Diamino acids estimation

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... 

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. 

The histones contain about 30 per cent, of diamino acids, and only in the case of thymus-histone has an estimation been made of the monoamino acids. They were supposed to be intermediate compounds between protamines and other proteins, and this supposition is confirmed by the results of analysis. 

Interestingly, Arduengo observed that Villa was stable with respect to dimerization in the absence of a Brpnsted or Lewis acid catalyst." Similarly, in the absence of an acid catalyst, dimerization of IXb is extremely slow and is first order in carbene. Therefore, the observed formal dimerization of Villa and IXb,c does not involve the coupling of two carbenes, but the nucleophilic attack of one carbene upon its conjugate acid, followed by proton elimination, as already suggested by Chen and Jordan (Scheme 8.13). It is important to keep in mind that even A,A-dialkylimidazolium ions have pXa values of 24 in DMSO, and based on the calculated proton acidity, Alder estimated the pXa values for acyclic diamino-carbenes to be from 2 to 6 pXa units higher than for imidazol-2-ylidenes. " ... 

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