Five-coordinate geometry is the standard binding mode of zinc porphyrins with pyridine ligands. Here we show that pseudo-octahedral six-coordinate zinc porphyrin complexes can also be formed in solution, by taking advantage of the chelate effect. UV–vis–NIR titrations indicate that the strength of this second coordination is ca. 6–8 kJ mol–1. We have used the formation of six-coordinate zinc porphyrin complexes to achieve the template-directed synthesis of a 3D π-conjugated spiro-fused array of 11 porphyrin units, covalently connected in a nontrivial topology. Time-resolved fluorescence anisotropy experiments show that electronic excitation delocalizes between the two perpendicular nanorings of this spiro-system within the experimental time-resolution of 270 fs.

Artificial influence: We describe a semi‐artificial adenosylpeptide B12 that behaves as a cofactor in B12‐dependent enzymatic reactions and demonstrate that the peptide backbone influences its chemical properties and modulates its bioactivity in vitro and in vivo. Inhibition of the growth of L. delbrueckii is demonstrated, thus providing a potentially powerful approach for the development of antibacterial and antiproliferative compounds.

This communication describes the anti-platelet effects of a new class of cis-rhenium(II)-dicarbonyl-vitamin B 12 complexes with tuneable CO releasing properties.Carbon-monoxide releasing molecules (CORMs) represent an innovative class of compounds which attract interest due to their potential therapeutic utility. Unlike most common drugs whose pharmacological action is dependent on their interaction with a macromolecular target and whose potency is dictated by the stability of the drug-target complex, CORMs exert their therapeutic action via the liberated CO molecules. [1][2][3][4][5] However, apart from the common scientific consensus that CORM-based therapy should not lead to significant carboxyhemoglobin (COHb) formation and to the inhibition of respiratory enzymes that are sensitive to CO, it is questionable whether CORMs should release CO slowly or rapidly and what kinetics of CO release is most advantageous for therapeutic applications. There are only few reports clearly showing the advantages of CORMs slowly releasing CO over those releasing CO instantly 6,7 and they relate to the anti-platelet effects of CORMs. Furthermore, it has proved chemically challenging to fine-tune the activation and the rate of CO release within a family of structurally similar CORM compounds. For all of these reasons, versatile classes of CORMs with tuneable release properties affording anti-platelet activity are highly desired for tackling these open questions in systematic structure-activity relationship studies. Such studies will facilitate the development of CORMs with optimal antiplatelet activity. -(CO) 2 fragment. This approach seemed to be reasonable because small variations in the coordination sphere of rhenium complexes have profound consequences on the electrochemistry, water stability and CO releasing properties of the dicarbonyl core.17 B 12 appeared to be attractive as ligand for the rhenium-based CORM entity mainly because of two reasons: (a) its cellular uptake properties can be exploited to deliver therapeutic agents specifically at disease sites; 18-20 (b) the electronic properties at the cobalt center can be selectively modified by introducing structural modifications at the corrin-p-system. 21-24Having the general design of the B 12 -ReCORMs derivatives in mind (Scheme 1), we synthesized and studied first a series of Scheme 1 General design concept for the tuneable activation of CORMbiovectors conjugates.

F430 is au nique enzymatic cofactor in the production and oxidation of methane by strictly anaerobic bacteria. The key enzyme methyl coenzyme Mreductase (MCR) contains ahydroporphinoid nickel complex with acharacteristic absorption maximum at around 430 nm in its active site.H erein, the three-step semisynthesis of ahybrid Ni II -containing corrinoid that partly resembles F430 in its structural and spectroscopic features from vitamin B 12 is presented. Ak ey step of the route is the simultaneous demetalation and ring closure reaction of a5 ,6-secocobalamin to metal-free 5,6-dihydroxy-5,6-dihydrohydrogenobalamin with cobaltocene and KCN under reductive conditions.Studies on the coordination chemistry of the novel compound support an earlier hypothesis why nature carefully selected ac orphin over acorrin ligand in F430 for challenging nickel-catalyzed biochemical reactions.

Cobalamins are essential cofactors for the metabolism in mammals. [1, 2] Since fast proliferating cancer cells are strong consumers of vitamin B 12 (1; abbreviated as B 12 ) the development of B 12 -bioconjugates has recently attracted considerable attention. [3][4][5][6][7][8][9][10] On the other hand, the synthesis of vitamin B 12 analogues with metals other than cobalt (met-balamins) is still an unsolved chemical challenge. Several attempts failed to remove the cobalt ion directly from B 12 . [11,12] Although these potential antivitamins have been obtained from biotechnologically produced metal-free corrinoids followed by metal insertion, [13] thorough chemical and biological investigations of these metbalamin derivatives are still lacking.A metal-templated macrocyclisation of suitable precursors in the form of the AD and BC fragments of the natural product would offer an attractive chemical alternative. This strategy is closely related to the first total synthesis of B 12 achieved by the combined effort of Woodward et al. and Eschenmoser et al. [14][15][16][17][18] During the synthesis of the B 12 precursor cobyric acid, a "western" AD fragment was successfully joined with an "eastern" BC fragment. The building blocks had to be synthesised in a linear sequence of 37 steps for the former and 17 steps for the latter fragment, respectively. [16,17] In our endeavour aimed at the development of metbalamins for biological studies, we explored an efficient one-pot strategy for the cleavage and demetallation of B 12 into the related AD and BC fragments 2 À and 3 (Scheme 1). Kräutler et al. and others investigated extensively the photo-oxidation of corrinoids [19][20][21] and demonstrated the efficient removal of the cobalt ion from oxidatively cleaved hydrophobic cobyric acid heptamethylester derivatives (abbreviated as cobester). [21][22][23] In contrast to the cobester, the synthesis and demetallation of the related natural counterpart, an oxidised bicyclic B 12 derivative, has not yet been A C H T U N G T R E N N U N G achieved. Considering the strong solvent dependence on the reactivity of singlet oxygen towards corrinoids, [21] we were able to efficiently photo-oxidise dicyano-B 12 at the C-5 and C-15 meso-positions to the desired dicyano-tetraoxo-