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),-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 CID. 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=(OH)-H-...-C-alpha intramolecolar H-bonds; (iii) the alternate formation of 1 - 4 intramolecular C=(OH)-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 beta-bend ribbon spiral, which is a subtype of the 3(10)-helix.