X-ray studies on crystalline complexes involving amino acids and peptides. XXVII. Effect of chirality, specific interactions and characteristic aggregation patterns in the structures of arginine and its complexes with formic acid

Topological analysis of DL-arginine monohydrate at 100 K

The charge density of DL-arginine monohydrate has been extracted from a synchrotron data set of 212121 reflections collected at 100 K with a Bruker CCD area detector up to a resolution of sin θ/λ = 1.40 Å-1 (d = 0.36 Å). DL-arginine monohydrate is characterised by a neutral α-amino group and a notable different hydrogen bond pattern compared to previously examined free arginine structures. All bond critical points of the electron density at the covalent and hydrogen bonds were located. We compare the local topological indices of ρ(r) with other amino acids. The conformation obtained from the experimental structure is found to be different from the Hartree-Fock (HF) optimization for the isolated molecule, thus the topology obtained from the static experimental charge density is compared to that derived from calculations at experimental and optimized geometry at HF and B3LYP levels of theory.

Salt bridge stabilization of charged zwitterionic arginine aggregates in the gas phase

The discovery of several new unusually stable aggregates of arginine that are intermolecularly bound by salt bridges is reported. Quadrupole ion-trap mass spectrometry provides evidence for the stability of arginine in the zwitterionic state, where the protonated guanidinium group of one arginine interacts strongly with the carboxylate of another to form stable noncovalent complexes, coordinated to either a cation or anion. Clusters of arginine with itself, sodium, potassium, lithium, magnesium, chloride, fluoride, bromide, iodide, and nitrate are observed. DFT calculations at the B3LYP/6-31G level are used to assess the structures and energetics of particularly prominent clusters. An examination of mixtures of D-arginine with isotopically labeled L-arginine indicates that the stability of these clusters does not depend on arginine enantiomeric purity. The cyclic trimers of arginine, capped with either Cl(-) or NO(3)(-), possess exceptional stability.

X-ray studies on crystalline complexes involving amino acids and peptides XXXIV. Novel mode of aggregation, interaction patterns and chiral effects in the maleic acid complexes of DL- and L- arginine

Amino acid - carboxylic acid complexes provide useful information in relation to molecular interactions in present day biological systems and to prebiotic self-organisation. The crystal structures of the complexes of maleic acid with DL- arginine (orthorhombic; Pca2(1); a=15.9829, b=5.4127, c=16.1885; R=0.0522 for 956 reflections) and L- arginine (triclinic; P1; a=5.2641, b=8.0388, c=9.7860, alpha=106.197, beta=97.275, gamma=101.64; R=0.039 for 1749 reflections) have been determined. The complexes are made up of positively charged zwitterionic arginine molecules and negatively charged semi-maleate ions which contain an intramolecular symmetric O-H-O hydrogen bond. In both the structures, the amino acid molecules aggregate into layers. In each layer, S2 head-to-tail sequences are interconnected through specific intermolecular interactions between alpha-carboxylate and guanidyl groups, an arrangement observed for the first time in crystal structures involving arginine. The carboxylate-guanidyl interactions are of different types in the two complexes and consequently aggregation patterns in them exhibit substantial differences. Interactions between the amino acid layers involve the semi-maleate ions in both the complexes. In addition, water-bridges also exist in the L complex. The full potential of the guanidyl group for specific interactions is realized in both the structures. The L complex contains an array of water-mediated salt bridges. The structures demonstrate that the effect of chirality on molecular aggregation can span a wide range.