hydroxylradicalen

Hydroxyl radicals, often written as OH•, are highly reactive chemical species that play a central role in oxidation processes in the atmosphere, water, and living organisms. The hydroxyl radical is a single, highly reactive atom with an unpaired electron on oxygen, making it extremely short-lived under ordinary conditions. Because of this, OH• acts as a powerful initiator of oxidative chain reactions, yet its presence is typically fleeting and localized.

Formation and occurrence are context-dependent. In the atmosphere, hydroxyl radicals form mainly through photochemical reactions driven by sunlight, including the photolysis of ozone in the presence of water vapor and related pathways that generate O(1D) which subsequently reacts with water. In aqueous systems and biology, OH• can be produced by radiolysis of water or by reactions such as the Fenton reaction, where transition metals catalyze the decomposition of hydrogen peroxide to yield OH•. OH• can also arise during combustion and various industrial processes.

The reactivity of OH• is among the highest of all reactive oxygen species. It reacts with nearly all organic molecules at diffusion-controlled rates, abstracting hydrogen atoms or adding to double bonds and forming a range of radical intermediates. These reactions frequently culminate in the formation of water and oxidized products. In living tissues, hydroxyl radicals can damage DNA, proteins, and lipids, contributing to oxidative stress and cellular injury if antioxidant defenses are overwhelmed.

Measurement and study of hydroxyl radicals employ techniques such as electron spin resonance with spin-trapping agents, as well as indirect assays that monitor the consequences of OH•-induced oxidation. Biological and environmental systems rely on antioxidant mechanisms to limit OH• formation and mitigate its damaging effects.