Guanidinium group

The introduction of another organic cation function, guanidinium group, into macrocyclic structures such as (IV)-(VI) produces ligands which also display affinity for phosphate anions58). 

Yang W, Yan J, Fang H et al (2003) The first fluorescent sensor for D-glucarate based on the cooperative action of boronic acid and guanidinium groups. Chem Commun 792-793... 

Fig. 8. Comparison of the pterin-bound and -free structures. In the absence of pterin with the SEITU inhibitor in the active site (right), L-Arg binds in the pterin site. L-Arg is an excellent mimic for H4B owing to the similar H-bonding pattern with the heme propionate and Serl04, and the ability of the L-Arg guanidinium group to aromatically stack with Trp449.

In the final step, isourea is released from the guanidinium group of the arginine by hydrolysis (not shown), and is immediately rearranged into urea. In addition, ornithine is regenerated and returns via the ornithine transporter into the mitochondria, where it becomes available for the cycle once again. 

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