The fluorescence increase on the deuterium oxide addition to the solvent medium 
was studied in various tryptophan- (or indole-) and/or tyrosine-containing model 
compounds. It was shown how the rates of the deuteration at the indole NH group 
of tryptophan and at the OH group of tyrosine could be followed independently of 
each other. The method was applied to a study of erabutoxin b molecule, a 
neurotoxic protein from a sea snake, to analyze the microenvironments of its 
single tryptophan and tyrosine residues. It was shown that the "functionally 
invariant" single tryptophan residue was exposed to the solvent in the surface 
of the molecule and that the "structurally conserved" single tyrosine residue 
was buried in the molecule. The rate of deuteration of the tyrosine residue (80 
s(-1) at pH 6.3 and 33 degrees C) was 1/20 of that of an exposed tyrosine. It 
was also found that the amino group of Lys-27 quenched the fluorescence of 
Trp-29 but its deuteration had no effect on the fluorescence.