Crystallisation amino acids

This interpretation is supported by literature studies on copper(II) complexes containing two -amino-acid ligands. For N-unsubstituted -amino-acid ligands, deductions as to position of the cis -trans geometrical equilibrium in solution are difficult as illustrated by the fact that for some -amino acids solid complexes have been isolated of both the ds and trans geometry. In contrast it seems as if copper(II) complexes containing two N-alkylated -amino-acid ligands crystallise exclusively in the trans form ". ... 

Because of their zwitterionic nature, amino acids are generally soluble in water. Their solubility in organic solvents rises as the fat-soluble portion of the molecule increases. The likeliest impurities are traces of salts, heavy metal ions, proteins and other amino acids. Purification of these is usually easy, by recrystallisation from water or ethanol/water mixtures. The amino acid is dissolved in the boiling solvent, decolorised if necessary by boiling with Ig of acid-washed charcoal/lOOg amino acid, then filtered hot, chilled, and set aside for several hours to crystallise. The crystals are filtered off, washed with ethanol, then ether, and dried. 

It appears as if an axiom of stereochemistry, the absolute identity of the most important chemical and physical properties of chiral isomers, is no longer valid. Experiments using the amino acid tyrosine (Tyr) showed unexpected differences in the solubility of D-and L-Tyr in water a supersaturated solution of 10 mM L-Tyr crystallised much more slowly than that of D-Tyr under the same conditions. The saturated solution of L-Tyr was more concentrated than that of D-Tyr. Supersaturated solutions of DL-Tyr in water formed precipitates containing mainly D-Tyr and DL-Tyr, so that there was an excess of L-Tyr in the saturated solution. The experiments were carried out with extremely great care in order to exclude the possibility of contamination. Further experiments will show whether this is a particular property of tyrosine, or whether other amino acids will show similar behaviour. Possible... 

These experimental results could not be confirmed by Lahav and co-workers they suggest that impurities in the starting materials have a much greater effect on the crystallisation process than the PVED (Parity Violating Energy Difference). Extensive experimental studies indicate the importance of small quantities of impurities, particularly in early phases of crystallisation nucleus formation. Amino acids from various sources were used, and the analyses were carried out using the enan-tioselective gas chromatography technique (M. Lahav et al 2006). 

The topical homochirality problem is presently being investigated in several research laboratories across the world. One new object of study is systems with eutectic mixtures. The addition of chiral dicarboxylic acids that co-crystallise with chiral amino acids to aqueous mixtures of d- and L-amino acids allows tuning of the eutectic composition of the amino acids in several cases, these systems yield new eutectic compositions of 98% ee or higher. Thus, solid mixed crystals with a ratio... 

Many compounds are less soluble as racemates than as their pure enantiomers. It thus appears probable that evaporation of an amino acid solution with a low ee should cause selective precipitation of the racemate crystals, which in turn should lead to an increase of the ee. Extremely simple manipulations, carried out in the chemistry department of Columbia University, led to a drastic increase in enantiomeric excess of phenylalanine 500 mg phenylalanine (with a 1 % ee of the L-component) was dissolved in water, and the resulting solution slowly evaporated until about 400 mg had crystallised out. The remaining solution contained a few mg of phenylalanine with 40% ee of the L-component (i.e., a 70 30 ratio of l to d). If 500 mg of such a solution (40% ee in water) is allowed to evaporate and is separated from the racemate, the result is about 100 mg, with 90% ee of the L-enantiomer (Breslow and Levine, 2006). 

The results found in the literature and from these experiments enabled us to select parameters which may give an indication of the degree of integration. These are resistance to diseases and pests, overall taste, phenols, ratio of proteins to free amino acids (physiological amino acid status), integration score on crystallisation pictures and species-typical colour ratio in spectral-range luminescence. 

As the result of hydrolysis a complex mixture of all, or nearly all, the previously mentioned units is obtained. These have been isolated by various methods based upon the fractional crystallisation of the compounds themselves, or of their copper, silver and other salts. Only when one or more of the amino acids occurred in somewhat large amounts was their isolation and characterisation effected their amount seldom reached a value higher than 20 per cent, of the total quantity and the remainder was represented by uncrystallisable syrups of unknown nature. A great advance was made when Drechsel discovered that the protein molecule contained diamino acids as well as monoamino acids, and to Kossel and Kutscher we owe our chief knowledge concerning their isolation and estimation. Emil Fischer, in igoi, by his... 

