Effects of the Surroundings on Molecular Transition Energies

DNA photolyases, which use the energy of blue light to split pyrimidine dimers formed by UV irradiation of DNA, provide other examples of large and variable shifts in the absorption spectmm of a bound chromophore. These enzymes contain a bound pterin (methylenetetrahydrofolate, MTHF) or deazaflavin, which serves to absorb light and transfer energy to a flavin radical in the active site [81]. The absorption maximum of MTHF occurs at 360 nm in solution, but ranges from 377 to 415 nm in the enzymes from different organisms [82]. 

Solvatochromic effects on the transition energies of molecules in solution often can be related phenomenologically to the solvent s dielectric constant and refractive index. The analysis is similar to that used for local-field correction factors (Sect. 3.1.5). Polar solvent molecules around the chromophore will be ordered in response to the chromophore s ground-state dipole moment (/taa), and the oriented 

Now suppose that excitation of the chromophore changes its dipole moment to Uhl,. Although the solvent molecules cannot reorient instantaneously in response, the dielectric constant includes electronic polarization of the solvent in addition to orientational polarization, and changes in electronic polarization can occur essentially instantaneously in response to changing electric fields. The high-frequency component of the dielectric constant is the square of the refractive index (n) (Sects. 3.1.4 and 3.1.5). If we subtract the part of the reaction field that is attributable to electronic polarization, the part due to orientation of the solvent Eor) can be written 

The energy of interactions of a molecule with its surrotmdings can be treated in a more microscopic way by using Eqs. (4.19-4.21) to describe the grotmd and excited-state wavefunctions, tpa and V - The solvation energy of the molecule in the ground state is 

If the stmcture of the surroundings is well defined, as it may be for a chromo-phore in a protein (see, for example, the visual pigments shown in Fig. 4.25), the electric potential at each point in the chromophore can be estimated by summing the contributions from the charges and dipoles of the surrounding atoms  

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