Synthesis and solution structure of an S‐glycosylated cyclic hexapeptide: Evidence for conformational change induced by glycosylation

Synthesis and conformational analysis of the S‐glycosylated cyclic hexapeptide cyclo(‐d‐Pro¹‐Phe²‐Cys³(tetra‐O‐acetyl‐β‐d‐galactopyranosyl)‐Trp⁴‐Lys(Z)⁵‐Phe⁶‐) I was carried out to examine the influence of a saccharide residue in position i of a standard β‐turn on the formation of reverse turns and on the biological activity. Synthesis was carried out in the liquid phase employing a galactosylated cysteine building block. The cyclization reagents DPPA/NaHCO₃ avoided high dilution conditions. Spectroscopic data were extracted from homo‐ and heteronuclear 2D‐NMR techniques (TOCSY, NOESY, HMQC, HMQC‐TOCSY, HMBCS‐270). For structural refinement restrained molecular dynamics (MD) simulations in vacuo and with explicit DMSO as solvent were performed. Finally, simulations in DMSO without experimental restraints provided insight in stability and dynamics of the structural model. A comparison of the S‐glycosylated Cys³ peptide with the analogous Thr³ peptide exhibits a similar overall conformation of the hexapeptide [βII’d‐Pro‐Phe and another β‐turn about Trp⁴‐Lys⁵(Z)]. However, the latter shows a distinct dynamic flip βI, βII in the glycopeptide, whereas the Thr‐analogue only populates βI. This influence is attributed to a βI stabilizing effect of a hydrogen bridge of Thr‐O, in position i to the NH of the amino acid in position i+ 2, which is lacking in the glycosylated compound.