Biosynthesis of vitamin B12. In search of the porphyrin-corrin connection

Three Manganese Complexes of Anionic N4-Donor Schiff-Base Macrocycles: Monomeric MnII and MnIII, and dimeric MnIV

Three manganese macrocyclic complexes of two anionic N4-donor [1+1] Schiff-base macrocycles that differ in ring size (14 versus 16 membered), HLEt and HLPr (obtained from condensation of diphenylamine-2,2′-dicarboxaldehyde and either diethylenetriamine or dipropylenetriamine), are reported. Specifically, a pair of monomeric complexes MnIILEt(NCS)(H2O) and [MnIIILPr(NCS)2]·0.5H2O, plus a dimeric complex [MnIV2LEt2(O)2](ClO4)2·3DMF have been synthesised and characterised. Single crystal structure determinations on [MnIIILPr(NCS)2]·0.5H2O and [MnIV2LEt2(O)2](ClO4)2·3DMF revealed octahedral manganese centres in both cases: N6-coordinated Jahn–Teller distorted MnIII in the former and a pair of N4O2-coordinated MnIV in the latter. UV-Vis, IR, and electron paramagnetic resonance spectroscopy as well as magnetic measurements are reported. These macrocyclic complexes feature a simple and original design, and could find future uses as models for manganese catalase or as building blocks for the assembly of larger supramolecular architectures.

Probing the rate of hole transfer in oxidized synthetic chlorin dyads via site-specific (13)C-labeling

Understanding electronic communication among interacting constituents of multicomponent molecular architectures is important for rational design in diverse fields including artificial photosynthesis and molecular electronics. One strategy for examining ground-state hole/electron transfer in an oxidized tetrapyrrolic array relies on analysis of the hyperfine interactions observed in the EPR spectrum of the pi-cation radical. This strategy has been previously employed to probe the hole/electron-transfer process in oxidized multiporphyrin arrays of normal isotopic composition, wherein (1)H and (14)N serve as the hyperfine "clocks", and in arrays containing site-specific (13)C-labels, which serve as additional hyperfine clocks. Herein, the hyperfine-clock strategy is applied to dyads of dihydroporphyrins (chlorins). Chlorins are more closely related structurally to chlorophylls than are porphyrins. A de novo synthetic strategy has been employed to introduce a (13)C label at the 19-position of the chlorin macrocycle, which is a site of large electron/hole density and is accessible synthetically beginning with (13)C-nitromethane. The resulting singly (13)C-labeled chlorin was coupled with an unlabeled chlorin to give a dyad wherein a diphenylethyne linker spans the 10-positions of the two zinc chlorins. EPR studies of the monocations of both the natural abundance and (13)C-labeled zinc chlorin dyads and benchmark zinc chlorin monomers reveal that the time scale for hole/electron transfer is in the 4-7 ns range, which is 5-10-fold longer than that in analogous porphyrin arrays. The slower hole/electron transfer rate observed for the chlorin versus porphyrin dyads is attributed to the fact that the HOMO is a(1u)-like for the chlorins versus a(2u)-like for the porphyrins; the a(1u)-like orbital exhibits little (or no) electron/hole density at the site of linker attachment whereas the a(2u)-like orbital exhibits significant electron/hole density at this site. Collectively, the studies of the chlorin and porphyrin dyads provide insights into the structural features that influence the hole/electron-transfer process.

Recent studies of enzymically controlled steps in B12 biosynthesis

The acquisition and sequencing of the genes encoding the enzymes for vitamin B12 biosynthesis in Salmonella typhimurium and Pseudomonas denitrificans has dramatically altered the direction of research on the pathway from uroporphyrinogen III to the corrinoids. Through a combination of molecular biology, organic chemistry and NMR spectroscopy, logical progression along the sequence is being made. Recent work from our laboratory is focused on the discovery and specificities of the methyltransferases connecting uroporphyrinogen III with cobyrinic acid, the temporal resolution of cobalt insertion and a comparison of the anaerobic pathway in S. typhimurium and the aerobic pathway in Ps. denitrificans. The implication of two parallel routes to corrins in these bacteria is discussed.

