Guanidino groups

The guanidine-type resonance, which is present in aliphatic guanidines, is stronger than that in the corresponding amidines, at least in the case of the cationic forms. 2-Aminopteridine and 2-aminoquinazoline cations are two examples of compounds in which hydration is stabilized by the presence of a guanidino group (32). 

The association of a specific pharmacologic activity with certain functionality was remarked on earlier. The guanidino group, for example, often yields compounds that show hypotensive activity because of peripheral sympathetic blockade (see, for example, bethanidine). Attachment of a piperidine group to the side chain proves compatible with retention of this activity. 

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. 

Evidently, the titratable functions of M are two very strong bases, since the equivalent weight of M calculated from the curve was 391, corresponding to approximately half the molecular weight (791) calculated from the analyses discussed above. Apparently, one of the functions was due to the guanidino group of the isolated... 

Importantly, the crystal structure of 34 complexed with N9 sialidase (Fig. 8) indicated differences in the orientation of the guanidino group in subsite S2, and in its interaction with the active site residues, compared to that of zanamivir (Babu et al. 2000). These differences have implications for cross-reactivity of 34 with zanamivir-resistant influenza viruses that have Glul 19 mutations in the sialidase S2 subsite (see Sect. 5.1). 

Figure 5.37 APG can be used to label specifically arginine residues in proteins, producing stable, cyclic Schiff base-like bonds with the side-chain guanidino groups. Photoactivation with UV light then causes ring expansion of the phenyl azide group, initiating covalent bond formation with amines.

Some studies document hydrolysis of an arginine residue to ornithine and/or citrulline [87], The reactions are insufficiently understood, but an attractive hypothesis is that they involve the hydrolysis of the imine bond of the guanidino group (see also Sect. 11.6). 

As shown in Fig. 6.24, the guanidino group of arginine (6.59) can exist in two tautomeric forms. According to the hypothetical mechanism present-... 

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