Alternatively, the F126 radical, or radical cation, may rearrange to neighbouring amino acids in processes mediated by hydrogen atom abstractions , or via oxidation of amide carbonyls (by the F126 radical cation) , yielding free radicals at peptide α-carbon sites on the protein backbone. Viehe et al have proposed that α-carbon radicals are especially stabilized thermodynamically by capto-dative effects, i.e. simultaneously by electron-withdrawing (-C=O) and electron-donating (-NR2) groups and, furthermore, that they readily combine with other radicals. Hence, depending on the lifetimes and the locations of these radicals on the protein backbone, "indirect" cross-links to U1 may form, yielding products that are identified by mass spectral analysis after controlled RNA-and protein-degradation (e.g. 5-7; Fig. ). These steps are consistent with the outcome of cross-linking reactions of F126 mutants, i.e. the exchange of phenylalanine for histidine and tyrosine produced similar direct and indirect cross-links, whereas leucine was mostly inactive since its aliphatic side chain cannot partake in the initial electron transfer.