Boric acid functions as a weak Lewis acid and a versatile buffering component widely used in biochemical and molecular biology applications. It is particularly relevant for RNA electrophoresis, boron-dependent enzymatic assays, and pH stabilization, owing to its unique ability to form reversible complexes with cis-diols such as ribose.
Chemical Properties
Boric acid (H₃BO₃ or B(OH)₃; molecular weight 61.83 g/mol) crystallizes as colorless triclinic structures with a density of 1.44 g/cm³ and decomposes at 170.9°C into metaboric acid. The boron atom exhibits trigonal planar geometry, coordinated by three hydroxyl groups via dative bonds (B–O ≈ 1.36 Å). It behaves as a weak monoprotic acid (pKa ≈ 9.24) through equilibrium with water, forming tetrahedral borate ions (B(OH)₄⁻) at alkaline pH. Solubility increases significantly with temperature, reaching approximately 39.7 g/100 mL at 100°C. These borate species are capable of forming stable complexes with cis-diol-containing molecules, including ribonucleosides. The compound is hygroscopic and exhibits mild antiseptic properties.
Biochemical Applications
In molecular biology, boric acid is commonly used in combination with Tris base to prepare TBE buffer (typically 0.5–1 M, pH ~8.3), enabling high-resolution separation of RNA and DNA in agarose gel electrophoresis. The formation of borate-ribose complexes contributes to maintaining consistent nucleic acid charge and improving band sharpness. In enzymology, boric acid at concentrations of 10–50 mM is utilized in studies involving boron-dependent oxidoreductases, including inhibition assays such as IC₅₀ determination for enzymes like aldose reductase. In protein chemistry, saturated boric acid solutions are applied for selective precipitation of glycoproteins through interaction with vicinal diol groups. Additionally, in histological applications, approximately 5% boric acid solutions are used to selectively stain RNA-rich structures by modulating methyl green binding.
