However, a study of the photo-induced intramolecular cyclization of 5-benzyluracil and 6-benzyluracil via benzyl and uracil radical intermediates suggests a plausible model for the cross-linking of U1 with F126 (Fig. ) . Hence, photo-induced electron transfer in the U1-F126 complex generates a shortlived anion/cation radical pair (exciplex) (Fig. ; 1 and 2). Subsequent protonation of the uracil radical anion can yield a neutral α-hydroxy radical , whereas ready deprotonation of the F126 radical cation will produce a stabilized benzylic radical. In the absence of oxygen, the major fate of these free radicals is recombination with formation of the direct U1-F126 cross-link (Fig. ). An analgous mechanism has been proposed for the reaction between free uracils/halognated uracils and tyrosine derivatives . (bonds in red are formed upon cyclisation). b) Possible mechanism of UV cross-linking between the stacked F126 und U1 of the FOXRRM (1). Photo-induced electron transfer leads to a radical ion pair (2). After protonation/deprotonation steps, the radicals on the benzylic position of F126 and C4 of uridine (3) recombine to yield direct cross-links (4). Indirect cross-links between U1 and R127 (5), P125 (6) or I124 (7) may form when the radical cation of F126 oxidizes amide carbonyls from flanking amino acids, which rearrange to radicals stabilized by capto-dative effects at the α-carbons (*).