Strong Ferromagnetic Exchange Coupling Mediated by a Bridging Tetrazine Radical in a Dinuclear Nickel Complex.

The radical bridged compound [(Ni- (TPMA))2-μ-bmtz•−](BF4)3·3CH3CN (bmtz = 3,6-bis- (2′-pyrimidyl)-1,2,4,5-tetrazine, TPMA = tris(2- pyridylmethyl)amine) exhibits strong ferromagnetic exchange between the S = 1 NiII centers and the bridging S = 1/2 bmtz radical with J = 96 ± 5 cm−1 (−2JNi‑radSNiSrad). DFT calculations support the existence of strong ferromagnetic exchange.Department of Energy Office of Science Graduate Fellowship, Instituto de Ciencia y Tecnología del Distrito Federal (ICyTDF) National Science Foundation (CHE-1310574) and the Robert A. Welch Foundation (A-1449)

The Disobeying 'Soldier': Use of an Achiral Group to Modulate Chiral Induction in PNA Duplexes

Peptide nucleic acid (PNA) is a synthetic analogue of DNA in which the natural nucleobases A, G, C, and T are linked to an achiral, charge neutral, pseudopeptide backbone. PNA strands can form double helices similar to DNA whose helical sense can be modulated by applying the 'sergeants-and-soldiers' principle. Attachment of a chiral amino acid (sergeant) at the C-terminus of PNA leads to the amplification of chirality of the sergeant onto the achiral PNA monomers (soldiers), resulting in an enantiomeric excess of either left- or right-handed PNA duplexes. In the present study we looked at the effect of an achiral N-terminal terpyridine (soldier) on the helicity of the double helix that contains L-lysine. We have found that terpyridine interferes with the chiral induction effect of the L-lysines, an effect that can be reverted upon coordination of Cu2+ ions to terpyridine

Metal Coordination to Ligand-Modified Peptide Nucleic Acid Triplexes

A challenging goal in nanotechnology is the precise and programmable arrangement of specific elements in nanosystems in the three-dimensional space. The use of ligand-modified nucleic acids represents an accurate and selective tool to achieve this goal when it comes to metal ion organization. The synthesis of peptide nucleic acid (PNA) monomers that contain ligands instead of nucleobases makes possible the creation of metal-mediated alternative base pairs and triplets at specific locations in PNA duplexes and triplexes, respectively. We report the formation of four- and six-coordinate metal complexes between PNA triplexes modified with 2,2′-bipyridine (<b>Bpy</b>) or 8-hydroxyquinoline (<b>Q</b>) ligands and 3d metal ions. These metal complexes function as alternative base triplets or pairs in that they increase the thermal stability of the triplexes if the stability constants of the metal complexes are relatively high. The increase in the triplex melting temperature correlates with the stability constants of the metal complexes with ligand-containing PNA determined by UV–vis titrations. The metal complexes coordinate two or three ligands although three bidentate ligands are in close proximity of each other within a triplex. Metal coordination to ligand-modified PNA triplexes was further studied by electron paramagnetic resonance (EPR) spectroscopy and circular dichrosim (CD) spectroscopy. EPR spectroscopy indicated the formation of a square planar [Cu<b>Q</b>₂] complex between Cu²⁺ and <b>Q</b>-containing PNA triplex. Taken together, the spectroscopic results indicate that in the presence of 1 equiv of Fe²⁺ or Ni²⁺ the majority, but not all, of the <b>Bpy</b>-containing PNA triplexes contain [M<b>Bpy</b>₃] complexes, with a minority of them being metal free. We attribute this behavior to a supramolecular chelate effect exerted by the triplex, which favors the formation of tris-ligand complexes, that is balanced by the steric interactions between the metal complex and the adjacent nucleobase triplets, which decrease the stability of the complex and triplex. In contrast, the very high stability of square planar [M<b>Q</b>₂] complexes of Cu²⁺ and Ni²⁺ leads to formation of bis-ligand complexes instead of tris-ligand complexes with <b>Q</b>₃-containing <b>PNA</b> triplexes. The metal-containing PNA triplexes have a terminal l-lysine and adopt a left-handed chiral structure in solution. The handedness of the PNA triplex determines that of the metal complexes formed with the <b>Bpy</b>-containing PNA triplexes