The importance of life-time changes in fluorescence depolarization

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Abstract
The rotational relaxation time, ρ, calculated from measurements of fluorescence depolarization, is clearly dependent on the assumed mean life-time, τ, of the excited state. However, variations in τ with experimental conditions (temperature and solvent composition) occur and the effect of such alterations in τ is demonstrated. In particular it should be noted that, unless life-time changes can be excluded, the occurrence of linear plots of reciprocal degree of polarization against the temperature/viscosity ratio does not necessarily indicate the absence of intramolecular freedoms. An attempt to correct for such life-time changes by measurement of the fluorescence intensity is made for the bovine serum albumin–1-dimethyl-aminonaphthalene-5-sulphonyl chloride system. The value of ρ/3τ thus obtained for this system at 20° is approx. 4·7, as against approx. 3·4 obtained by several workers in the absence of life-time corrections.