Quantum Dynamics of Electronic Excitations in Biomolecular Chromophores: Role of the Protein Environment and Solvent

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• 作者：Joel Gilmore ; Ross H. McKenzie
• 刊名：Journal of Physical Chemistry A
• 出版年：2008
• 出版时间：March 20, 2008
• 年：2008
• 卷：112
• 期：11
• 页码：2162 - 2176
• 全文大小：792K
• 年卷期：v.112,no.11(March 20, 2008)
• ISSN：1520-5215

文摘

A biomolecular chromophore can be viewed as a quantum system with a small number of degrees of freedominteracting with an environment (the surrounding protein and solvent) which has many degrees of freedom,the majority of which can be described classically. The system-environment interaction can be described bya spectral density for a spin-boson model. The quantum dynamics of electronic excitations in the chromophoreare completely determined by this spectral density, which is of great interest for describing quantum decoherenceand quantum measurements. Specifically, the spectral density determines the time scale for the "collapse" ofthe wave function of the chromophore due to continuous measurement of its quantum state by the environment.Although of fundamental interest, there very few physical systems for which the spectral density has beendetermined experimentally and characterized. In contrast, here, we give the parameters for the spectral densitiesfor a wide range of chromophores, proteins, and solvents. Expressions for the spectral density are derived forcontinuum dielectric models of the chromophore environment. There are contributions to the spectral densityfrom each component of the environment: the protein, the water bound to the protein, and the bulk solvent.Each component affects the quantum dynamics of the chromophore on distinctly different time scales. Ourresults provide a natural description of the different time scales observed in ultrafast laser spectroscopy,including three pulse photon echo decay and dynamic Stokes shift measurements. We show that even if thechromophore is well separated from the solvent by the surrounding protein, ultrafast solvation can be still bedominated by the solvent. Consequently, we suggest that the subpicosecond solvation observed in somebiomolecular chromophores should not necessarily be assigned to ultrafast protein dynamics. The magnitudeof the chromophore-environment coupling is sufficiently strong that the quantum dynamics of electronicexcitations in most chromophores at room temperature is incoherent, and the time scale for "collapse" of thewave function is typically less than 10 fs.