The blue-light sensitive photoreceptor, phototropin, is a flavoprotein which regulates thephototropis
m response of higher plants. The photoinduced triplet state and the photoreactivity of the flavin-
mononucleotide (FMN) cofactor in two LOV do
mains of
Avena sativa,
Adiantum capillus-veneris, and
Chlamydomonas reinhardtii phototropin have been studied by ti
me-resolved electron para
magneticresonance (EPR) and UV-vis spectroscopy at low te
mperatures (
T 
mages/entities/le.gif"> 80 K). Differences in the electronicstructure of the FMN as reflected by altered zero-field splitting para
meters of the triplet state could becorrelated with changes in the a
mino acid co
mposition of the binding pocket in wild-type LOV1 and LOV2as well as in
mutant LOV do
mains. Even at cryogenic te
mperatures, ti
me-resolved EPR experi
ments indicatephotoreactivity of the wild-type LOV do
mains, which was further characterized by UV-vis spectroscopy.Wild-type LOV1 and LOV2 were found to for
m an adduct between the FMN cofactor and the functionalcysteine with a yield of 22% and 68%, respectively. The absorption
maxi
mu
m of the low-te
mperaturephotoproduct of wild-type LOV2 is red-shifted by about 15 n
m as co
mpared with the FMN C(4a)-cysteinyladduct for
med at roo
m te
mperature. In light of these observations, we discuss a radical-pair reaction
mechanis
m for the pri
mary photoreaction in LOV do
mains.
