Emission anisotropy excitation polarization spectrum

Since electronic transitions differ from one excitation wavelength to another, the value of P would change with excitation wavelength. Emission generally occurs from the lowest excited state Si Vo, and so one can measure anisotropy or polarization along the absorption spectrum at a fixed emission wavelength. We obtain a spectrum called the excitation polarization spectrum or simply the polarization spectrum (Figure 11.2). 

The use of 1-dimethylamino-5-naphthyl sulphonate ("dansyl") as a label was first introduced in 1952 for the determination of the rotational diffusion of protein molecules from the anisotropy of the dansyl fluorescence (1). Somewhat later, (2) it was found that the emission intensity of fluorophores of this type is strongly increased when they are adsorbed from water solution on proteins This increase in quantum yield of emission (accompanied by a blue-shift of the emission spectrum) could be correlated with a decreasing polarity of the medium (3). The effect can be explained as follows If the dipolar interaction of the excited chromophore with the solvent is smaller than this interaction in the ground state, the emitted quantum will be increased at the same time, the probability of nonradiative deactivation will be reduced, increasing the quantum yield of fluorescence (4). If the fluorescent label can be attached to a well-defined site on a protein, its emission characteristics will "report" about the polarity of this site (5) and the use of... 

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