Fig. 9 “Photoinduced decarboxylation and conformational changes in the chromophore vicinity.
Coordinates of the wild-type structure (PDB 1gfl) are shown with carbon atoms in green , whereas
the structure of the photoproduct GFP R is shown with carbon atoms in yellow . Electron density
shown is the refined 2Fo-Fc map contoured at 1.6 s for the 1.8 ˚ resolution structure.
(a) Conformational changes of Thr 203 and His 148. (b) Absence of electron density for the
O e 1, O e 2 and Cd atoms of Glu 222 in GFP R .(c) Proposed hydrogen bonds in the chromophore
vicinity of chain A in the GFP 483 structure include solvent molecules 173[Z], 175[Z] and 203[Z].
(d) Chromophore structure in GFP A . The hydrogen-bonding network includes solvent atoms 25
and 63” (reproduced with permission [ 19 ])
transfer of charge between the phenolic- and imidazolidinone rings of the chromo-
phore will compete (efficiently) with electron transfer from Glu 222 to the chromo-
phore. If an electron is transferred, the g-carboxyl radical of Glu 222 will
decarboxylate via the “Kolbe” mechanism [ 19 ]. This then leads to retro-transfer of
an electron and a proton, or alternatively a hydrogen radical (Fig. 10 ).An experi-
mental test for the accumulation of a radical intermediate was negative, which shows
that at least at 200K, the lifetime of the carboxylate radical intermediate as well as
the predicted chromophore radical intermediate is less than ~7 s. Considering the
distances involved, these processes can proceed in the ns time domain, which is also
in agreement with the low quantum efficiency of the reaction. Together with the low-
and room-temperature photoconversion experiments, a two-step mechanism of the
photoconversion of GFP A to GFP R was proposed.