Investigation of the free radical scavenging ability of l-tryptophan and its derivatives using experimental methods and quantum chemical calculations†

The free radical scavenging ability of L-tryptophan (LP) and 5-hydroxy-L-tryptophan (HLP) was evaluated using experimental and theoretical methods. The impact of antioxidant concentration on the scavenging of DPPH˙ and ABTS˙⁺ free radicals was assessed for both compounds. The results indicated that HLP exhibited superior scavenging ability, with IC₅₀ values of 31.96 × 10⁻⁷ ± 0.85 × 10⁻⁷ M for DPPH˙ and 8.69 × 10⁻⁶ ± 0.95 × 10⁻⁶ M for ABTS˙⁺. In contrast, LP showed higher IC₅₀ values of 9.51 × 10⁻³ ± 0.53 × 10⁻³ M for DPPH˙ assay and 8.91 × 10⁻⁴ ± 0.73 × 10⁻⁴ M for ABTS˙⁺ assay, indicating less effective scavenging. Theoretical calculations, performed by analyzing frontier molecular orbitals and molecular electrostatic potential, revealed that electron-donating regions were primarily distributed across the aromatic rings and heteroatoms. At the same time, electron-accepting zones were only located at nitrogen heteroatoms. The hydrogen atoms within the hydroxyl and amine groups of LP and HLP molecules were preferential positions for nucleophilic attacks. Furthermore, thermodynamic and kinetic analyses suggested that hydrogen atom transfer was the predominant mechanism governing the reaction of LP and HLP with free radicals. The presence of the OH group in the HLP molecule significantly enhanced its free radical scavenging ability compared to LP.