Aram M. Petrosyan

Salts of Amino Acids

Within this chapter, the fundamental properties of amino acids are presented. The basic chemical features of amino acids are the ubiquitous amino and acid groups; the residue of the molecule is usually referred to as side chain. Of an infinite number of conceivable amino acids, twenty (plus a few more or less frequent) members are found in proteins of living beings and thus play a crucial role in the chemistry of life. From a chemical point of view, the chirality of most amino acids is an important feature, which is discussed in regard with the nomenclature systems conventionally employed. Chirality is related with symmetry, both of the molecule and the crystal structure of amino acids (or their salts). Moreover, the chemical flexibility of amino acids, both in terms of symmetry and in terms of their amphoteric nature, is reviewed, thus forming the frame of reference for the following chapters which deal with the actual amino acid salts.

Hexafluorosilicates of alanine

Abstract The existence of three types of salts of amino acid hexafluorosilicates 2 A+ · SiF62–, A2+ · SiF62– and 2(A ··· A+) · SiF62–, where A is an amino acid in zwitterionic state, A+ is a singly charged cation, A2+ is a doubly charged cation and (A ··· A+) is a dimeric cation with a short hydrogen bond, have been established, in the literature (Ghazaryan, V. V.; Fleck, M.; Petrosyan, A. M.: Salts of amino acids with hexafluorosilicate anion. J. Cryst. Growth (2011), doi: 10.1016/j.jcysgro.2011.11.017). Here we report the investigation of hexafluorosilicates of α-alanine. Both L- and DL-alanine form salts according to the main 2 A+ · SiF62– type: 2L-Ala+ · SiF62– · 3 H2O and 2DL-Ala+ · SiF62– · 2 H2O. In addition we found that L-alanine forms a new A+ · (A ··· A+) · SiF62– type of salt: L-Ala+ · (L-Ala ··· L-Ala+) · SiF62– · H2O. Results of structural and vibrational spectroscopic studies are described and discussed.

Mixed salts of amino acids: Syntheses, crystal structure and vibrational spectra of L-histidinium(2+) nitrate-perchlorate and L-histidinium(2+) nitrate-tetrafluoroborate

Abstract Crystals of L-histidinium(2+) nitrate-perchlorate (L-His2+ · NO3– · ClO4–) and L-histidinium(2+) nitrate-tetrafluoroborate (L-His2+ · NO3– · BF4–) have been obtained by crystallization from aqueous solutions. For both compounds, single-crystal XRD analysis as well as ATR FTIR and Raman spectroscopy was performed to determine and investigate the crystal structures, which turned out to be closely related, albeit not isotypic, although symmetry (space group P212121, Z = 4) and unit cell pa rameters of both compounds match to some extent. In each case the asymmetric unit contains a doubly charged L-His2+ cation with the charge counterbalanced by NO3– as well as ClO4– or BF4– anions. The structural differences between both compounds are subtle, mainly expressed in different conformations of the L-histidinium(2+) cations, which leads to different hydrogen bond networks. The L-His2+ cations form O—H···O hydrogen bonds to the nitrate anions and weak N—H···O and N—H···F hydrogen bonds. Powder SHG tests confirm considerable second order nonlinear optical activity for L-His2+ · NO3– · BF4– but not for L-His2+ · NO3– · ClO4–, this discrepancy being a result of the structural difference.

Crystal structure at 296 and 150 K, vibrational spectra and thermal behaviour of sarcosine sarcosinium nitrate

Abstract Crystals of sarcosine sarcosinium nitrate (2 Sar · HNO3) have been grown and characterized at 296 and 150 K by single crystal XRD and vibrational spectroscopy. The species was found to be triclinic (space group P-1, a = 5.320(1) Å, b = 6.578(1) Å, c = 15.957(2) Å, α = 95.448(6)°, β = 95.944(6)°, γ = 93.410(6)°, V = 551.6(1) Å3, and Z = 2 at 296 K), with a dimeric sarcosine ... sarcosinium cation and a nitrate anion in the asymmetric unit. A very strong hydrogen bond connects the molecules within the dimer. In addition, the tensor of thermal expansion was determined for the temperature range of 200 to 290 K. A strong anisotropy was detected, with thermal expansion along the c-axis approximately 14 times as strong as along the a-axis.

