Scaling the Amphiphilic Character and Antimicrobial Activity of Gramicidin S by Dihydroxylation or Ketal Formation.

The acid lability of aliphatic ketals, which often serve as protection groups for 1,2-diols, is influenced by their local structural environment. The acetonide of the protected amino acid cis-dihydroxyproline (Dyp) is a typical protecting group cleavable by traces of TFA. The tricyclic acetonide of the dipeptide d-Hot═Tap is resistant to TFA and thus can serve as a bioorthogonal modification of bioactive peptides. With the aim of improving antimicrobial activity and hemolytic properties, we use these reactivity differences to scale the membrane affinity of the decapeptide Gramicidin S cyclo(d-Phe-Pro-Val-Orn-Leu-)2 (GS). The cis-dihydroxylated amino acids are used to increase the polarity of GS or obversely decrease the polarity by stereoselective ketal formation with an aliphatic ketone. While Dyp (GS mimetic 15) has only minimal influence on the biological properties of GS, d-Hot═Tap at the position of d-Phe1-Pro2 eradicates the biological activity (GS mimetic 16). The acid-stable ketals 17-19 are bioorthogonal modifications which reconstitute the biological activity of GS. We describe an improved synthesis of orthogonally protected Fmoc-Dyp-acetonide (9) and of several Fmoc-d-Hot═Tap-ketals for solid-phase peptide synthesis.

Self-assembly of peptide boroxoles on cis-dihydroxylated oligoamide templates in water.

We develop templates that can be used to stabilize consistent oligomers of a bioactive peptide. In the present study, we synthesize oligomers of an antibody epitope from the amyloidogenic prion protein. Dynamic covalent chemistry is the basis for the spontaneous condensation of 2, 3, 4 or 6 peptides with qualified polyol templates presenting the required number of bioorthogonal ligation sites. To study this process in aqueous solution, the N-terminal amino acid of a 13-mer peptide is first acylated with 4-carboxy-benzoboroxole (1-hydroxy-1,3-dihydrobenzo[c][1,2] oxaborole-5-carboxylic acid) and then mixed with the template to obtain self-assembled miniamyloids of specified degree of oligomerization. The template is assembled from bicyclic dipeptides of alternating d- and l-stereochemistry. The cis-diol group of this dipeptide hot=Tap (hot: d-hydroxythreonine, Tap: l-thiaproline) has sufficiently high affinity for boroxoles in water. A single N3 -hot=Tap-OMe dipeptide template forms a 1 : 1 complex with 4-carboxy-benzoboroxole with excellent diastereoselectivity. The oligomeric template N3 -(hot=Tap)n -OMe (n = 2, 3, 4 or 6) presents a regular pattern of 2, 3, 4 or 6 cis-diol groups for the spontaneous esterification with the same number of boronic acids. Nuclear magnetic resonance identifies the homogenous regioselectivity and stereoselectivity of this ligation process. The combination of electron-poor benzoboroxoles with this optimized cis-diol template allows for the complete ligation under high-dilution conditions in water with only 1.3 equivalents of peptide-boroxole per diol functionality. Copyright © 2017 European Peptide Society and John Wiley & Sons, Ltd.

Optimization of Cyclic Plasmin Inhibitors: From Benzamidines to Benzylamines.

New macrocyclic plasmin inhibitors based on our previously optimized P2-P3 core segment have been developed. In the first series, the P4 residue was modified, whereas the 4-amidinobenzylamide in P1 position was maintained. The originally used P4 benzylsulfonyl residue could be replaced by various sulfonyl- or urethane-like protecting groups. In the second series, the P1 benzamidine was modified and a strong potency and excellent selectivity was retained by incorporation of p-xylenediamine. Several analogues inhibit plasmin in the subnanomolar range, and their potency against related trypsin-like serine proteases including trypsin itself could be further reduced. Selected derivatives have been tested in a plasma fibrinolysis assay and are more effective than the reference inhibitor aprotinin. The crystal structure of one inhibitor was determined in complex with trypsin. The binding mode reveals a sterical clash of the inhibitor's linker segment with the 99-hairpin loop of trypsin, which is absent in plasmin.

Improved δ-valerolactam templates for the assembly of Aß-miniamyloids by boronic ester formation.

The 6,7,8,8a-cis (all-cis) substituted δ-valerolactams of type 10, 11 and 12 are high-affinity diols for boronic ester formation, superior to the corresponding 6,7-trans analogues 1, 3 and 4. X-ray and NMR structure analysis have identified the differences of the six-membered ring conformations which cause the improved esterification properties of the all-cis stereoisomers. The homooligomeric all-cisδ-valerolactams 46-48 are used as polyol templates for the self-assembly of peptidic oligomers 49-52 by dynamic covalent chemistry. The templates have a diol spacing of approximately 5 Å, suitable for the assembly of branched peptides from the quantitative reaction between the peptide of interest, 2-formylphenylboronic acid and the respective template. According to this strategy, the tetrameric Aß-miniamyloid 52 formed spontaneously from nine individual molecules in a three-component system. A detailed NMR analysis based on the complete sequential assignment of the trimeric Aß(32-40)-miniamyloid 51 identified its three-dimensional structure in solution.