Peculiar properties of homoleptic Cu complexes with dipyrromethene derivatives.

In view of preparing Cu polynuclear complexes with dipyrromethene ligands, the mononuclear complexes [Cu(II)(dipy)2] (dipyH = 5-phenyldipyrromethene) and [Cu(II)(dpdipy)2] (dpdipyH = 1,5,9-triphenyldipyrromethene) have been prepared and characterized by X-ray crystallography, mass spectrometry and EPR spectroscopy. Their peculiar redox and spectroscopic (absorption/emission) behaviours are discussed. In contrast to Cu(II) complexes of 1,1'-bidypyrrin, the reduction electrolysis of [Cu(II)(dpdipy)2] leads to decomposition products on a time scale of a few hours. Moreover in relation to this observation, [Cu(I)(dpdipy)2](-) could not be synthesized in spite of the Cu(I) core protection by the phenyl substituents in ortho position of the nitrogen atoms. Theoretical calculations provide some explanations for this instability. Interestingly [Cu(II)(dipy)2] and [Cu(II)(dpdipy)2] display weak luminescence at room temperature, attributed to a ligand centered emission.

How does hydroxyl introduction influence the double helical structure: the stabilization of an altritol nucleic acid:ribonucleic acid duplex.

Altritol nucleic acids (ANAs) are a promising new tool in the development of artificial small interfering ribonucleic acids (siRNAs) for therapeutical applications. To mimic the siRNA:messenger RNA (mRNA) interactions, the crystal structure of the ANA:RNA construct a(CCGUAAUGCC-P):r(GGCAUUACGG) was determined to 1.96 Å resolution which revealed the hybrid to form an A-type helix. As this A-form is a major requirement in the RNAi process, this crystal structure confirms the potential of altritol-modified siRNAs. Moreover, in the ANA strands, a new type of intrastrand interactions was found between the O2' hydroxyl group of one residue and the sugar ring O4' atom of the next residue. These interactions were further investigated by quantum chemical methods. Besides hydration effects, these intrastrand hydrogen bonds may also contribute to the stability of ANA:RNA duplexes.

The crystal structure of the CeNA:RNA hybrid ce(GCGTAGCG):r(CGCUACGC).

Cyclohexenyl nucleic acids (CeNA) are characterised by the carbon-carbon double bond replacing the O4'-oxygen atom of the natural D-2'-deoxyribose sugar ring in DNA. CeNAs exhibit a high conformational flexibility, are stable against nuclease activity and their hybridisation is RNA selective. Additionally, CeNA has been shown to induce an enhanced biological activity when incorporated in siRNA. This makes CeNA a good candidate for siRNA and synthetic aptamer applications. The crystal structure of the synthetic CeNA:RNA hybrid ce(GCGTAGCG):r(CGCUACGC) has been solved with a resolution of 2.50 Å. The CeNA:RNA duplex adopts an anti-parallel, right-handed double helix with standard Watson-Crick base pairing. Analyses of the helical parameters revealed the octamer to form an A-like double helix. The cyclohexenyl rings mainly adopt the (3)H(2) conformation, which resembles the C3'-endo conformation of RNA ribose ring. This C3'-endo ring puckering was found in most of the RNA residues and is typical for A-family helices. The crystal structure is stabilised by the presence of hexahydrated magnesium ions. The fact that the CeNA:RNA hybrid adopts an A-type double helical conformation confirms the high potential of CeNAs for the construction of efficient siRNAs which can be used for therapeutical applications.

Crystallization and preliminary X-ray study of the D-altritol oligonucleotide GTGTACAC.

In altritol nucleic acids (ANAs), the natural five-membered ribose ring of RNA is replaced by the six-membered D-altritol ring. ANAs are good candidates to act as siRNAs in the RNA-interference pathway. Crystals of the fully modified altritol self-complementary octamer GTGTACAC were grown by the hanging-drop vapour-diffusion technique at 289 K. Diffraction data were recorded on SLS beamline X06DA and processed to 3.0 A resolution. The crystals belonged to the hexagonal space group P6(1)22 or P6(5)22, with unit-cell parameters a = 25.05, c = 117.58 A.

Homoselenacalix[n]arenes.

A novel family of homoselenacalix[n]arenes (n = 3-8), with bridging CH(2)SeCH(2) groups connecting the aryl subunits, has been synthesized via two different approaches employing nucleophilic Se species. The macrocycles are adequately characterized, including single-crystal X-ray structures for the homoselenacalix[4]- and homoselenacalix[6]arene homologues. The combined features of a calixarene-like macrocyclic scaffold and the presence of multiple selenium atoms create appealing (biomimetic) supramolecular opportunities.

Efficient fragment coupling approaches toward large oxacalix[n]arenes (n = 6, 8).

The first rational, stepwise synthesis of enlarged oxacalix[n]arenes (n > 4) is described. Variously substituted oxacalix[3]arene[3]pyrimidines were prepared rather selectively by a straightforward [3 + 3] fragment coupling approach after a thorough search for the optimum nucleophilic aromatic substitution conditions. Similar procedures also allowed facile synthesis of unsymmetrical oxacalix[4]- and oxacalix[8]arenes.

Efficient post-macrocyclization functionalizations of oxacalix[2]arene[2]pyrimidines.

Diversely functionalized oxacalix[2]arene[2]pyrimidines have been synthesized starting from a bis(methylsulfanyl)-substituted oxacalix[4]arene by two efficient post-macrocyclization pathways. Functionalized aryl groups were introduced on the pyrimidine building block via Liebeskind-Srogl cross-coupling reactions, while a variety of O-, S-, N-, and C-nucleophiles were inserted on the calixarene skeleton by nucleophilic aromatic substitution reactions on the bis(methylsulfonyl)oxacalix[4]arene analogue.