Crystal structure and conformation of polypeptides: L-leucylglycylglycylglycine

Abstract
Crystals of L-leucylglycylglycylglycine, LGGG (C12H22N4O5), grown from an ethanol-water solution, are orthorhombic, space groups P212121, with unit cell dimensions (at 22 .+-. 3.degree.) a = 9.337(1), b = 10,995(1), c = 15.235(1).ANG., v = 1563.4.ANG.3, Z = 4 with density of Dobs = 1.29 g .cntdot. cm-3 and Dcalc = 1.279 g .cntdot. m-3. The crystal structure was solved by the application of direct methods and refined to an R value of 0.029 for 1018 reflections with I .gtoreq. 2.sigma.. The molecule exists as a zwitterion in the crystal. The trans peptide backbone takes up a folded conformation at the middle glycylglycyl link accompanied by a significant nonplanarity up to .DELTA..omega. of 8.degree. at the middle peptide and is relatively more extended at the two ends. The molecules are linked together intermolecularly in an infinite sequence of head to tail 1.sbd.4'' hydrogen bonds, as is typical of charged peptides. It is interesting to note that while glycylglycylglycine takes up an extended .beta.-sheet conformation, addition of Leu to the N-terminal results in a bent conformation.

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