Principles of Fluorescence Spectroscopy

544 PROTEIN FLUORESCENCE
Figure 16.23. Emission spectra of the M13 procoat protein. The positions
of the tyrosine and tryptophan residues are shown on the figure.
Excitation at 280 (dashed) or 295 nm (dotted). The difference spectra
are shown as solid lines. Revised from [91].
tyrosine and tryptophan dominate the emission of most proteins,
and proteins are usually not excited at 260 nm where
phenylalanine absorbs. Phenylalanine emission was detected
from an unusual histone-like protein, HTa, from the thermophilic
archaebacterium Thermoplasma acidophilum. 82
HTa associates strongly with DNA to protect it from thermal
degradation. This highly unusual protein is a tetramer,
with five phenylalanine residues and one tyrosine residue in
each monomer, and no tryptophan residues (Figure 16.26).
In this class of organisms the more thermophilic variants
have higher phenylalanine contents in their histone-like
proteins. An experimentally useful property of this protein
is the ability to remove the tyrosine residues, located at the
Figure 16.24. Excitation spectra of mutant M13 procoat proteins. The
lower panel shows the quantum yields normalized according to eq.
16.2. Revised from [91].
third position from the carboxy terminus, by digestion with
carboxypeptidase A.
The emission spectrum of HTa excited at 252 nm is
shown in Figure 16.26 (solid, top panel). The emission
Figure 16.25. Structure and RET efficiencies for the mutant M13 procoat
proteins. Data reprinted with permission from [91]. Copyright ©
2001, American Chemical Society.