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The MeTr structure consists of a TIM barrel that embeds methyl-H4folate and cobamide. All related methyltransferases are predicted to fold into a similar TIM barrel pattern and have a similar pterin and cobamide binding site. The observed structure is consistent with either a 'front' (N5) or 'back' (C8a) side protonation of CH3-H4folate, a key step that enhances the electrophilic character of the methyl group, activating it for nucleophilic attack by Co(I).

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Biphenylene dimers, which may be considered as "macrocyclic" 2 × 2 fragments of net 1 and dimeric fragments of a double-stranded polymer, 5, are prepared. X-ray crystallography on two polymorphs of the dimer 9 reveals that its bond alternation is analogous to that in biphenylene; the tetraphenylene part of 9 has approximately C 2h symmetry with a small dihedral angle (<12°) between the benzene rings in the adjacent biphenylenes. Cyclic voltammetry of 9 gives two reversible waves corresponding to a radical anion and a dianion; the lithium and potassium salts of these anions, 9 •-,M + (M ) Li, K) and 9 2-,2M + (M ) Li, K), were generated and characterized by ESR, NMR, and UV-vis spectroscopies. The spectral data for the dianions are compatible with classical structures; no evidence for "in-plane aromaticity" is found. For polymer 5, a localized band and large density of states near the Fermi level are found, using extended Hu ¨ckel theory calculations.