Answer:
Due to the resonance structures
Explanation:
In the question:
"<em>Explain why, when the guanidino group of arginine is protonated, the double-bonded nitrogen is the nitrogen that accepts the proton. There is a scheme of a reversible reaction, where one equivalent of the reactant reacts with two equivalents of H plus</em>"
We have to take into account the structure of the <u>amino acid</u> arginine. In which, we have the amino and the carboxylic groups in the right and the <u>guanidine group in the left</u>.
In this group, we have a central carbon with three nitrogen atoms around and a double bond with the nitrogen on the top. This nitrogen on the top will accept the proton because the structure produced will have a positive charge on this nitrogen. Then, the double bond with the carbon can be delocalized into the nitrogen producing a positive charge in the carbon.
In this structure (<u>the carbocation</u>), we can have several resonance structures. In the <em>blue option</em>, we can produce a double bond with the nitrogen on the right. In the <em>purple option</em>, we can produce a double bond with the nitrogen on the left.
In conclusion, if the nitrogen in the top on the guanidine group accepts an hydrogen atom and we will have <u>several resonance structures that can stabilize the molecule.</u> Due to this, the nitrogen in the top its the best option to accept hydrogens.
See figure 1
I hope it helps!