From peptides to their alternating ester-urea analogues: synthesis and influence of hydrogen bonding motif and stereochemistry on aggregation.

Peptide-mimicking scaffolds with an incorporated ester-urea motif, replacing two adjacent amide residues, were synthesized and their aggregation behavior was studied in dependence of hydrogen bonding sites as well as backbone stereochemistry. Two oligomer series containing either 50% or 100% ester-urea units and either all-(l) or (d)-alt-(l) backbone configuration were prepared via ester and amide couplings, using a divergent/convergent exponential growth strategy. Their aggregation behavior in organic solution was investigated by means of concentration-dependent NMR spectroscopy and compared to the parent peptide series. Interestingly, the naturally occurring peptide scaffold exhibits the largest tendency to associate in combination with the strongest difference in aggregation behavior between all-(l) and (d)-alt-(l) backbone stereochemistry. With increasing incorporation of the ester-urea motif the aggregation strength decreases and become much less dependent on the backbone configuration. The obtained structure-aggregation relationships reveal the importance of the commensurability and multivalency of hydrogen bonding sites as well as conformational restriction for peptide association and should hence aid the design of peptide mimics, such as beta-sheet breakers or gelators.