Three of the amino acids, naniely, tyrosine, cystine and diaminotrioxy-dodecanic acid are characterised by their extremely slight solubility in neutral aqueous solutions. They are therefore easily obtained after hydrolysis by acids by neutralising and concentrating the solution, when they crystallise out. 

Leucine.—The greater part of the leucine is contained in the ester fractions, which boil between 70° and 90° C. It generally occurs in considerable amounts in the protein, and is obtained by crystallisation from water, in which it is less soluble than the other amino acids which may be present. It is seldom present in its pure, optically active form, as this is easily racemised, and the various crops of crystals most probably also contain isoleucine. It is more easily isolated by completely racemis-ing the mixture of amino acids contained in this fraction by heating in an autoclave with baryta to 160-180° C., and then, after removal of the baryta, separating it by crystallisation. The difficulty of separating it from the other amino acids, especially valine and isoleucine, makes an exact quantitative estimation almost impossible. The values which have been found are therefore minimal ones, and they will also include in many cases the yield of isoleucine. 

Proline.—This is the only product of hydrolysis obtained from an ester fraction which is soluble in alcohol it is also much more easily soluble in water than the other products with which it is present and therefore is somewhat easily separated, as it remains in the mother-liquor after these have crystallised out. The solution, in which it is contained, is evaporated to dryness and extracted with absolute alcohol the combined alcoholic extracts from the several fractions are evaporated to dryness and taken up by absolute alcohol several times, so as to remove small amounts of the other amino acids, which, though insoluble in alcohol, are dissolved when proline is present. 

Glutamic Acid.—The greater part of the glutamic acid is isolated as hydrochloride before the mixture of amino acids is esterified. It is contained with aspartic acid ester in the aqueous solution after the phenylalanine ester has been extracted by ether, and it is separated from aspartic acid, after hydrolysis by baryta, by conversion into its hydrochloride from this it is obtained by treatment with the calculated quantity of soda to combine with the hydrochloric acid and by crystallisation from water, in which it is soluble with some difficulty. 

Only in a few cases has this compound been isolated from the products of hydrolysis of proteins, since its separation is extremely laborious. It can only be effected after all the other amino acids have been removed by crystallisation and by the ester method, and after the diamino acids have been precipitated by phosphotungstic acid. From the last mother-liquors it is obtained by crystallisation, and is best identified in the form of its /8-naphthalene sulpho-derivative. 

The separation and estimation of the two main groups of amino acids can be carried out in one experiment, instead of separately as described. The protein is hydrolysed by sulphuric acid, the tyrosine, cystine and diaminotrioxydodecanic acid are removed by crystallisation, and the diamino acids are precipitated by phosphotungstic acid. From this precipitate they are obtained by decomposition with baryta, and they are then separated by means of their silver compounds by Kossel, Kustcher and Patten s method. The filtrate from the phosphotungstic acid precipitate is freed from the excess of phosphotungstic acid by means of baryta, and the solution is treated by Fischer s ester method for the monoamino acids. 

Hippuric acid, or benzoylglycine, has been known for a long time, and by preparing the benzoyl derivatives of the other amino acids, Fischer found that their acidic character was greatly increased, and that they then combined with the optically active bases brucine, strychnine, cinchonine, morphine, forming stable salts. These salts were much less soluble and their power of crystallising much greater than the salts of the amino acids themselves, and consequently they were more easily isolated further, they were easily reconverted into the amino acids. 

L-Histidine [71-00-]] M 155.2, m 287°(dec), [a] -39.7" (HjO), -hl3.0" (6M HCI). Likely impurity is arginine. Adsorbed from aqueous soln on to Dowex 50-H+ ion-exchange resin, washed with 1.5M HCI (to remove other amino acids), then eluted with 4M HCI as the dihydrochloride. Histidine is also purified as the dihydrochloride which is finally dissolved in water, the pH adjusted to 7.0, and the free zwitterionic base crystallises out on addition of EtOH. 

