Lifetime of tyrosine fluorescence in nucleosome core particles.

The fluorescence intensity and fluorescence lifetime of the tyrosine residues in calf thymus nucleosome core particles have been determined as functions of the ionic strength of the solvent. For interpreting the results, in the first approximation, the 30 tyrosine residues involved in the particle are classified into two groups. About 12 belong to class I; they are distributed in the protein core with an average distance of 2.0 nm from its center. In the intact particle (in 20 mM to 0.4 M salt solution), a Förster-type energy transfer is considered to take place from these class-I tyrosine residues to the DNA bases, but this no longer occurs on elevating the salt concentration to about 1.4 m. The remaining tyrosine residues (about 18, called class II) are considered to be involved in hydrogen bonds or in some other intramolecular interactions in the intact core particles, so that their fluorescence is completely quenched. On elevating the salt concentration to 2.0 M, this quenching is partially removed. Implications of these dynamic and static quenching are discussed in terms of the structure of the core particle.