24. Pyrimidine, piridazine, pirazine-structure. Biologically active substances with pyrimidine ring – pyrimidine bases, nucleotides, vitamins and medicines.

Heterocyclic compounds with two nitrogen atoms. (1,3-diazole). Structure.

• Structure:
• Imidazole is a five-membered heterocyclic compound, also known as 1,3-diazole.
• It contains two nitrogen atoms and three carbon atoms in a ring structure.
• Properties:
• Aromatic: Imidazole exhibits aromatic character due to its cyclic structure and electron delocalization.
• Amphoteric: Imidazole can act as both an acid (H+ donor) and a base (H+ acceptor) due to the presence of two nitrogens with different properties.
• One nitrogen atom (N1) can donate a proton (acidic).
• The other nitrogen atom (N2) lacks a lone pair and cannot accept a proton (not basic).
• Reactions:
• Imidazole can undergo substitution reactions (SN2 mechanism) at positions 4 and 5 of the ring.

Tautomerism in imidazole derivatives. Histidine and histamine.

• Tautomerism: Imagine a molecule that can exist in two very similar forms, like a shape-shifter. These forms, called tautomers, share the same atoms but have a flipped proton and its electron pair within the molecule.
• Imidazole: This five-membered ring with two nitrogens is particularly good at this shape-shifting due to the presence of a lone pair on one nitrogen.
• The Flip: The lone pair can attract a proton from the other nitrogen, creating two possible forms:
• N-H Tautomer: Proton sits on a specific nitrogen (N).
• N+ Tautomer: Proton is on the other nitrogen, with a positive charge (N+).
• Fast Change: These forms are constantly switching back and forth, making it difficult to isolate one form.
• Why It Matters: Tautomerism affects the reactivity of imidazole derivatives and plays a role in the function of biomolecules like histidine.
• Histidine:
• Amino Acid: Histidine is an essential amino acid, meaning your body can't produce it and needs to obtain it from food.
• Structure: It has a unique structure with an imidazole ring (a five-membered ring containing two nitrogens) attached to its central carbon backbone.
• Histamine:
• Biogenic Amine: Histamine is a signaling molecule with various functions in the body.
• Neurotransmitter: It acts as a neurotransmitter in the brain, involved in sleep-wake regulation, learning, and memory.
• Immune Response: Histamine also plays a crucial role in the immune response, especially allergic reactions. It triggers inflammation, itching, and mucus production when released by immune cells.
• Symptoms: When released excessively during allergic reactions, histamine can cause:
• Diarrhea
• Headache
• Asthma
• Hypotension (low blood pressure)
• Treatment: These symptoms can be alleviated by antihistamines, medications that block histamine receptors.
• Synthesis: Histamine is formed by a simple process called decarboxylation, where a carboxyl group (COOH) is removed from histidine by an enzyme.
• The Connection: