Crystal alanine

Crystalline Structures. Crystal shape of amino acids varies widely, for example, monoclinic prisms in glycine and orthorhombic needles in L-alanine. X-ray crystallographic analyses of 23 amino acids have been described (31). L-Glutamic acid crystallizes in two polymorphic forms (a and P) (32), and the a-form is mote facdely handled in industrial processes. The crystal stmeture has been determined (33) and is shown in Figure 1. 

Commercially preferred crystals for use as flavor enhancement are obtained by crystallization in the presence of amino acids such as alanine (7). 

Zwitterionic L-alanine ( HjN—CfCHj)—CO2—) is a dipolar molecule that forms large well-ordered crystals in which the molecules form hydrogen-bonded columns. The strong interactions lead to the presence of well-defined intra- and intermolecular vibrations that can usefully be described using hannonic theory. 

From studies in solution and in the sohd state, a number of structures both sheet-like [14] and helical [19, 21, 24—36] have been proposed over the years for these polymers. Poly(/9-alanine) 3 for example crystallizes as extended chains [14]... 

Examination of the backbone torsion angles in a number of crystal stractures of /9-alanine-containing peptides reveals that the conformation around the C(a)-C(/9) bond of /9-alanine residues is essentially gauche or trans (anti) with values close to 60° or 180°, respectively [158]. Populating the gauche conformation of /9-ami-... 

Fig. 2.11 Preferential conformations around the C(a)-C(/9) bond in the crystal structures of /9-alanine-containing peptides [158]...

To check this prediction, a number of MaxEnt charge density calculations have been performed with the computer program BUSTER [42] on a set of synthetic structure factors, obtained from a reference model density for a crystal of L-alanine at 23 K. The set of 1500 synthetic structure factors, complete up to a resolution of 0.555 A [45], was calculated from a multipolar expansion of the density, with the computer program VALRAY[ 46],... 

BUSTER chooses the minimal grid necessary to avoid aliasing effects, based on the prior prejudice used and on the fall-off of the structure factor amplitudes with resolution for the 23 K L-alanine valence density reconstruction the grid was (64 144 64). The cell parameters for the crystal are a = 5.928(1)A b = 12.260(2)A c = 5.794(1) A [45], so that the grid step was shorter than 0.095 A along each axis. 

Another hypothesis on homochirality involves interaction of biomolecules with minerals, either at rock surfaces or at the sea bottom thus, adsorption processes of biomolecules at chiral mineral surfaces have been studied. Klabunovskii and Thiemann (2000) used a large selection of analytical data, provided by other authors, to study whether natural, optically active quartz could have played a role in the emergence of optical activity on the primeval Earth. Some researchers consider it possible that enantioselective adsorption by one of the quartz species (L or D) could have led to the homochirality of biomolecules. Asymmetric adsorption at enantiomor-phic quartz crystals has been detected L-quartz preferentially adsorbs L-alanine. Asymmetrical hydrogenation using d- or L-quartz as active catalysts is also possible. However, if the information in a large number of publications is averaged out, as Klabunovskii and Thiemann could show, there is no clear preference in nature for one of the two enantiomorphic quartz structures. It is possible that rhomobohedral... 

Cheam, T. C. 1993. Normal Mode Analysis of Alanine Dipeptide in the Crystal Conformation Using a Scaled Ab Initio Force Field. J. Mol. Struct. 295,259-271. 

Hardness data for only two amino acids were found in the literature. They are glycine and alanine. They are the smallest of the amino acids. Both consist of rather flat tablet-like collections of atoms that form layered crystal structures in which the molecular sub-groups within the layers are held together by hydrogen bonds (Albrecht and Corey, 1939), and the molecules by London forces. Their hardnesses are ... 

The hardness value reported for alanine seems rather high, whereas the glycine value is typical for molecular crystals. 

The instability of these chiral monolayers may be a reflection of the relative stabilities of their bulk crystalline forms. When deposited on a clean water surface at 25°C, neither the racemic nor enantiomeric crystals of the tryptophan, tyrosine, or alanine methyl ester surfactants generate a detectable surface pressure, indicating that the most energetically favorable situation for the interfacial/crystal system is one in which the internal energy of the bulk crystal is lower than that of the film at the air-water interface. Only the racemic form of JV-stearoylserine methyl ester has a detectable equilibrium spreading pressure (2.6 0.3dyncm 1). Conversely, neither of its enantiomeric forms will spread spontaneously from the crystal at this temperature. 

The EPR spectra of Cu(II)-doped /-alanine single crystals show a well resolved triplet structure which is due to a nitrogen ligand nucleus182. Moreover, in the partially... 

Fig. 35. Projection in the be crystal plane of an /-alanine single crystal. Proposed positions for the Cu(II) impurities are marked with Roman letters. (Adapted from Ref. 63)...

Figure 13. Packing arrangement of N-( -cinnamoyl)-(S)-alanine delineated by the faces observed in the pure crystal (a) for clarity, only three molecules shown (b) complete arrangement.

Figure 14. Computer-drawn pictures of measured crystals of N-( -cinnamoyIM5)-alanine (a) pure (b) grown in the presence of the methyl ester (c) grown in the presence of -cinnamoyl-(R)-alanine.

Figure 30. Optical microscope pictures of the 010 faces of a glycine crystal after partial dissolution in the presence of (fl)-alanine (a) (010) face (6) (0T0) face.

Lock (113) found that TGS crystals grown in the presence of resolved alanine are unipoled that is, they contain only one enantiomorphous phase. Thus it should be possible to assign the absolute configuration of the occluded additive by measuring the electric dipole of the crystal, provided that one can predict which enantiomorph of TGS is induced by the resolved additive. 

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