Structures of L-Phenylalanine Dimers, N-(tert-Butoxycarbonyl)-L-phenylalanyl-L-phenylalanine Benzyl Ester (Boc-Phe-Phe-OBzl) and L-Phenylalanyl-L-phenylalanine Ethyl Ester Trifluoroacetate (Phe-Phe-OEt*Tfa)

Article abstract of DOI:10.1107/S0567740882009522

The crystal structures of the title compounds have been determined by the direct method and refined by the block-diagonal least-squares method.Boc-Phe-Phe-OBzl has space group P2₁, with a = 14.363 (3), b = 19.016 (5), c = 5.077 (1) Angstroem, β = 97.02 (2) deg, V = 1376.3 (8) Angstroem³, Z = 2, and R = 0.070 for 1921 reflections; Phe-Phe-OEt*Tfa has space group P2₁2₁2₁, with a = 16.507 (4), b = 24.927 (7), c = 5.650 (1) Angstroem, V = 2325 (1) Angstroem³, Z = 4, and R = 0.074 for 1544 reflections.The former crystallizes in the parallel β pleated sheet.Deviation of the main-chain conformation from that of the typical β sheet due to the substitution of bulky terminal groups is observed.The latter molecule is not in the β sheet, but has a column structure with a hydrophilic core of hydrogen bonds and a hydrophobic shell.