Tyrosine-to-tryptophan energy transfer and the structure of calf gamma-II crystallin.

The efficiency of electronic energy transfer from tyrosine to tryptophan residues in the lens protein calf gamma II crystallin has been determined from measurements of fluorescence excitation spectra at 25 degrees C and independently from phosphorescence spectroscopy at 77 K. The total transfer efficiency from the fifteen tyrosines to the four tryptophans in native calf gamma II was found to be t = 78 +/- 10%, at 25 degrees C. The expected value based on Förster theory and the X-ray structure of the protein is t = 83%, in good agreement with the present experimental result. The transfer efficiency measured in denatured calf gamma II at 25 degrees C was t = 20 +/- 10%, in good agreement with the value t = 25% expected for the completely denatured protein, based on Förster theory and the known amino acid sequence. These results are of interest for several reasons. First, energy transfer can provide a simple , experimental confirmation of lens protein structures determined from X-ray data or from computer modeling studies. Second, the present studies show that energy transfer measurements can be used to monitor the effect of denaturants on lens protein structure, an aspect not readily investigated by X-ray crystallography. Third, the present electronic energy transfer studies may be relevant to understanding the mechanism of UV photodamage in lens proteins and hence in whole lenses.