One is enough: The dipeptide Boc-L-Phe-D-Oxd-OBn (Boc = tert-butoxycarbonyl, Phe = phenylalanine, Oxd = 4-methyl-5-carboxy oxazolidin-2-one, Bn = benzyl; see picture; gray C, white H, red O, blue N) spontaneously forms uniform fibers consisting of parallel infinite linear chains arising from single intermolecular N-H···O=C hydrogen bonds. This is the absolute borderline case of a parallel β-sheet structure. (Figure Presented). © 2008 Wiley-VCH Verlag GmbH & Co. KGaA.