Amino acids, isolation salts

Equilibration experiments have been carried out to assess the degree of chiral recognition exhibited by (iJ/J)-(28) and (/ /2)-(29) towards racemic amino acid ester salts and other substituted ammonium salts [4,30,51,52], The optically pure hosts dissolved in CHCI3 were equilibrated with HjO solutions of the racemic salts at 0°C and the amounts of each enantiomer extracted into the CHCI3 layers were estimated polarimetrically after isolation. An Enantiomeric Distribution Constant (EDC), defined as the ratio, where and are, respectively, the distribution... 

Miscellaneous Reactions. Sodium bisulfite adds to acetaldehyde to form a white crystalline addition compound, insoluble in ethyl alcohol and ether. This bisulfite addition compound is frequendy used to isolate and purify acetaldehyde, which may be regenerated with dilute acid. Hydrocyanic acid adds to acetaldehyde in the presence of an alkaU catalyst to form cyanohydrin the cyanohydrin may also be prepared from sodium cyanide and the bisulfite addition compound. Acrylonittile [107-13-1] (qv) can be made from acetaldehyde and hydrocyanic acid by heating the cyanohydrin that is formed to 600—700°C (77). Alanine [302-72-7] can be prepared by the reaction of an ammonium salt and an alkaU metal cyanide with acetaldehyde this is a general method for the preparation of a-amino acids called the Strecker amino acids synthesis. Grignard reagents add readily to acetaldehyde, the final product being a secondary alcohol. Thioacetaldehyde [2765-04-0] is formed by reaction of acetaldehyde with hydrogen sulfide thioacetaldehyde polymerizes readily to the trimer. 

Although a C—CN bond is normally strong, one or two cyano groups in TCNE can be replaced easily, about as easily as the one in an acyl cyanide. The replacing group can be hydroxyl, alkoxyl, amino, or a nucleophilic aryl group. Thus hydrolysis of TCNE under neutral or mildly acidic conditions leads to tricyanoethenol [27062-39-17, a strong acid isolated only in the form of salts (18). 

From a practical point of view, literature data indicate that it is not necessary in most cases to isolate the title salts prior to their reaction. Tliis can be readily understood since their preparation is nearly quantitative and sulfur dioxide is the sole product evolved during their formation. In fact, it is highly advisable to recourse to a two-step procedure during the reactions between A -(l-chloroalkyl)pyridinium chlorides and amino acids (Section IV,C,6) (97BSB383). 

Reactions between A -(l-chloroalkyl)pyridinium chlorides 33 and amino acids in organic solvents have a low synthetic value because of the low solubility of the amine partner. A special protocol has been designed and tested in order to circumvent this drawback. Soon after the preparation of the salt, an aqueous solution of the amino acid was introduced in the reaction medium and the two-phase system obtained was heated under reflux for several hours. However, this was not too successful because sulfur dioxide, evolved during the preparation of the salt, was converted into sulfite that acted as an 5-nucleophile. As a result, A -(l-sulfonatoalkyl)pyridinium betaines such as 53 were obtained (Section IV,B,3) (97BSB383). To avoid the formation of such betaines, the salts 33 were isolated and reacted with an aqueous solution of L-cysteine (80) to afford thiazolidine-4-carboxylic acids hydrochlorides 81 (60-80% yields). 

Detection of Variants With Altered Functional Properties. When substitutions In either a-or 3-chains Involve amino acid residues that participate In the contact with heme or the contact between chains, changes In functional properties can occur and the determination of the oxygen affinity of the blood sample or of an Isolated hemoglobin variant Is desirable. Oxygen affinity Is affected by temperature, pH, salt concentration, the level of 2,3-dlphosphoglycerate (2,3-DPG), and to a lesser extent by the concentration of the hemoglobin. The concentration of 2,3-DPG In blood changes rather rapidly after collection and a... 

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... 

Valine.—This amino acid is contained mixed with leucine in the fractions of the esters which boil between 6o° and 90° C. Its isolation and separation from leucine is of extreme difficulty, since these compounds, as well as their copper salts into which they are converted by boiling with freshly precipitated cupric oxide, tend to form mixed crystals. Its isolation was only effected by these means in certain cases, and its amount is really much more than the figures represent from its yield. It is best characterised by conversion into its phenylhy-dantoine derivative by treatment with phenyl isocyanate in alkaline solution. The phenylureido acid is first formed, and this loses a molecule of water, as shown by Mouneyrat, and is changed into its anhydride or phenylhydantoine by treatment with hydrochloric acid. The following reactions occur —... 

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. 

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