[Structure: see text] The self-assembly in the crystal state of the terminally protected, linear dipeptide Boc-(S,S)c3diPhe-(R,R)c3diPhe-NHiPr (1) through intermolecular hydrogen bonds leads to the formation of a supramolecular helix of large diameter (18 A), internally decorated with phenyl rings. As a result, a hollow helical channel large enough to accommodate guest molecules is observed. This supramolecular structure differs from previous examples of peptide nanotubes. Compound 1 incorporates a highly restricted cyclopropane phenylalanine analogue (c3diPhe) with remarkable conformational properties.