Phenylalanine-to-tyrosine singlet energy transfer in the archaebacterial histone-like protein HTa

The Archaebacterium Thermoplasma acidophilum has a histone-like protein (HTa) abundantly associated with its deoxyribonucleic acid. Each native tetrameric complex of HTa contains 20 phenylalanine residues, 4 tyrosine residues, and no tryptophan. When the protein was excited by radiation at 252 nm, which is a wavelength absorbed predominantly by phenylalanine, the fluorescent emission was mostly from tyrosine. According to the excitation spectrum for this tyrosine fluorescence, the cause was energy transfer from phenylalanine, which occurred with about 50% efficiency. When the tyrosine residues were removed enzymatically, the excited-state lifetime of the phenylalanine residues nearly doubled. Because of energy transfer, the tyrosine emission had two apparent fluorescence decay lifetimes; one lifetime (3.9 ns) was that of tyrosine while the second (12.1 ns) corresponded to the excited state of phenylalanine.