Direct observation of long-lived radical pair between flavin and guanine in single- and double-stranded DNA-oligomers.

The mechanism by which cryptochrome (CRY) proteins are capable of sensing weak magnetic fields (e.g., the geomagnetic field: ~50 μT) was suggested to be mediated by spin-correlated radical pairs (SCRPs) comprising a flavin adenine dinucleotide (FAD) radical and a tryptophan (Trp) radical which are formed simultaneously by light-induced electron transfer (ET). Here, we provide evidence for direct photoinduced ET that leads to long-lived SCRPs comprising a flavin (Fl) radical and a guanine (G) radical in flavin-tethered single- and double-stranded DNA oligomers by using time-resolved electron paramagnetic resonance (TREPR) spectroscopy. Transient absorption (TA) spectroscopy and its magnetic field effect (MFE) identified RP generation via a triplet-state precursor, in contrast to RP generation via a singlet-state precursor in CRY. Our findings of RPs in Fl-DNA oligomers having microsecond-long lifetimes and capable of exerting a large MFE at room temperature may significantly impact on our understanding of biological magnetoreception.