Resonance energy transfer between cysteine-34 and tryptophan-214 in human serum albumin. Distance measurements as a function of pH.

The single cysteine residue (Cys-34) of human serum albumin was modified with the organic mercurial [4-[p-(dimethylamino)phenyl]azo]phenyl]mercuric acetate. Introduction of this chromophore into the protein results in the quenching of the protein tryptophan fluorescence spectrum due to energy transfer from the tryptophan residue to the mercurial. Since human albumin contains only a single tryptophan, it was then possible to calculate distances between the mercurial bound at Cys-34 and Trp-214 under various conditions. This distance contracted during the course of the N leads to F transition, being 34-35 A in the N conformation (pH 6-7.5) and 29.9 A in the F conformation (pH 3.6). The distance increased substantially during the course of the F leads to E transition occurring between pH 3.6 and pH 1.9 and was found to be nearly 37 A at pH 1.9. The distance between Cys-34 and Trp-214 was found to undergo a slight contraction during the N leads to B transition occurring between pH 7.0 and pH 9.0. At pH 8.5-9 where the protein is predominately in the B form, the distance was found to be slightly more than 31 A.