Abstract
Both deprotonated and neutral achiral title dipeptides assume similar structures of two conformations, which are related by a unit-cell inversion centre. Two mol-ecules of both conformations of the metal-free neutral dipeptide are linked by two hydrogen bonds, while two mol-ecules of both conformations of the ionized form coordinate a calcium ion in calcium(II) bis-[2-(2-{[(benz-yl-oxy)carbon-yl]amino}-acetamido)-2-methyl-propano-ate] monohydrate, 0.5Ca2+·C14H17N2O5-·0.5H2O, which lies on an inversion centre and forms a distorted octa-hedral complex with the metal ion. These CaII complexes are connected in the crystal via hydrogen bonds in the b- and c-axis directions, whereas in the a-axis direction, they stack via apolar contacts. In the metal-free crystal, namely 2-(2-{[(benz-yloxy)carbon-yl]amino}-acetamido)-2-methyl-propanoic acid, C14H18N2O5, mol-ecules are hydrogen bonded in the a- and c-axis directions, and stack in the b-axis direction via apolar contacts.
Highlights
Supporting information: this article has supporting information at journals.iucr.org/e aIMBB-FORTH, 70013 Heraklion, Greece, and bDepartment of Food Sciences, Interdisciplinary Research Center, JustusLiebig-University of Giessen, 35392 Giessen, Germany. *Correspondence e-mail: petratos@imbb.forth.gr. Both deprotonated and neutral achiral title dipeptides assume similar structures of two conformations, which are related by a unit-cell inversion centre. Two molecules of both conformations of the metal-free neutral dipeptide are linked by two hydrogen bonds, while two molecules of both conformations of the ionized form coordinate a calcium ion in calcium(II) bis[2-(2-{[(benzyloxy)carbonyl]amino}acetamido)-2-methylpropanoate] monohydrate, 0.5Ca2+ÁC14H17N2O5ÀÁ0.5H2O, which lies on an inversion centre and forms a distorted octahedral complex with the metal ion
These CaII complexes are connected in the crystal via hydrogen bonds in the b- and c-axis directions, whereas in the a-axis direction, they stack via apolar contacts
In the metal-free crystal, namely 2-(2-{[(benzyloxy)carbonyl]amino}acetamido)-2-methylpropanoic acid, C14H18N2O5, molecules are hydrogen bonded in the a- and caxis directions, and stack in the b–axis direction via apolar contacts
Summary
The presence of Gly and Aib ( -aminoisobutyric acid) combines a residue with the greatest conformational flexibility (Gly) with a severely restricted residue (Aib) because of the second methyl group attached to the C atom. Because of the absent side-chain atoms, Gly can adopt almost all conformations in contrast to all other residues. This makes Gly a conserved residue in peptides and proteins because a mutation of Gly could change the flexibility necessary for function or cause significant alteration of the secondary structure. Peptides composed of Aib and Gly only show an enormous structural flexibility (Gessmann et al, 1991; Gessmann, Bruckner, Aivaliotis et al, 2015; Gessmann, Bruckner & Petratos, 2015) and normally normally do not yield suitable sized crystals for structure analysis with X-rays.
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