A structure is proposed for F430M, a non-cristalline methanolysis product of isolates of the nickel-containing, porphinoid factor F430 from Methanobacterium thermoautotrophicum.Crucial to the structure determination are five incorporation experiments with M . thermoautotrophicum (strain Marburg) in which the specifically m~n o -~~C -l a beled biosynthetic precursors (2-13C)-, (3-I3C)-, (4-I3C)-, (5-I3C) ALA (ALA = 6-amino-levulinic acid) and ~-(methyl-'~C)methionine were incorporated into F430 with high efficiency. The I3C-NMR.-spectra of the specifically labeled F430M samples derived therefrom, together with the UV./VIS. spectral data of F430M, contain all the information necessary for the deduction of the constitution of the F430M chromophore, assuming the established pattern of porphinoid biosynthesis to be operative in F430 biosynthesis. IH-NMR. spectroscopy and, in particular, 'H-NMR.-NOE-difference spectroscopy corroborates and completes the constitutional assignments and, furthermore, makes possible an almost complete derivation of the molecule's relative configuration. Schemes 3 and 4 summarize the results of 'H-NMR. spectroscopy, presenting them within the context of the proposed structure for F430M. The assignment of absolute configuration implied in the formula is given preference because of F430M's very close structural and (assumed) biosynthetic relationship to sirohydrochlorin and vitamin BI2 (with respect to ring C, the assignment is based on degradative evidence).According to the proposed structure, the nickel complex F430M possesses an uroporphinoid (Type 111) ligand skeleton with an additional carbocyclic ring and a chromophore system not previously encountered among natural porphinoids. It can be considered to be a (tetrahydro) derivative of the corphin system, combining structural elements of both porphyrins and corrins. I ) Arbeitsgruppe f u r physikalische organische Chemie (Leitung Prof. J . F. M. Oh), Organisch-chemisches Laboratorium der ETHZ. 0018-019X/82/3/0828-38$01.00/0 0 1982 Schweizerische Chemische Gesellschaft HELVETICA CHIMICA ACTA -Vol. 65, Fasc. 3 (1982) -Nr. 81 829 14) Die Strukturen I, J und K zeigen die hier als UV./VIS.-Modellsysteme massgebenden Chromophortypen. u b e r deren UV./VIS.-Spektren vgl. (151. I J K