Amino groups, liberation

Another important method of following protein hydrolysis is that due to Van Slyke, and consists in estimating the free amino groups liberated by treatment with nitrous acid, whereby gaseous nitrogen is evolved and measured in a special apparatus. 

Nitro groups have been reduced to amino groups, whilst amino groups in the 3- and 6-positions of pyrido[2,3-f ]pyrazines and in the 5-position of the [3,4-f ] isomers have been hydrolyzed to the corresponding hydroxy derivatives with alkali. Protected amino groups have been liberated by hydrolysis or reduction in deazapteridine syntheses. 

After the A-acetyl-protecting group has served its purpose, it may be removed by hydrolysis, liberating the amino group ... 

The determination of amino nitrogen before and after acid hydrolysis of urine has frequently been used for the quantitative estimation of the amount of urinary peptides (H5, M4). The number of liberated a-amino groups represents, in fact, the whole of formerly combined amino groups, not necessarily attached to a second amino acid partner. Besides, considerable losses connected with decomposition of some amino acids occur in the course of hydrolysis thus limiting the true quantitative value of this procedure. 

FIGURE 5.7 Merrifield resin obtained by polymerization of styrene in the presence of divinylbenzene followed by reaction with CH3OCH2Cl.3 The amino-methyl resin is obtained by transformation or reaction of the copolymer with A -hydroxymethylphthalimide, followed by hydrazinolysis to liberate the amino groups.31... 

Once it is part of a cyclic dipeptide, the prolyl residue becomes susceptible to enantiomerization by base (see Section 7.22). The implication of the tendency of dipeptide esters to form piperazine-2,5-diones is that their amino groups cannot be left unprotonated for any length of time. The problem arises during neutralization after acidolysis of a Boc-dipeptide ester and after removal of an Fmoc group from an Fmoc-dipeptide ester by piperidine or other secondary amine. The problem is so severe with proline that a synthesis involving deprotection of Fmoc-Lys(Z)-Pro-OBzl produced only the cyclic dipeptide and no linear tripeptide. The problem surfaces in solid-phase synthesis after incorporation of the second residue of a chain that is bound to the support by a benzyl-ester type linkage. There is also the added difficulty that hydroxymethyl groups are liberated, and they can be the source of other side reactions. 

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