Based on the observation that an increased substrate size is paralleled by an enhanced diastereoselectivity, a late-stage dihydroxylation protocol toward the 21mer CMP (collagen model peptide) Ac-(Pro-Hyp-Gly)-Pro-Dyp-Gly-(Pro-Hyp-Gly)-NH is presented. C3 and C4 hydroxylation have a converse effect on the triple-helical stability of collagen. Their combined influence on the melting temperature was studied by NMR spectroscopy.

The combinationo fs upramolecular aggregation of collagen model peptidesw ith reversible covalent endcappingo ft he formed triple helix in as ingle experimental set-up yielded minicollagens, whichw ere characterizedb y as ingle melting temperature.I ns pite of the numerous possible reaction intermediates, as pecific synthetic collagen with al eading, middle and trailings trand is formed in ahighly cooperative self-assembly process.[a] C. Priem,P rof. Dr.A .G eyer

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-) (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.