A protein from a species of microscopic algae makes an unusual C–C cross-link between valine and phenylalanine, reports a group from Oregon State University (Science, DOI: 10.1126/science.1205687). Many proteins form cross-links between amino acids to generate a catalytic cofactor or stabilize the structure, but they typically involve nitrogen, oxygen, or sulfur functional groups. Led by Daniel J. Arp and P. Andrew Karplus, the Oregon State group studied the protein symerythrin from Cyanophora paradoxa. The function of symerythrin is unknown, but on the basis of its similarity to other proteins, it may be involved in reducing reactive oxygen species. To form the cross-link, the researchers propose that a diiron site in the enzyme forms an Fe(IV)2O2 intermediate, which abstracts a hydrogen atom from a methyl group on a nearby valine residue to produce an alkyl radical. The radical species then adds to the phenyl ring of a phenylalanine, generating a cyclohexadienyl radical. The cross-linking reaction finishes with deprotonation coupled with transfer of an electron back to the diiron center. The cross-link anchors the N-terminal tail to the core of the protein and creates a substrate-binding pocket, the group suggests.