Nitric acid functions as a powerful oxidizing mineral acid in biochemical protocols, particularly for nitration of aromatic amino acids, selective protein oxidation, and nitrocellulose preparation for chromatography.
Chemical Properties
Nitric acid (HNO₃, MW 63.01 g/mol) forms a colorless-to-yellow fuming liquid (70% w/w aqueous, density 1.42 g/cm³, bp 121°C) that completely dissociates per HNO3 → H⁺ + NO₃⁻ (pKa -1.4). Concentrated acid exhibits oxidizing properties via NO₃⁻ reduction to NO₂, NO, or NH₄⁺ depending on the substrate; trigonal planar structure (N central, three O atoms: one double bond, resonance-stabilized NO₃⁻). It forms an azeotrope (68%, bp 120.5°C) and fumes that redden on standing due to NO₂ formation.
Biochemical Applications
Protein chemistry employs fuming HNO₃ (1:3 with H₂SO₄) for tyrosine nitration to 3-nitrotyrosine (1–5 min), useful for monitoring oxidative stress biomarkers via LC-MS. Nucleic acid work uses 0.1–1 M nitric acid for depurination controls in sequencing ladders. Carbohydrate laboratories apply dilute acid (2 M, 100°C) for total reducing sugar hydrolysis from polysaccharides. Histology stains leverage 10% HNO₃ for calcium removal from bone sections prior to von Kossa silver staining.
