Fig. 3 Chromophore-surrounding structure of GdFP in comparison to EGFP and ECFP. The
schematic representation of interactions between the protein matrix and the chromophore of GdFP
(a) and ECFP (b; major configuration A 0 ) are plotted. Note the additionally appearing possible
interaction between Phe165 and the amino group of the chromophore in GdFP. (c) Stereo-view of
the chromophores in EGFP ( green ) ECFP ( blue ) and GdFP ( yellow ) and their environments.
Tyr145 and His148 of the ECFP minor-form are shown in red . Note a slight shift of the amino
indole moiety of the GdFP chromophore toward the Phe165
are 3.2-4.5 ˚ apart, which makes interactions between the amino group and the
aromatic ring of Phe165 possible. This novel interaction was assigned as an
additional factor that might further contribute to more effective intramolecular
charge transfer in the excited state of the GdFP chromophore when compared to
the ground state. Thus, it cannot be excluded that this novel interaction is also
involved in the occurrence of highly red-shifted absorbance and fluorescence as
well as the large Stokes shift.
Interestingly, careful inspection of the residue configurations in the chromo-
phore vicinity revealed that the chromophore of GdFP has less hydrophobic
contacts with other amino acids in its proximity than the ECFP chromophore
(Fig. 3a, b ). This could be associated with higher flexibility of the chromophore
substantially contributing to the increase in radiationless fluorescence decay [ 23 ].
Indeed, this was found in GdFP since the quantum yield was lower when com-