A Bis(guanidinium)alcohol Attached to a Hairpin Polyamide: Synthesis, DNA Binding, and Plasmid Cleavage.

Bis(guanidinium)alcohols have been designed to react with phosphodiester substrates in a fast transphosphorylation step, a quasi-intramolecular process taking place in contact ion pairs. Here the attachment of such compounds to Dervan-type hairpin polyamides is described. The resulting conjugate 1 binds to AT-rich DNA duplexes with affinity similar to that of the parent polyamide as shown by UV melting experiments and CD titrations. Conjugate 1 nicks plasmid DNA at concentrations ranging from micromolar to high nanomolar.

Cleavage of phosphodiesters and of DNA by a bis(guanidinium)naphthol acting as a metal-free anion receptor.

Phosphoric acid diesters form anions at neutral pH. As a result of charge repulsion they are notoriously resistant to hydrolysis. Nucleophilic attack, however, can be promoted by different types of electrophilic catalysts that bind to the anions and reduce their negative charge density. Although in most cases phosphodiester-cleaving enzymes and synthetic catalysts rely on Lewis acidic metal ions, some exploit the guanidinium residues of arginine as metal-free electrophiles. Here we report that a combination of two guanidines and a hydroxy group yields highly reactive receptor molecules that can attack a broad range of phosphodiester substrates by nucleophilic displacement at phosphorus in a single-turnover mode. Some stable O-phosphates were isolated and characterized further by NMR spectroscopy. The bis(guanidinium)naphthols also cleave plasmid DNA, presumably by a transphosphorylation mechanism.

Enantioselective synthesis of (+)-estrone exploiting a hydrogen bond-promoted Diels-Alder reaction.

Starting from Dane's diene and methylcyclopentenedione, (+)-estrone is synthesized along the Quinkert-Dane route in 24% total yield. The key step is an enantioselective Diels-Alder reaction promoted by an amidinium catalyst as efficiently as by a traditional Ti-TADDOLate Lewis acid.

A practical synthesis of the 16C/15N-labelled tripeptide N-formyl-Met-Leu-Phe, useful as a reference in solid-state NMR spectroscopy.

A mild synthetic method for N-formyl-Met-Leu-Phe-OH (1) is described. After Fmoc solid phase peptide synthesis, on-bead formylation and HPLC purification, more than 30 mg of the fully (13)C/(15)N-labelled tripeptide 1 could be isolated in a typical batch. This peptide can be easily crystallised and is therefore well suited as a standard sample for setting up solid-state NMR experiments.

C2-symmetric bisamidines: chiral Brønsted bases catalysing the Diels-Alder reaction of anthrones.

C(2)-symmetric bisamidines 8 have been tested as chiral Brønsted bases in the Diels-Alder reaction of anthrones and N-substituted maleimides. High yields of cycloadducts and significant asymmetric inductions up to 76% ee are accessible. The proposed mechanism involves proton transfer between anthrone and bisamidine, association of the resulting ions and finally a cycloaddition step stereoselectively controlled by the chiral ion pair.

Tripeptides from synthetic amino acids block the Tat-TAR association and slow down HIV spread in cell cultures.

Non-natural amino acids with aromatic or heteroaromatic side chains were incorporated into tripeptides of the general structure Arg-X-Arg and tested as ligands of the HIV RNA element TAR. Some of these compounds could compete efficiently with the association of TAR and Tat and downregulated a TAR-controlled reporter gene in HeLa cells. Peptide 7, which contains a 2-pyrimidinyl-alkyl chain, also inhibited the spread of HIV-1 in cell cultures. NMR studies of 7 bound to HIV-2-TAR gave evidence for contacts in the bulge region.

Synthesis of C2-symmetric bisamidines: a new type of chiral metal-free Lewis acid analogue interacting with carbonyl groups.

The chiral bisamidine 5 has been prepared in just two steps from malonodinitrile. In the monocationic form this compound adopts a planar conformation with an almost convergent orientation of two N-H groups. Ketones, aldehydes, and nitro compounds are assumed to bind to this strongly polar cleft via hydrogen bonds, resulting in a Lewis-acid-like activation of the carbonyl groups. A broad scope of reactions (Diels-Alder, hetero-Diels-Alder, Friedel-Crafts) can be catalyzed. The observed accelerations surpass the rate effects of neutral hydrogen-bond donors such as thioureas or TADDOLs.

Counting the monomers in nanometer-sized oligomers by pulsed electron-electron double resonance.

In a lot of cases active biomolecules are complexes of higher order, thus methods capable of counting the number of building blocks and elucidating their geometric arrangement are needed. Therefore, we experimentally validate here spin-counting via 4-pulse electron-electron double resonance (PELDOR) on well-defined test samples. Two biradicals, a symmetric and an asymmetric triradical, and a tetraradical were synthesized in a convergent reaction scheme via palladium-catalyzed cross-coupling reactions. PELDOR was then used to obtain geometric information and the number of spin centers per molecule in a single experiment. The measurement yielded the expected distances (2.2-3.8 nm) and showed that different spin-spin distances in one molecule can be resolved even if the difference amounts to only 5 A. The number of spins n has been determined to be 2.1 in both biradicals, to 3.1 and 3.0 in the symmetric and asymmetric triradicals, respectively, and to 3.9 in the tetraradical. The overall error of PELDOR spin-counting was found to be 5% for up to four spins. Thus, this method is a valuable tool to determine the number of constituting spin-bearing monomers in biologically relevant homo- and heterooligomers and how their oligomerization state and geometric arrangement changes during function.

Enantioselective synthesis of non-natural aromatic alpha-amino acids.

We present two complementary methods for the stereoselective synthesis of non-natural alpha-amino acids with aromatic or heteroaromatic side chains. One approach is based on the chemical transformation of methionine, whereas the other applies the stereoselective Myers alkylation of glycine. The resulting product types differ in the linker length between glycine and the aromatic substituent. Since methionine and pseudoephedrine are available in both absolute configurations, R- or S-configured enantiopure amino acids with either C(2) or C(3) linkers can be obtained on gram scales. In each case the key step of the synthesis is hydroboration of the unsaturated building blocks 9 and 17, followed by palladium-catalyzed Suzuki cross-coupling with aryl halides. Attention must in certain cases be paid to the stereochemical integrity when basic Suzuki conditions are applied. Our initial difficulties are reported as well as the final "racemization-proof" procedures. The protecting groups chosen for the alpha-amino acids should be compatible with solid-phase peptide synthesis. This was confirmed by the successful synthesis of a series of tripeptides.