Oxazolidin-2-one-Containing Pseudopeptides That Fold into β-Bend Ribbon Spirals

Three sets of oligomers containing the 4-carboxy-5-methyloxazolidin-2-one (Oxd) moiety have been synthesized with the aim of checking whether these molecules are able to fold in ordered structures: A set [Boc-(l-Ala-l-Oxd)n-OR], B set [Boc-(l-Ala-d-Oxd)n-OR], and C set [Boc-(Aib-l-Oxd)n-OR] preferential conformations have been analyzed with IR absorption, NMR, and CD. We have noticed that in these oligomers three stabilizing effects are active: (i) the rigid Oxd −CO−N(CH<)−CO− moiety, which always tend to assume a trans conformation; (ii) the formation of Oxd CO···HαC intramolecolar H-bonds; (iii) the alternate formation of 1 ← 4 intramolecular CO···HN H-bonds. Through the analysis of the experimental data, we could demonstrate that only the oligomers of the B set are able to meet all three requirements listed above. By a deeper insight into the CD spectra, we gathered that the secondary structure adopted by the B set oligomers is a β-bend ribbon spiral, which is a subtype of the 310-helix.