Protein molecules absorption rate constants

The refractive index n in Eqs. (7.20) and (7.21) pertains to the bulk medium in which the absorption and emission spectra of the chromophores are measured. This factor cancels out when Eqs. (7.20) and (7.21) are combined in Eq. (7.23). The factor of that remains in Eqs. (7.23), (7.24), (7.27), and (7.28) enters through H2 ei) and reflects the attenuation of high-frequency electrostatic interactions between the energy donor and acceptor. It therefore pertains to the protein or other material that surrotmds the interacting molecules, and not to the solvent used to measure the individual spectra. The appropriate value of the refractive index usually is not known accurately, but for proteins probably is comparable to that of 7/-methylacetamide (1.43 at 589.3 run). Using the refractive index of water (1.33) would increase the calculated rate constant for energy transfer [(1.43/1.33)" 1.34], but the resulting error in Rq or / 2i would be relatively small [(1.43/1.33)" 1.05]. 

---