Amino acids, crystallization

When low-temperature studies are performed, the maximum resolution is imposed by data collection geometry and fall-off of the scattered intensities below the noise level, rather than by negligible high-resolution structure factor amplitudes. Use of Ag Ka radiation would for example allow measurement of diffracted intensities up to 0.35 A for amino-acid crystals below 30 K [40]. Similarly, model calculations show that noise-free structure factors computed from atomic core electrons would be still non-zero up to O.lA. 

Cu(II) impurity complexes in amino acid single crystals have been the subject of several EPR studies181-183. Since nitrogen and proton hf structures are only partially resolved in the EPR spectra, no detailed information about the electronic properties of the complex in the neighborhood of the metal ion can be evaluated. ENDOR spectroscopy has therefore been applied58,63 to draw detailed pictures of the positions and the molecular environment of Cu(II) impurities in amino acid crystals. 

There are several indications that a crystalline solid is the most appropriate state to model the protein interior (Chothia, 1984). The very fact that protein structures can be determined to high resolution by X-ray diffraction is indicative of the crystalline nature of the protein. Additionally, the packing density and volume properties of amino acid residues in proteins are characteristic of amino acid crystals (Richards, 1974, 1977). In spite of the apparent crystallinity of the protein interior, most model compound studies have investigated either the transfer of compounds from an organic liquid into water (see, for example, Nozaki and Tanford, 1971 Gill et al., 1976 Fauch-ere and Pliska, 1983), or the association of solute molecules in aqueous solution (see, for example, Schellman, 1955 Klotz and Franzen, 1962 Susi et al., 1964 Gill and Noll, 1972). Both these approaches tacitly assume a liquidlike protein interior. 

The OH 0 hydrogen bonds in the amino acid crystal structures show the same characteristics as in the carboxylic acids. There are more extensive neutron data available in this category, as shown in Thble 7.6. The H O distances range... 

Table 8.6. Geometry of three-center bonds In the amino acid crystal structures from neutron diffraction data...

Intramolecular hydrogen bonds in the amino acid crystal structures occur as the minor components of three-center bonds in the configuration... 

C-H 0=C hydrogen bonds may inhibit zwitterion formation in the crystalline state. This suggestion has been made on the basis of isonicotinic acid [464], and of nicotinyl glycine [465]. In these crystal structures, the observed C-H 0=C interactions with H 0 distances from 2.35 to 2.66 A and C-ft 0 angles in the range 123 0 to 170° appear to be preferred and suppress the formation of the more common NH 0=0 bonds. Since, however, there is no evidence of this in the amino acid crystal structures, this effect, if real, must be associated with the properties of the pyridine ring. 

In a survey of a representative group of 49 amino acid crystal structures which were determined by neutron diffraction or good precision X-ray diffraction analyses, there were 10 in class I, 25 in class II, 14 in class III and none in class IV. The absence of a representative of configuration IV is likely to be due to steric repulsions arising from molecular overcrowding. Examples of the three classes are illustrated in the following [74]. 

Class /. Of the ten amino acid crystal structures belonging to this class, five have carboxylate or hydroxyl oxygens as acceptor groups A, as shown in Fig. 14.2 for L-cysteine [LCYSTN12] and in Fig. 14.3 for L-lysine-L-aspartate [LYSASP], In two of the cases, A is a carboxylate or water oxygen or chloride ion, as shown in... 

Data on NH2+ hydrogen bonding are more limited. There are only three amino acid crystal structure analyses containing NH groups, one of which is a neutron diffraction study. They are all based on the amino acid proline ... 

Structural studies of a-amino acid crystals by H CRAMPS NMR... 

A study example concerning the solid structure of a-amino acid, polypeptide and a protein is given to introduce the basis of the chemical shift of proton NMR precisely, and studies to do this have only recently been undertaken. Some interesting work has been done, including the discrimination of amino acid crystal polymorphism, conformational analysis of polypeptides and fibrous proteins, and the determination of the N—H bond length in polypeptides. 

From these results, it was concluded that the polymorphic forms of L-histidine can be readily distinguished by H CRAMPS NMR spectra, even when it is quite difficult to distinguish using the C and N CP-MAS NMR spectra. From the H CRAMPS NMR measurement, it is concluded that the H chemical shifts of a-amino acid crystals are very sensitive to a small difference in the magnetic surroundings of protons as well as that in the hydrogen-bond network. Accordingly, the solid H CRAMPS NMR is a very useful means for structural analysis of L-histidine crystals. 

In conclusion, the polymorphic forms of some a-amino acid crystals such as a glycine and L-histidine have been successfully studied by H CRAMPS NMR spectra. For glycine crystals, the H" signals of a-glycine splits into two peaks... 

In this context, it is fascinating that 18 of the natural amino acids crystallize as racemates in a ratio of 1 1, except for threonine and arginine that crystallize as pure d- and pure L-crystals. Serine is particularly unusual as it is formed with an ee of 99% starting from an initial racemate in aqueous solution (Klussmann et al. 2006). Putting the data into perspective, one may conclude that it was likely that meteoritic impact delivered defined molecules from space to Earth. Among them, stereoisomers of a-methyl amino acids were in ee. These optically active compounds were the starting material for the transfer of chirality to other biomolecules. This... 

The ordering of the molecules within one sheet of an amino acid crystal may be straight or zig-zag (Fig. 9.2.2). In the straight arrangement, the molecules are all in the same orientation (translational symmetry) in the zig-zag arrangement, the molecules are ordered on a screw-axis (helical symmetry). 

Most hydrophobic amino acid crystals produce two straight sequences in two planes, whereas many hydrophilic amino acids have one plane with a straight and one plane with a zig-zag sequence. There is no obvious correlation between packing of the pure amino acids in crystals and favored secondary structures in proteins helix breakers may, for example, form helices in crystals of the pure amino acid. 

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