Amino Acid Structures

Although this book deals with salts of amino acids, this chapter discusses the structures of pristine amino acids, since these molecular structures are also found in salts, and factors as solubility and conditions of crystal growth are important for the synthesis of amino acid salts as well as the pure form. In this context, the impact of minimal changes in conditions (such as impurities) on the growth of amino acid crystals is noted. The structural variation is very high, as not only amino acids differ from each other, but many amino acids exist in more than one form. This refers to the fact that enantiopure crystals can be grown as well as racemates (so-called dl-amino acids). Moreover, often more than one hydration state is found: Anhydrous forms are common, but many amino acids form hydrated crystals. For some amino acids (e.g., glycine, proline, methionine), more than one polymorph of the same hydration state is found. Some of these polymorphs form at ambient condition, often due to minuscule changes in conditions. For others, variation in temperature and pressure has been found to be the cause of the formation of different polymorphs. High-pressure data are available for some amino acids (e.g., alanine, serine, cysteine). Most amino acids are found to form a so-called head-to-tail motif, where acid and amino groups connect to form infinite chains. In some of the larger, nonpolar amino acids, a bilayer pattern is found, where polar groups of opposing molecules face each other, forming a layer, with the hydrophobic side chain facing outward (this motif is found in phenylalanine, methionine, leucine, or isoleucine). Not all amino acids crystallize readily, as is proved by the crystal structure of l-arginine, which could be determined only very recently, and that of lysine, which could not be solved at all to date. In addition to the standard 20, some nonstandard amino acids are discussed. Finally, notes on remarkable physical effects (such as piezoelectric data) or possible applications (as for instance the interactions of amino acids with carbon nanotubes) are presented.

Comment on "A study on structural, optical, mechanical and ferroelectric properties of tri-glycine barium nitrate single crystals"

We argue that the “tri-glycine barium nitrate” crystal reported by R. Ezhil Vizhi and D. Rajan Babu, Ferroelectrics Letters Section 40, 1–10 (2013) is not a new ferroelectric but a dubious crystal.

L-Argininium phosphite – a new candidate for NLO materials

In order to investigate the possibility of salt formation in the L-Arg-H3PO3-H2O system, single crystals of L-argininium phosphite, C6H15N4O2(+)·H2PO3(-), were prepared by evaporation of an aqueous solution containing equimolar quantities of L-arginine and phosphorous acid. The asymmetric unit contains one L-argininium(+) cation and one phosphite [HPO2(OH)](-) anion. The phosphite anions form chains parallel to [010] by O-H...O hydrogen bonding, with an O...O distance of 2.630 (3) Å. The protonated amine and guanidyl groups of the L-argininium(+) cations form N-H...O hydrogen bonds with the carboxylate groups and anions. The IR and Raman spectra are discussed in relation to the crystal structure. The salt displays nonlinear optical (NLO) properties. Another salt was obtained from a solution with a 1:2 molar ratio of components, but was characterized by vibrational spectra only.

On the existence of “l-arginine acetamide”

We argue that the “l-arginine acetamide” crystal reported by Anitha et al. (J Therm Anal Calorim 119:785–789, 2015) is actually the well-known monoclinic form of anhydrous l-arginine hydrochloride crystal.

On the influence of L-threonine on thiourea and urea on L-threonine

Abstract We show that the papers on ‘L-threonine doped thiourea’ (Thilakavathi et al., Materials Research Innovations 20 (2016) 254–258) and on obtaining a compound ‘urea L-threonine’ (Jaikumar and Kalainathan, Crystal Research Technology 43 (2008) 565–571) are completely erroneous.

Comments on ‘Co-60 γ-irradiation effect on linear, nonlinear optical and electrical properties of a semiorganic L-alanine barium nitrate (LABN) crystal’

Abstract We prove that a so-called L-alanine barium nitrate NLO single crystal described by Suresh Kumar et al. (Materials Research Innovations, DOI:10.1080/14328917.2017.1323990) is a dubious crystal and the title paper is erroneous.