Melting point amino acids

Melting Point. Amino acids are soHds, even the lower carbon-number amino acids such as glycine and alanine. The melting points of amino acids generally He between 200 and 300°C. Frequentiy amino acids decompose before reaching their melting points (Table 2). 

In this form, the amino acid is said to be a zwitterion, which is a net neutral compound that exhibits charge separation. An amino acid can be thought of as an internal salt, and as such, it will exhibit many of the physical properties of salts. For example, amino acids are highly soluble in water and have very high melting points. Amino acids are also said to be amphoteric, because they will react with either acids or bases. When treated with a base, an amino acid will function as an acid by giving up a proton (the ammonium group is deprotonated). 

Complexation by the amino acids prior to polymerization alters the kinetics of chain propagation, which is consistent with the presumed catalytic role of the metal in these reactions (57), but amino acid salt formation has further implications from the standpoint of chemical evolution the polymerizations may be performed at temperatures that more closely resemble those found on the Earth s surface. Amino acids form salts with carboxylic acids (e.g. formic and acetic), and these salts tend to melt at lower temperatures than the corresponding amino acid. For example, a nrixtuie of the formate salts of glutamic acid and glycine (analogous to the first Harada-Fox melt, reference 27) fuses at 150°C, and the yield of polymer after foiu hours exceeds the typical yield obtained with the corresponding melt of amino acids. This trend towards lower melting points continues as one increases the number of amino acids in the mixture as a mixture of their formate salt, the Fox-Harada mixture of 16 amino acids (cf. reference 28) is a viscous paste at room temperature. The formation of peptides and ultimately precipitation of macromolecules from a concentrated aqueous solution of amino acid salts can be effected at 50°C. 

Note that the amino-acids, because of their salt-like nature, usually decompose on heating, and therefore seldom have sharp melting-points. Furthermore, all naturally occurring amino-acids are a-amino-acids, and consequently, with the exception of glycine, can exist in optically active forms. 

The melting points of the derivatives of a number of amino acids are collected in Table 111,132. Most a-amino acids decompose on heating so that the melting points would be more accurately described as decomposition points the latter vary somewhat with the rate of heating and the figures given are those obtained upon rapid heating. 

Amino acid Melting point, °C Density, [ ]d t, °C f,% Solvent... 

Amino-l,2,4-triazole has been prepared by evaporating formylguanidine nitrate with sodium carbonate, and from 5(3)-amino-1,2,4-triazole carboxjdic acid-3(S) by heating above its melting point, or by a long digestion with acetic acid. ... 

Amino acids have high melting or decomposition points and are best examined for purity by paper or thin layer chromatography. The spots are developed with ninhydrin. Customary methods for the purification of small quantities of amino acids obtained from natural sources (i.e. l-5g) are ion-exchange chromatography (see Chapter 1). For general treatment of amino acids see Greenstein and Winitz [The Amino Acids, Vols 1-3, J.Wiley Sons, New York 1961] and individual amino acids in Chapters 4 and 6. 

The melting point of the ethyl ester of 2-amino-4-methylselena-zole-5-carboxylic acid is given as 180-181 °C and that of the free acid as 181-182°C. In addition, the melting points of the acetyl derivatives are very close to each other. These facts led King and... 

Sehlogl has shown that para-aminophenylglycine, para-amino-phenyloxamic acid, and para-aminoacetanilide form compounds with aldehydes, which have sharp melting-points and are suitable for the characterisation of aldehydes. With p-aminophenylglycine condensation takes place when the glycine, mixed with alcohol and the aldehyde in question, is warmed. For the purpose of condensing with p-aminophenyloxamic acid and with p-aminoacetaldehyde the alcoholic suspension of the amino-hody is acidulated slightly with hydrochloric acid and the solution is warmed after the aldehyde has heen added. This method yields the hydrochloride of the condensation products. 

To obtain the corresponding amino derivative, 109 g of base are heated under agitation in a round-bottomed flask with 300 cc of 35-36% concentrated hydrochloric acid and 600 cc of water. It is heated on a water bath until dissolution is complete, then maintained at boiling point for 90 minutes, cooled, diluted with 1 liter of water, and neutralized with about 350 cc of 30% soda. The N-(2-diethylaminoethyl)-2-methoxy-4-amino-5-chlorobenzamide formed crystallizes, is centrifuged and washed in water. Its melting point is 122°C and the yield is 74%. 

