Arginine functional group

Arginine, the most basic of the 20 common amino acids, contains a guanidino functional group in its side chain. Explain, using resonance structures to show how the protonated guanidino group is stabilized. 

Some amino acids such as aspartic acid and glutamic acid contain an additional acid functional group, while amino acids such as lysine, arginine, and histidine contain additional basic groups. The presence of these units will confer to the protein tendencies to move toward the anode or cathode. The rate of movement is dependent on a number of factors including the relative abundance and accessibility of these acid and base functional groups. 

FIGURE 3-8 Uncommon amino acids, (a) Some uncommon amino acids found in proteins. All are derived from common amino acids. Extra functional groups added by modification reactions are shown in red. Desmosine is formed from four Lys residues (the four carbon backbones are shaded in yellow). Note the use of either numbers or Creek letters to identify the carbon atoms in these structures, (b) Ornithine and citrulline, which are not found in proteins, are intermediates in the biosynthesis of arginine and in the urea cycle. 

In summary, protein molecules may contain up to nine amino acids that are readily derivatizable at their side chains aspartic acid, glutamic acid, lysine, arginine, cysteine, histidine, tyrosine, methionine, and tryptophan. These nine residues contain eight principal functional groups with sufficient reactivity for modification reactions primary amines, carboxylates, sulfhydryls (or disulfides), thioethers, imidazolyls, gua-nidinyl groups, and phenolic and indolyl rings. All of these side chain functional groups in addition to the N-terminal a-amino and the C-terminal a-carboxylate form the full complement of polypeptide reactivity within proteins (Fig. 12). 

We found that the color yielded for primary alkyl amine positive tests is strongly influenced by the presence of other functional groups. Specifically we could not establish a clear positive for arginine, asparagine, and cysteine when their lateral chains were deprotected. A similar effect was observed in the case of secondary amines or sterically hindered amines such as Aib. 

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