1. Conformational switching caused by biphenyl substitution at the Calpha position: ethyl 2-benzyl-2-(formylamino)-3-phenylpropionate and ethyl 3-(1,1'-biphenyl-4-yl)-2-(formylamino)-2-(4-phenylbenzyl)propionate
Lakshminarasimhan Damodharan, Vasantha Pattabhi, Manoranjan Behera, Sambasivarao Kotha Acta Crystallogr C. 2004 Jul;60(Pt 7):o527-30. doi: 10.1107/S0108270104013150. Epub 2004 Jul 30.
The title compounds, C19H21NO3 and C31H29NO3, are derivatives of alpha-aminoisobutyric acid, with benzyl and dibenzyl substitution. The pseudo-peptide formed by the N-formyl and ethyl ester substitution at the Calpha position switches from a trans-trans to a trans-cis configuration as a result of biphenyl substitution. The packing of the compounds is stabilized by N-H...O and C-H...O hydrogen bonds.
2. Parallel zippers formed by alpha-helical peptide columns in crystals of Boc-Aib-Glu(OBzl)-Leu-Aib-Ala-Leu-Aib-Ala-Lys(Z)-Aib-OMe
I L Karle, J L Flippen-Anderson, K Uma, P Balaram Proc Natl Acad Sci U S A. 1990 Oct;87(20):7921-5. doi: 10.1073/pnas.87.20.7921.
The crystal structure of the decapeptide Boc-Aib-Glu(OBzl)-Leu-Aib-Ala-Leu-Aib-Ala-Lys(Z)-Aib-OMe (where Aib is alpha-aminoisobutyryl, Boc is t-butoxycarbonyl, OBzl is benzyl ester, and Z is benzyloxycarbonyl) illustrates a parallel zipper arrangement of interacting helical peptide columns. Head-to-tail NH...OC hydrogen bonding extends the alpha-helices formed by the decapeptide into long columns in the crystal. An additional NH...OC hydrogen bond in the head-to-tail region, between the extended side chains of Glu(OBzl), residue 2 in one molecule, and Lys(Z), residue 9 in another molecule, forms a "double tooth" on the side of the column. These double teeth are repeated regularly on the helical columns with spaces of six residues between them (approximately 10 A). The double teeth on a pair of parallel columns (all carbonyl groups pointed in the same direction) interdigitate in a zipper motif. All contacts in the zipper portion are of the van der Waals type. The peptide, with formula C66H103N11O17.H2O, crystallizes in space group P2(1)2(1)2(1) with a = 10.677(4) A, b = 16.452(6) A, and c = 43.779(13) A; overall agreement R = 10.2% for 3527 observed reflections (magnitude of /F0/ greater than 3 sigma); resolution 0.9 A.