A 30 g (0.15 mol) quantity of 4-chloro-2.6-diaminopyrimidine is dissolved in 600 ml of hot 3A alcohol, the solution cooled to 0°C to 10°C and 41.8 g (0.24 mol) of m-chloroperbenzoic acid is added. The mixture is held at 0°C to 10°C for 4 hours and filtered. The solid is shaken for 2 hours in 0.24 mol of 10% sodium hydroxideand filtered. The solid is washed with water and dried to yield 19.3 g of crude product. This product Is extracted for 1 hour with 900 ml of boiling acetonitrile to yield 14.8 g (44.7% yield) of 6-amino-4-chloro-1,2-dihydro-1 -hy-droxy-2-iminopyrimidine, melting point 193°C. 

A solution of N-(2-aminobenzvl)-1-phenyl-2-metKylaminoethanol-1 was prepared by the reaction of a-bromo-acetophenone and (2-nitrobenzyl)methylamine, followed by hydrogenation of the nitro group by means of nickei on diatomaceous earth at room temperature and reduction of the CO group by means of sodium borohydride. The intermediate thus produced was dissolved in 100 ml of methylene chloride and introduced dropwise into 125 ml of sulfuric acid at 10° to 15°C. After a short standing, the reaction mixture was poured onto ice and rendered alkaline by means of a sodium hydroxide solution. Dy extraction with ether, there was obtained 1,2,3,4-tetrahydro-2-methyl-4-phenyl-8-amino-iso-quinoline. The base is reacted with maleic acid to give the maleate melting point of the maleate 199° to 201°C (from ethanol). 

Amino-5-methoxy pyrimidine is obtained having a melting point of about 300°C by condensation of methoxymalonic acid ester with guanidine carbonate in the presence of sodium ethylate. The resultant reaction product is then converted to 2-amino-5-methoxy-4,6-di-chloropyrimidine (melting point 216°C to 217°C) by heating this reaction product with phosphorus oxychloride. The dichloro compound is then suspended in water with zinc dust and... 

After the said 2 to 3 hours the liquid is cooled and the bottom sediment, which has a greenish color, is filtered off. The liquid sucked off eventually is treated with active carbon, filtered and made slightly acid by means of acetic acid, at which 2-amino-benzolsulfon-amido-5-methyl-1,3,4-thiodiazol (melting point 204° to 206°C) is precipitated. 

The total amount of the hydrochloride obtained Is stirred with 50 cc of water and the mixture is mixed with 15 cc of 45% caustic soda solution. After complete dissolution, the mixture is treated with decolorizing carbon and the filtrate is brought to a pH value of 5.5 by means of hydrochloric acid, 1 7.6 g of p-aminobenzenesulfonyl-2-amino-4,5-dimethyloxazole are obtained as colorless crystals with a melting point of 193°C to 194°C (corrected), corresponding to a yield of 65.9% calculated on the basis of the 2-amino-4,5-dimethyloxazole used. 

Amino acid zwitterions are internal salts and therefore have many of the physical properties associated with salts. They have large dipole moments, are soluble in water but insoluble in hydrocarbons, and are crystalline substances with relatively high melting points. In addition, amino acids are amphiprotic they can react either as acids or as bases, depending on the circumstances. In aqueous acid solution, an amino acid zwitterion is a base that accepts a proton to yield a cation in aqueous base solution, the zwitterion is an add that loses a proton to form an anion. Note that it is the carboxylate, -C02-, that acts as the basic site and accepts a proton in acid solution, and it is the ammonium cation, -NH3+, that acts as the acidic site and donates a proton in base solution. 

The charged functional groups of amino acids ensure that they are readily solvated by—and thus soluble in— polar solvents such as water and ethanol but insoluble in nonpolar solvents such as benzene, hexane, or ether. Similarly, the high amount of energy required to disrupt the ionic forces that stabilize the crystal lattice account for the high melting points of amino acids (> 200 °C). 

Table 2 shows melting points of representative examples of the keto acids (66) having a tertiary amino group at 4-position. 2-(4-Diethylamino-2-hydroxybenzoyl)benzoic acid (66b) is the most popular among the keto acids. 

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