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In a de­hydro­amino acid with a C=C bond between the α- and β-C atoms, the amino acid residues are linked trans to each other and there are no strong intra­molecular hydrogen bonds. The torsion angles indicate a non-helical conformation of the mol­ecule.

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In the crystal structure of the de­hydro­dipeptide (Boc-Phe-ΔAla-OiPr), the mol­ecule has a trans configuration of the N-methyl­amide group. Its geometry is different from saturated peptides but is in excellent agreement with other de­hydro­alanine compounds. In the crystal, an N—H...O hydrogen bond links the mol­ecules in a herringbone packing arrangement.

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In the title hydrated mol­ecular salt, C3H8N+·C2HO4·0.5H2O, the water O atom lies on a crystallographic twofold axis. The C=C—C—N torsion angle in the cation is 2.8 (3)° and the dihedral angle between the CO2 and CO2H planes in the anion is 1.0 (4)°. In the crystal, the hydrogen oxalate ions are linked by O—H...O hydrogen bonds, generating [010] chains. The allyl­ammonium cations bond to the chains through N—H...O and N—H...(O,O) hydrogen bonds. The water mol­ecule accepts two N—H...O hydrogen bonds and makes two O—H...O hydrogen bonds. Together, the hydrogen bonds generate (100) sheets.

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The asymmetric unit of the title compound, C2H8N+·C4H5O4, consists of two allyl­ammonium cations and two hydrogen succinate anions (Z′ = 2). One of the cations has a near-perfect syn-periplanar (cis) conformation with an N—C—C—C torsion angle of 0.4 (3)°, while the other is characterized by a gauche conformation and a torsion angle of 102.5 (3)°. Regarding the anions, three out of four carboxilic groups are twisted with respect to the central C–CH2–CH2–C group [dihedral angles = 24.4 (2), 31.2 (2) and 40.4 (2)°], the remaining one being instead almost coplanar, with a dihedral angle of 4.0 (2)°. In the crystal, there are two very short, near linear O—H...O hydrogen bonds between anions, with the H atoms shifted notably from the donor O towards the O...O midpoint. These O—H...O hydrogen bonds form helical chains along the [011] which are further linked to each other through N—H...O hydrogen bonds (involving all the available NH groups), forming layers lying parallel to (100).

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In the title hydrated mol­ecular salt, C4H12N+·C2HO4·0.5H2O, the O atom of the water mol­ecule lies on a crystallographic twofold axis. The dihedral angle between the CO2 and CO2H planes of the anion is 18.47 (8)°. In the crystal, the anions are connected to each other by strong near-linear O—H...O hydrogen bonds. The water mol­ecules are located between the chains of anions and iso­butyl­amine cations; their O atoms participate as donors and acceptors, respectively, in O—H...O and N—H...O hydrogen bonds, which form channels (dimensions = 4.615 and 3.387 Å) arranged parallel to [010].

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The title salt, 2C3H8N+·C2O42−, crystallized with six independent allyl­ammonium cations and three independent oxalate dianions in the asymmetric unit. One of the oxalate dianions is nearly planar [dihedral angle between CO2 planes = 1.91 (19)°], while the other two are twisted with angles of 11.3 (3) and 26.09 (13)°. One cation has a synperiplanar (cis) conformation with an N—C—C—C torsion angle of 0.9 (3)°, whereas the five remaining cations are characterized by gauche arrangements, with the N—C—C—C torsion angles ranging from 115.9 (12) to 128.8 (3)°. One of the allyl­ammonium cations is positionally disordered (fixed occupancy ratio = 0.45:0.55). In the crystal, the cations and anions are connected by a number of strong N—H...O and N—H...(O,O) hydrogen bonds, forming layers parallel to (001), with the vinyl groups protruding into the space between the layers.
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