Flavin mononucleotide (FMN) is a ubiquitous blue-light
pigment
due to its ability to drive one- and two-electron transfer reactions.
In both light-oxygen-voltage (LOV) domains of phototropin from the
green algae Chlamydomonas reinhardtii, FMN is noncovalently bound. In the LOV1 cysteine-to-serine mutant
(C57S), light-induced electron transfer from a nearby tryptophan occurs,
and a transient spin-correlated radical pair (SCRP) is formed. Within
this photocycle, nuclear hyperpolarization is created by the solid-state
photochemically induced dynamic nuclear polarization (photo-CIDNP)
effect. In a side reaction, a stable protonated semiquinone radical
(FMNH·) forms undergoing a significant bathochromic
shift of the first electronic transition from 445 to 591 nm. The incorporation
of phototropin LOV1-C57S into an amorphous trehalose matrix, stabilizing
the radical, allows for application of various magnetic resonance
experiments at ambient temperatures, which are combined with quantum-chemical
calculations. As a result, the bathochromic shift of the first absorption
band is explained by lifting the degeneracy of the molecular orbital
energy levels for electrons with alpha and beta spins in FMNH· due to the additional electron.