understand the effect of a surrounding globular protein and solvent on the quantum dynamics of electronic excitations in a biological chromophore. For these models we derive expressions for the frequency dependent spectral density which describes the coupling of the electronic levels in the chromophore to its environment. The magnitude and frequency dependence of the spectral density determines whether or not the quantum dynamics is coherent or incoherent, and thus whether on not one can observe quantum interference effects such as Rabi oscillations. We find the contributions to the spectral density from each component of the chromophore environment: the bulk solvent, protein, and water bound to the protein. The relative importance of each component to the quantum dynamics of the chromophore is determined by the time scale on which one is considering the dynamics. Our results provide a natural explanation and model for the different time scales observed in the spectral density extracted from the solvation dynamics probed by ultra-fast laser spectroscopy techniques such as the dynamic Stokes shift and three pulse photon echo spectroscopy. Our results are used to define under what conditions the dynamics of the excited chromophore is dominated by the surrounding protein and when it is dominated by dielectric fluctuations in the solvent. We show that even when the chromophore is shielded from the solvent by the protein ultra-fast solvation can be dominated by the solvent. Hence, we suggest that the ultra-fast solvation recently seen in some biological chromophores should not necessarily be assigned to ultra-fast protein dynamics. The magnitudes of the spectral density that we estimate from our continuum models and extracted from experiment suggest that most quantum dynamics of electronic excitations is incoherent. A possible exception is transfer of excitons between neigbouring chromophores in photosynthetic systems.

We give a theoretical treatment of the interaction of electronic excitations (excitons) in biomolecules and quantum dots with the surrounding polar solvent. Significant quantum decoherence occurs due to the interaction of the electric dipole moment of the solute with the fluctuating electric dipole moments of the individual molecules in the solvent. We introduce spin boson models which could be used to describe the effects of decoherence on the quantum dynamics of biomolecules which undergo light-induced conformational change and on biomolecules or quantum dots which are coupled by Förster resonant energy transfer.

Previously reported excitation spectra for eumelanin are sparse and inconsistent. Moreover, these studies have failed to account for probe beam attenuation and emission reabsorption within the samples, making them qualitative at best. We report for the first time quantitative excitation spectra for synthetic eumelanin, acquired for a range of solution concentrations and emission wavelengths. Our data indicate that probe beam attenuation and emission reabsorption significantly affect the spectra even in low-concentration eumelanin solutions and that previously published data do not reflect the true excitation profile. We apply a correction procedure (previously applied to emission spectra) to account for these effects. Application of this procedure reconstructs the expected relationship of signal intensity with concentration, and the normalized spectra show a similarity in form to the absorption profiles. These spectra reveal valuable information regarding the photophysics and photochemistry of eumelanin. Most notably, an excitation peak at 365 nm (3.40 eV), whose position is independent of emission wavelength, is possibly attributable to a 5,6-dihydroxyindole-2-carboxylic acid (DHICA) component singly linked to a polymeric structure.

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Articles you may be interested inThe quantum yield of synthetic eumelanin is known to be extremely low and it has recently been reported to be dependent on excitation wavelength. In this paper, we present quantum yield as a function of excitation wavelength between 250 and 500 nm, showing it to be a factor of 4 higher at 250 nm than at 500 nm. In addition, we present a definitive map of the steady-state fluorescence as a function of excitation and emission wavelengths, and significantly, a three-dimensional map of the "specific quantum yield": the fraction of photons absorbed at each wavelength that are subsequently radiated at each emission wavelength. This map contains clear features, which we attribute to certain structural models, and shows that radiative emission and specific quantum yield are negligible at emission wavelengths outside the range of 585 and 385 nm ͑2.2 and 3.2 eV͒, regardless of excitation wavelength. This information is important in the context of understanding melanin biofunctionality, and the quantum molecular biophysics therein.