Modular design of synthetic protein mimics. Crystal structures, assembly, and hydration of two 15- and 16-residue apolar, leucyl-rich helical peptides

Two I5- and 16-residue peptides containing α-aminoisobutyric acid (Aib) have been synthesized, as part of a strategy to construct stereochemically rigid peptide helices, in a modular approach to design of protein mimics. The peptides Boc-(Val-Ala-Leu-Aib)4,-OMe ( I ) and Boc-Val-Ala-Leu-Aib-Val-Ala-Leu-(Val-Ala-Leu-Aib)2-OMe (II) have been crystallized. Both crystals are stable only in the presence of mother liquor or water. The crystal data are as follows. I: C78H140N16019.2H20,P21, a = 16.391 (3) Å, b = 16.860 (3) Å, c = 18.428 (3) Å, β = 103.02 (I)°, Z = 2, R = 9.6% for 3445 data with lFol > 3σ(F), resolution 0.93 Å. 11: C74H133N15018.7.5H20, C2221, a = 18.348 ( 5 ) Å, b = 47.382 (1 1) Å, c = 24.157 ( 5 ) Å, Z =8, R = l0,6%, for 3147 data with lFol > 3σ(F), resolution 1.00 Å. The 15-residue peptide (11) is entirely a helical, while the 16-residue peptide ( I ) has a short segment of 310 helix at the N terminus. The packing of the helices in the crystals is rather incfficicnt with no particular attractions between Leu-Leu side chains, or any other pair. Both crystals have fairly large voids, which are filled with water molecules in a disordered fashion. Water molecule sites near the polar head-to-tail regions are well detcrmined, those closer to the hydrophobic side chains less so and a number of possible water sites in the remaining "empty" space are not determined. No interdigitation of Leu side chains is observed in the crystal as is hypothesized in the "leucine zipper" class of DNA binding proteins