Dissolve 0-01 g. equivalent of the amino acid in 20 ml. of N sodium hydroxide solution and add a solution of 2 g. of p-toluenesulphonyl chloride in 25 ml. of ether shake the mixture mechanically or stir vigorously for 3-4 hours. Separate the ether layer acidify the aqueous layer to Congo red with dilute hydrochloric acid. The derivative usually crystallises out rapidly or will do so on standing in ice. Filter off the crystals and recrystallise from 4-5 ml. of 60 per cent, alcohol. 

Recently, studies of the conformation of oligomers were extended to peptides derived from /3-methyl-L-aspartates. Their synthesis (n = 2 up to 14) was described by Goodman and Boardman (82), and later the specific rotations of their solutions in dimethyl formamide, dichloro-acetic acid and in chloroform were determined (83). The oligomers exist in a random-coil form in the first two solvents, but helices become stable in chloroform for n — 11 and 14. These peptides are unusual since their L-amino-acid residues produce a left-hand helix (84, 85) whereas most of the investigated polyamino acids crystallise as a right-hand helix (86). 

To obtain free alanine the alanine ester is heated for about 6 hours with 5 times its weight of water on a water bath, until the alkaline reaction has disappeared. The solution is evaporated on a water bath till crystallisation begins. The liquid is allowed to stand at 0°, when about 30 gms. alanine separate optical examination shows this to consist of almost pure d-compound. From the mother liquor a second crop of 20—25 gms. may be obtained, and this still consists of fairly pure active amino-acid, so that the total yields amount to 50—55 gms. The last mother liquor still contains a fair amount of active alanine, but it is mixed with so much racemic substance that it cannot be separated from it by mere recrystallisation from water. The first two crops are dissolved once more in hot water, and the liquid evaporated on the water bath till it begins to crystallise. At 0° a large quantity of the pure, active amino-acid separates out. 

The benzylbromomalonic acid containing water is now heated in an oil bath to 125°—130°, and the fused mass evolves carbon dioxide and a certain amount of hydrobromic acid. The reaction is complete in the course of 30—45 minutes. The residue is a yellow oil, which even at a low temperature does not crystallise, and which in the main consists of phenyl-a-bromopropionic acid. For the purpose of purification it is washed with water, taken up in ether, and dried with anhydrous sodium sulphate the ether is then distilled off. The mobile, almost colourless oil remaining is dissolved in 5 times its volume (excess) of 25% aqueous ammonia, and either heated for 3 hours to 100° in a sealed tube or allowed to stand for 3 to 4 days at ordinary temperature. On evaporation of the ammo-niacal solution an almost colourless residue is left, and this chiefly consists of ammonium bromide and phenylalanine. On boiling with absolute alcohol the amino-acid is left undissolved and is recrystallised from hot water. 

A major problem, until recently, was the determination of the protein primary structure, but with the advent of modern analysis of DNA this has become comparatively easy. One of the first structures to be described was that of insulin which contains 60 amino-acids and has a molecular weight of 12,000. Once the primary structure is known, it is possible to predict the secondary and tertiary structures using additional information obtained through X-ray crystallography of the crystallised protein. 

The crude product contains some of the aromatic amino acid tyrosine which is present in the original hydrolysate. A portion of this is removed by the charcoal treatment and by the hot-water washing, but the final recrystallised cystine may be contaminated with tyrosine if the suggested 5-6 hour period for crystallisation is greatly exceeded. 

Experiment 5.221 RESOLUTION OF dl-ALANINE Benzoyl DL-alanine. Dissolve lOOg (1.1 mol) of DL-alanine (Expt 5.180) in 400 ml of water containing 44.5 g (1.1 mol) of sodium hydroxide and cool the solution in an ice bath. Add 175g (1.2 mol) of benzoyl chloride and a solution of 49 g (1.2 mol) of sodium hydroxide in 200 ml of water to the stirred, cooled, amino acid solution, alternately and in portions during 2 hours continue to stir for a further 2-hour period. Boil the reaction mixture with 10 g of decolourising charcoal, filter, cool the clear yellow filtrate to 0 °C and acidify carefully to Congo red with concentrated hydrochloric acid. Triturate a portion of the oil which separates with water to induce crystallisation and then seed the bulk of the acidified solution with crystals and leave in an ice bath to complete the crystallisation process. Filter off the product, wash the filter cake with 500 ml of ice-cold water and recrystallise from about 3.5 litres of boiling water. The yield of benzoyl-DL-alanine, m.p. 162-164 °C, is 194.5 g (90%). 

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