The reaction of an aromatic heterocycle with a proton is called a protonation. One of articles about this theory is 《Hydroxylation and Ring-Opening Mechanism of an Unusual Flavoprotein Monooxygenase, 2-Methyl-3-hydroxypyridine-5-carboxylic Acid Oxygenase: A Theoretical Study》. Authors are Tian, Boxue; Tu, Yaoquan; Strid, Aake; Eriksson, Leif A..The article about the compound:5-Hydroxynicotinic acidcas:27828-71-3,SMILESS:O=C(O)C1=CN=CC(O)=C1).Reference of 5-Hydroxynicotinic acid. Through the article, more information about this compound (cas:27828-71-3) is conveyed.
Hybrid meta-GGA d. functional theory (the MPWB1K functional) was used to study the hydroxylation and ring-opening mechanism of 2-methyl-3-hydroxypyridine-5-carboxylic acid oxygenase (MHPCO). This enzyme catalyzes the conversion of 2-methyl-3-hydroxypyridine-5-carboxylic acid (MHPC) to α-(N-acetylaminomethylene)succinic acid (AAMS), which is the essential ring-opening step in the bacterial degradation of vitamin B6. MHPCO belongs to the flavin-containing aromatic hydroxylases family. However, MHPCO is capable of catalyzing a subsequent aromatic ring-cleavage reaction to give acyclic products rather than hydroxylated aromatic ones. Our calculations show that the re-aromatization of the hydroxylated intermediate occurs spontaneously in aqueous solution; this implies that the ring-opening process occurs inside the enzyme’s active site, in which limited water is available. The instability of the hydroxylated intermediate of MHPCO is the main reason why acyclic products are formed. Previously proposed mechanisms for the ring-opening step were studied, and were shown to be less likely to occur (ΔΔG≠298 > 35 kcal mol-1). Two new pathways with reasonable barrier heights (ΔΔG≠298 <15 kcal mol-1) are reported herein, which are in accordance with all exptl. information present to date. There are many compounds similar to this compound(27828-71-3)Reference of 5-Hydroxynicotinic acid. if you want to know more, you can check out my other articles. I hope it will help you，maybe you’ll find some useful information.
Reference:
Chapter 1 An introduction to palladium catalysis,
Palladium/carbon catalyst regeneration and mechanical application method