Conformational Properties of a Peptidic Catalyst: Insights from NMR Spectroscopic Studies.

Peptides have become valuable as catalysts for a variety of different reactions, but little is known about the conformational properties of peptidic catalysts. We investigated the conformation of the peptide H-dPro-Pro-Glu-NH2, a highly reactive and stereoselective catalyst for conjugate addition reactions, and the corresponding enamine intermediate in solution by NMR spectroscopy and computational methods. The combination of nuclear Overhauser effects (NOEs), residual dipolar couplings (RDCs), J-couplings, and temperature coefficients revealed that the tripeptide adopts a single predominant conformation in its ground state. The structure is a type I ß-turn, which gains stabilization from three hydrogen bonds that are cooperatively formed between all functional groups (secondary amine, carboxylic acid, amides) within the tripeptide. In contrast, the conformation of the enamine intermediate is significantly more flexible. The conformational ensemble of the enamine is still dominated by the ß-turn, but the backbone and the side chain of the glutamic acid residue are more dynamic. The key to the switch between rigidity and flexibility of the peptidic catalyst is the CO2H group in the side chain of the glutamic acid residue, which acts as a lid that can open and close. As a result, the peptidic catalyst is able to adapt to the structural requirements of the intermediates and transition states of the catalytic cycle. These insights might explain the robustness and high reactivity of the peptidic catalyst, which exceeds that of other secondary amine-based organocatalysts. The data suggest that a balance between rigidity and flexibility, which is reminiscent of the dynamic nature of enzymes, is beneficial for peptidic catalysts and other synthetic catalysts.

Is more better? A comparison of tri- and tetrapeptidic catalysts.

From an enzymatic perspective, there is a general notion that the bigger and more complex a catalytically active peptide is the more enzyme-like and the better it should become. But is this really true? We have tackled this question firstly by screening split-and-mix-libraries of tri- and tetrapeptides for members that catalyze aldol reactions. Then, the catalytic performance of all possible diastereoisomers of related tri- and tetrapeptidic catalysts of the type H-Pro-Pro-Glu/Asp-NH2 and H-Pro-Pro-Glu/Asp-Pro-NH2 in aldol and conjugate addition reactions was compared.

Peptide-catalyzed 1,4-addition reactions of aldehydes to nitroolefins.

Conjugate addition reactions of aldehydes to nitroolefins provide synthetically useful gamma-nitroaldehydes. Here we summarize our research on peptide-catalyzed conjugate addition reactions of aldehydes to differently substituted nitroolefins. We show that peptides of the general type Pro-Pro-Xaa (Xaa = acidic amino acid) are not only highly active, robust and stereoselective catalysts but have also remarkable chemoselectivities.

Optical coherence tomography for process control of laser micromachining.

In situ surface imaging for nondestructive evaluation (NDE) by optical coherence tomography (OCT) before, during, and after ablative laser processing is presented. Furthermore, it is shown that the ability of in situ characterization is beneficial for samples such as optical fibers, which are difficult to handle in the standard analysis. Surface images taken by the OCT are compared with these common analysis tools such as scanning electron microscopy (SEM), reflected-light, and confocal microscopy. An axial resolution of approximately 126 nm for surface detection and a lateral resolution <2.5 microm are obtained and the potential of the setup to imaging structures with high aspect ratio is demonstrated.

Enamine catalysis with low catalyst loadings--high efficiency via kinetic studies.

Kinetic studies on enamine catalysis provided insight into the rate determining step(s) of peptide catalyzed conjugate addition reactions between aldehydes and nitroolefins. They demonstrate that not enamine formation but both the reaction of the enamine with the electrophile and hydrolysis of the resulting imine are rate limiting. These results allowed for reducing the catalyst loading by a factor of 10 to as little as 0.1 mol %. This is the lowest catalyst loading that has been achieved so far in enamine catalysis with low molecular weight catalysts for a broad range of substrates.

Tripeptides of the type H-D-Pro-Pro-Xaa-NH2 as catalysts for asymmetric 1,4-addition reactions: structural requirements for high catalytic efficiency.

Analysis of the structural and functional requirements within the asymmetric peptidic catalyst H-D-Pro-Pro-Asp-NH(2) led to the development of the closely related peptide H-D-Pro-Pro-Glu-NH(2) as an even more efficient catalyst for asymmetric conjugate addition reactions of aldehydes to nitroolefins. In the presence of as little as 1 mol % of H-D-Pro-Pro-Glu-NH(2), a broad range of aldehydes and nitroolefins react readily with each other. The resulting gamma-nitroaldehydes were obtained in excellent yields and stereoselectivities at room temperature. Within the structure of the peptidic catalysts, the D-Pro-Pro motif is the major contributor to the high stereoselectivities. The C-terminal amide and the spacer to the carboxylic acid in the side-chain of the C-terminal amino acid are responsible for the fine-tuning of the stereoselectivity. The peptidic catalysts not only allow for highly effective asymmetric catalysis under mild conditions, but also function in the absence of additives.

Peptide catalyzed asymmetric conjugate addition reactions of aldehydes to nitroethylene--a convenient entry into gamma2-amino acids.

The peptide H-D-Pro-Pro-Glu-NH2 is a highly effective catalyst for conjugate addition reactions between aldehydes and nitroethylene. Only 1 mol % of H-d-Pro-Pro-Glu-NH2 and a 1.5-fold excess of aldehyde with respect to nitroethylene suffice to obtain gamma-nitroaldehydes and, after reduction, monosubstituted gamma-nitroalcohols in excellent yields and optical purities. The products can be readily converted into gamma2-amino acids, thereby opening an effective direct entry into this important class of compounds.

Separating stereoisomers of di-, tri-, and tetrapeptides using capillary electrophoresis with contactless conductivity detection.

The separation and detection of small oligopeptides in CE with contactless conductivity detection were demonstrated. A strongly acidic separation buffer (0.5 M acetic acid) was employed in order to render the species cationic. Separation of the stereoisomers was achieved in typically 10-15 min by using either dimethyl-beta-CD (DM-beta-CD), (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid (18C(6)H(4)), a combination of the two substances, or of histidine, as buffer additives. Calibration curves were determined for isomers of Gly-Asp and H-Pro-Asp-NH(2), in the range of 0.05-0.5 mM and 0.1-1 mM, respectively, and were found to be linear. LODs were determined to be in the order of 1.0 microM. The determination of isomeric impurities down to about 1% was found possible. Species showing good separation could also be successfully determined on an electrophoretic lab-on-chip device, with analysis times of a few minutes.

Porphyrin-DNA: a supramolecular scaffold for functional molecules on the nanometre scale.

We are pursuing the aim to use DNA as a supramolecular scaffold for the creation of electronically functional molecules on the nanometre scale. Here, we give a review on our results on porphyrin modified nucleotides used for this purpose. A general synthetic route to porphyrin-nucleotides has been devised, and the building blocks can be incorporated into oligonucleotides using standard solid phase synthesis methods. Up to 11 porphyrins were incorporated into DNA, reaching a length of approximately 4 nm in the array. The spectroscopic data are consistent with a porphyrin induced secondary structure stabilisation in the single strands.