Fluorescence-polarization spectrum and electronic-energy transfer in proteins

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Abstract
The polarization spectrum of insulin, ribonuclease and zein is qualitatively identical with that of tyrosine or cresol, but the lower absolute values of the principle polarization indicate the existence of energy transfer among the tyrosine residues. The polarization spectrum of proteins containing tryptophan is similar to that of N-glycyltryptophan but differs from it in the lower values of the polarization in the 270 m[mu] region. The ratio of the polarization on excitation by 305 m[mu] and by 270 m[mu] (P305/P270 ratio), which varies from 1.4 to 1.7 in the simple indole derivatives, is greater than 2 in 9 out of 10 globular proteins studied. The polarization spectrum in 50% propylene glycol-water at -70[degree] shows an increased P305/P270 ratio (2.3-3.0), and the polarization spectrum in 8[image]-urea shows this ratio decreased to the range 1.4-1.9 in 8 out of 10 proteins studied. The changes in urea are only partially reversible. The soluble fractions obtained in 3 proteins had a distinctly changed polarization spectrum with a lowered P305/P270 ratio. Though energy transfer among the tryptophan residues may play a part in the observed effects, it is believed that a change in the relative intensities of the G-S1 and G-S2 transitions in tryptophan can explain these effects equally well.