Meso-13C-Labeled Porphyrins for Studies of Ground-State Hole Transfer in Multiporphyrin Arrays

Understanding electronic communication among interacting chromophores provides the foundation for a variety of applications. The ground-state electronic communication in diphenylethyne-linked zinc-porphyrin dyads has been investigated by a novel molecular design strategy that entails introduction of a 13C-atom (*) at specific sites of the porphyrins where there is substantial electron density in the relevant frontier (highest occupied) molecular orbital. The site of 13C substitution is at a meso-position, either the site of attachment of the linker (proximal, "P") or the site trans to the linker (distal, "D"). The substituents (R) at the non-linking meso-positions are mesityl, tridec-7-yl ("swallowtail"), or p-tolyl groups. Altogether five isotopically labeled porphyrin dyads have been prepared. The hole/electron-transfer properties of one-electron oxidized dyads have been examined by electron paramagnetic resonance (EPR) spectroscopy. The introduction of the meso-13C label provides a "clock" (via the hyperfine interactions) that allows investigation of a time scale for hole transfer that is 3-4 times shorter than that provided by the natural abundance 14N nuclei of the pyrrole nitrogen atoms. The EPR studies indicate that the hole transfer, which has been previously shown to be fast on the time scale of the 14N hyperfine clock ( approximately 220 ns), remains fast on the time scale of the 13C hyperfine clock ( approximately 50 ns).

5-Aminolevulinic acid formation from glutamate via the C5 pathway in Clostridium thermoaceticum

A cell-free extract of the anaerobic eubacterium, Clostridium thermoaceticum, catalyzes the synthesis of 5-aminolevulinic acid (ALA) from glutamate via the C5 pathway. The enzyme reaction resembles that of higher plants and algae in cofactor requirements and sensitivity to ribonuclease. From the phylogenetic distribution it is proposed that the C5 pathway evolved earlier than the ALA synthase pathway.

Biosynthesis of coenzyme F430 in methanogenic bacteria. Identification of 15,17(3)-seco-F430-17(3)-acid as an intermediate

Coenzyme F430 is a hydroporphinoid nickel complex present in all methanogenic bacteria. It is part of the enzyme system which catalyzes methane formation from methyl-coenzyme M. We describe here that under certain conditions a second nickel porphinoid accumulates in methanogenic bacteria. The compound was identified at 15,17(3)-seco-F430-17(3)-acid. The structural assignment rests on 14C-labelling experiments, fast-atom-bombardment mass spectra, 1H-NMR spectra of the corresponding hexamethyl ester, and ultraviolet/visible spectral comparison with model compounds. In cell extracts and in intact cells of methanogenic bacteria, 15,17(3)-seco-F430-17(3)-acid was converted to F430. These findings indicate that the new nickel-containing porphinoid is an intermediate in the biosynthesis of coenzyme F430.

Cloning of multiple genes involved with cobalamin (Vitamin B12) biosynthesis in Bacillus megaterium

An effective shotgun cloning procedure was developed for Bacillus megaterium by amplifying gene libraries in Bacillus subtilis. This technique was useful in isolating at least 11 genes from B. megaterium which are involved with cobalamin (vitamin B12) biosynthesis. Amplified plasmid banks were transformed into protoplasts of both a series of Cob mutants blocked before the biosynthesis of cobinamide and Cbl mutants blocked in the conversion of cobinamide into cobalamin. Amplification of gene libraries overcame the cloning barriers inherent in the relatively low protoplast transformation frequency of B. megaterium. A family of plasmids was isolated by complementation of seven different Cob and Cbl mutants. Each plasmid capable of complementing a Cob or Cbl mutant was transformed into each one of the series of Cob and Cbl mutants; many of the plasmids isolated by complementation of one mutation carried genetic activity for complementation of other mutations. By these criteria, four different complementation groups were resolved. At least six genes involved in the biosynthesis of cobinamide are carried on a fragment of DNA approximately 2.7 kilobase pairs in length; other genes involved in the biosynthesis of cobinamide were located in two other complementation groups. The physical and genetic data permitted an ordering of genes within several of the complementation groups. The presence of complementing plasmids in mutants blocked in cobalamin synthesis resulted in restoration of cobalamin biosynthesis.