The first crystal structure of an oligoproline adopting an all-trans polyproline II (PPII) helix is presented. The high-resolution structure provides detailed insight into the dimensions and conformational properties of oligoprolines that are important for, e.g., their use as "molecular rulers" and "molecular scaffolds". The structure also showed that the amides interact with each other within a PPII helix and that water is not necessary for PPII helicity.

The "azido gauche effect" was examined both experimentally and theoretically and was found to determine the conformation of, for example, (4R)- and (4S)-azidoproline (Azp) derivatives. For (4R)Azp derivatives, the azido gauche effect induces a preferred C(4)-exo conformation of the pyrrolidine ring, which leads to stabilization of the s-trans amide conformer of, e.g., Ac-(4R)Azp-OCH(3) (5R) via an n-->pi interaction between the nonbonding electrons of the oxygen of the acetyl group and the carbonyl group of the ester. For (4S)Azp derivatives, the azido gauche effect results in a C(4)-endo conformation of the pyrrolidine ring that does not allow for this stabilizing n-->pi interaction of the s-trans conformer. Consequently, a significantly higher s-trans:s-cis amide conformer ratio is observed for (4R)Azp compared to (4S)Azp derivatives (e.g., 6.1:1 versus 2.6:1 in D(2)O for Ac-(4R)Azp-OCH(3) (5R) compared to Ac-(4S)Azp-OCH(3) (5S)). These conformational preferences are reflected in the higher tendency of (4S)Azp-containing peptides to form cyclic peptides with all-cis amide bonds compared to (4R)Azp derivatives. Ab initio calculations demonstrate that the strength of the azido gauche effect is comparable to that of the well-known "fluorine gauche effect". For azidoethane derivatives N(3)-CH(2)CH(2)-X (X = N(3), NHCOH, NHAc, or N(CH(3))Ac), the ab initio calculations revealed energy differences of 5-13 kJ mol(-)(1) between the anti and gauche conformations in favor of the gauche conformer. Calculations were also performed for the (4R)Azp and (4S)Azp derivatives 5R and 5S, supporting the experimentally observed data.

The conformational stability of the polyproline II (PPII) helix with respect to the functional groups at the C- and N-termini was examined both experimentally and theoretically. Oligoprolines AcN-[Pro](12)-CONH(2) (1), HN-[Pro](12)-CONH(2) (2), AcN-[Pro](12)-CO(2)H (3), and HN-[Pro](12)-CO(2)H (4) with charged and capped termini served as model compounds, and the relative ease with which they switch from the PPII to the polyproline I (PPI) helix was used as a measure to analyze their conformational stabilities. CD spectroscopic studies demonstrate that a positively charged N-terminus and a negatively charged C-terminus destabilize the PPII helix and favor the PPI helix, whereas capped termini favor the PPII over the PPI helix. These experimental findings are supported by the energy differences between the PPII and PPI helices of oligoprolines 1-4 computed by ab initio methods including electron-correlation effects (second-order Møller-Plesset perturbation theory, MP2). Furthermore, these quantum-chemical calculations show that differences in charge-dipole interactions are responsible for the experimentally and computationally observed relative stabilities. Although these electrostatic interactions between the terminal charges and the amide dipoles stabilize both helices, they are significantly stronger in the PPI helix where the amide bonds are oriented almost linear to the helix axis as compared to the PPII helix in which the amides are nearly perpendicular to the axis. Moreover, we demonstrate that a negative charge at the C-terminus has a more pronounced effect on the relative stability as compared to a positive charge at the N-terminus due to destabilization of the PPII helix by repulsive interaction between the C-terminal carboxylate with the neighboring amide bond. Studies at different pH values verified the electrostatic nature of the observed effects and demonstrate how changes in the protonation state can be used to deliberately stabilize the PPII helix over the PPI helix or vice versa.

[reaction: see text] Peptides containing a secondary amine and a carboxylic acid in a specific orientation to each other are presented as highly efficient catalysts for asymmetric aldol reactions: (1) their activity is considerably higher compared to that of proline, and (2) the enantioselectivity of the peptidic catalysts can be changed from (R)- to (S)-selectivity by simple modifications of the secondary structure.

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The functionalizability and conformational properties of azidoproline (Azp)-containing collagen model peptides (CMPs) were studied. The results show that (4R)Azp has a similar stabilizing effect on the collagen triple helix as (4R)hydroxyproline and that functionalized CMPs are readily accessible by "click" chemistry. The resulting triazole-functionalized CMPs form stable triple helices, demonstrating that sterically demanding moieties in three symmetry-related positions in all strands are tolerated. The straightforward synthesis and facile functionalizability of the Azp-containing CMPs are intriguing for the development of functional collagen-based materials.

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.

The attraction of H‐bonding: Proline derivatives with a C4‐endo ring pucker and a preference for the trans amide conformer are introduced as building blocks to tune the cis/trans amide conformer ratio in Xaa‐Pro bonds within peptides. Noncovalent interactions, such as hydrogen bonding between the substituent at C4 of the proline ring and the amide backbone are key for favoring or disfavoring the trans conformer (see picture).