Question

Explain why, when the imidazole ring of histidine is protonated, the double-bonded nitrogen is the nitrogen that accepts the proton. (hint: localized electrons are more apt to be protonated than delocalized electrons.)

Answer 1

When histidine is protonated, the double-bonded nitrogen accepts a proton because its electrons are localized and more reactive. Localized electrons on a nitrogen atom make it more susceptible to form bonds with a proton, acting as a Brønsted-Lowry base.

Step-by-step explanation:

The imidazole ring of histidine has two nitrogen atoms, one of which is part of a double bond while the other is not. When histidine is protonated, the double-bonded nitrogen is the one that accepts the proton. This occurs because the electrons on the double-bonded nitrogen are localized and therefore more available for bonding to a proton, while the electrons on the other nitrogen are delocalized across the ring, making them less available for protonation.

Localized electrons are those which are more confined to a particular atom, making them more apt to react and form new bonds, such as with a proton to form a substituted ammonium ion. In contrast, delocalized electrons are spread out over several atoms, like in the ring structure of histidine, and are less reactive towards protonation because their charge is spread out and not concentrated on a single atom.

This behavior aligns with the properties of amines and ammonia (NH3), where the nitrogen atom has a lone pair of electrons that can accept a proton, making these molecules act as Brønsted-Lowry bases. In histidine, the protonation occurs at a specific pH where the amine group's pKa indicates its readiness to accept a proton.

Answer 2

The important thing to note is the reason why electron react is due to the instability of the electrons. All elements wants to aim the electron configuration of the noble gases. This is the most stable form in which each of the orbitals are sufficiently filled. When it comes to bonding, the order of reactivity is: alkynes > alkenes > alkanes. Alkynes are compounds with triple bonds, alkenes with double bonds and alkanes with single bonds. The single bonds are called saturated hydrocarbons. This is because they have reached stability, so it is quite difficult to react this with reducing or oxidizing agents. Alkynes and alkenes are unsaturated hydrocarbons. They readily react with reducing and oxidizing agents so as to become saturated, as well. The underlying principle for this is that single bonds contain sigma bonds which is the head-on overlapping of electrons. These is the strongest type of covalent bond. Double and triple bonds contain pi bonds which is the side overlapping of electrons orbitals. Hence, these electrons would be easily separated making it more reactive especially during protonation.