Depolarization and Energy Transfer

Energy transfer is a source of depolarization. For example, a high energy-transfer efficiency in hemoproteins can be evidenced by plotting Perrin plot at different temperatures. 

The cytochrome b2 core from the yeast Hansenula anomala has a molecular mass of 14 kDa, and its sequence shows the presence of two tryptophan residues. Their fluorescence intensity decay can be adequately described by a sum of three exponentials. Lifetimes obtained from the fitting are equal to 0.054,0.529, and 2.042 ns, with fractional intensities equal to 0.922, 0.068, and 0.010. The mean fluorescence lifetime, r0, is 0.0473 ns. 

The data yield a rotational correlation time equal to 38 ps instead of 5.9 ns calculated theoretically for the cytochrome b2 core, with an extrapolated value A(o) of 0.208, lower than that (0.265) usually found for Trp residues at Xex = 300 nm at —45°C. The fact that the extrapolated anisotropy is lower than the limiting anisotropy means that the system is depolarized as a result of global and local motions within the protein. In this case, the value of the apparent rotational correlation time ( a) calculated from the Perrin plot is lower than the global rotational time of the protein ( bp). However, the fact that 4 a is 1000 times lower than 4 p indicates that a third process different than the global and local rotations is 

Albrecht, A.C. (1961). Polarizations and assignments of transitions The method of photoselection. Journal of Molecular Spectroscopy, 6, 84. 

Andres, S.F. and Atassi, M.Z. (1970). Conformational studies on modified proteins and peptides. 

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