A hydroxyl radical (•OH) is a highly reactive and unstable molecule. It consists of an oxygen atom bonded to a hydrogen atom, possessing an unpaired electron. This unpaired electron makes it a type of reactive oxygen species (ROS). Despite sharing the “hydroxyl” name, it is distinct from the stable hydroxyl group (-OH) found in compounds like alcohols, which lacks the unpaired electron and extreme reactivity.
Formation of Hydroxyl Radicals
Hydroxyl radicals emerge from both internal biological processes and external environmental exposures. Within the body, they are generated as byproducts of normal metabolic activities. One pathway involves the Fenton reaction, where hydrogen peroxide interacts with transition metals, particularly iron, forming hydroxyl radicals. This reaction occurs when ferrous iron (Fe2+) reacts with hydrogen peroxide (H2O2). Another internal source is the mitochondria during cellular respiration, where incomplete oxygen reduction can lead to superoxide and hydrogen peroxide, precursors to hydroxyl radicals.
External sources also contribute. Ultraviolet (UV) radiation from sunlight can break down water molecules or other compounds in the skin, generating these reactive species. Ionizing radiation, such as gamma rays or X-rays, can also split water molecules within tissues, yielding hydroxyl radicals. Additionally, exposure to certain air pollutants and chemicals in cigarette smoke can initiate reactions that produce hydroxyl radicals.
Biological Impact of Oxidative Stress
Once formed, hydroxyl radicals can inflict widespread damage throughout biological systems, a process termed oxidative stress. This occurs when reactive oxygen species overwhelm the body’s antioxidant defenses. The extreme reactivity of hydroxyl radicals enables them to abstract electrons from various cellular components. They are particularly detrimental to DNA, causing strand breaks, base modifications, and cross-linking, which can lead to mutations and impaired genetic function.
Hydroxyl radicals also target lipids, initiating a chain reaction known as lipid peroxidation within cell membranes. This process damages cell membrane integrity and function, compromising cellular compartmentalization and signaling. Proteins are another common target, as the radicals can modify amino acid residues, leading to fragmentation, aggregation, or changes in their three-dimensional structure. Such alterations can impair enzyme activity and the function of structural proteins. Over time, cumulative damage from oxidative stress contributes to cellular aging and has been implicated in the development of various chronic diseases.
The Body’s Defense Mechanisms
The body possesses mechanisms to neutralize hydroxyl radicals and their precursors, mitigating oxidative stress. A primary defense involves non-enzymatic antioxidant molecules. For instance, glutathione, a tripeptide, directly reacts with hydroxyl radicals, also participating in enzymatic antioxidant pathways. Vitamin C, a water-soluble antioxidant, readily donates an electron to neutralize various reactive oxygen species, including hydroxyl radicals. Vitamin E, a fat-soluble antioxidant, protects cell membranes by interrupting lipid peroxidation chain reactions, scavenging radicals within lipid environments.
Beyond molecular antioxidants, enzymatic systems prevent hydroxyl radical formation. Superoxide dismutase (SOD) enzymes convert superoxide radicals, a precursor to hydrogen peroxide, into less harmful oxygen and hydrogen peroxide. Catalase enzymes then break down hydrogen peroxide into water and oxygen, preventing its participation in the Fenton reaction that would otherwise generate hydroxyl radicals. These coordinated defenses maintain a delicate balance, protecting cellular components from oxidative damage.
Applications in Purification and Disinfection
The potent reactivity of hydroxyl radicals, which poses a threat in biological systems, is intentionally harnessed for beneficial applications in purification and disinfection technologies. These applications are often part of Advanced Oxidation Processes (AOPs). In wastewater treatment, AOPs generate hydroxyl radicals to break down persistent organic pollutants resistant to conventional methods. The radicals non-selectively attack and mineralize complex molecules, converting them into simpler, less harmful substances like carbon dioxide and water.
Hydroxyl radicals are also employed in purifying drinking water. They eliminate a wide range of contaminants, including pesticides, pharmaceuticals, and endocrine-disrupting compounds, ensuring water safety and quality. In air purification systems, AOPs use hydroxyl radicals to degrade volatile organic compounds (VOCs) and unpleasant odors. The radicals react with airborne pollutants, breaking their chemical bonds and transforming them into benign byproducts, improving indoor air quality.