Solvated Helical Backbones: X-Ray Diffraction Study of Boc-Ala-Leu-Aib-Ala-Leu-Aib-Ome.$H_2O$

A second example of insertion of a water molecule into the helical backbone of an apolar peptide is presented here and compared to a similar occurrence in a longer peptide with the same type of sequence of residues, i.e., $Boc-Aib-{(Ala-Leu-fib)}_3-Ome$. The backbone of the title compound assumes an approximate $3_{10}$-helical form with three 4 \rightarrow 1 hydrogen bonds. In the place of a fourth 4 \rightarrow 1 hydrogen bond, a water molecule is inserJed between 0(1) and N(4), and acts as a bridge by forming hydrogen bonds N(4) . . . W(1) (2.95 A) and W(l) . . . O(1) (2.81 A). The water molecule participates in a third hydrogen bond with a neighboring peptide molecule, W(1) . . . O(4) (2.91 A). The insertion of the water molecule causes the apolar peptide to mimic an amphiphilic helix. Crystals grown from ethyl acetate/petroleum ether (reported here)p from methanol/water solution are in space group ${P2}_12_12_1$ with a = 12.024(4) A, b = 15.714(6) A, c = 21.411(7) A, Z = 4 and $d_{calc}$ = 1.124 g/$cm^3$ for $C_{32}H_{58}N_6O_9$ . $H_2O$. The overall agreement factor R is 6.3% for 2707 reflections observed with intensities > 3 \sigma (F) and the resolution is 0.90 A.