Oxygen free radicals and clinical

Author: China Medical Science Press
Publisher:
Publish Date: 2000-01-01
Features: > Free radical medicine is a rapidly developing emerging discipline in the past three decades. Due to the deepening of basic research and clinical practice, the free radical theory has now permeated many clinical disciplines, providing new theories for the etiology and pathogenesis of many diseases, and opening up new approaches and prospects for the diagnosis, treatment, and prevention of many diseases. Free radicals have attracted widespread attention in the field of medicine. Based on this, the author has compiled the book "Oxygen Free Radicals and Clinical Practice" after reviewing a large number of domestic and international new related materials and combining his own research achievements. The book is divided into three parts: Part 1 introduces the basic knowledge of oxygen free radicals; Part 2 introduces the correlation between oxygen free radicals and diseases, involving internal medicine, surgery, gynecology, pediatrics, ophthalmology and otolaryngology, emergency medicine, etc.; Part 3 introduces the detection methods, clinical significance of oxygen free radicals, free radical reaction products, and free radical-scavenging enzymes, as well as the classification, mechanism of action, usage principles, and precautions of free radical scavengers. The content of this book is scientific, advanced, and highly targeted and practical, making it suitable for clinical physicians, free radical researchers, and related professionals to read and reference. Excerpt: The Concept of Oxygen Free Radicals
Definition of Radicals and Free Radicals
The term "radical" (·) is commonly used in chemistry to represent different atomic groups, such as the carbonate group (CO?2?), nitrate group (NO??), and methyl group (CH??). A "free radical" refers to any atom or atomic group that can exist independently and contains one or more unpaired electrons. The presence of one or more unpaired electrons makes free radicals susceptible to magnetic attraction (i.e., paramagnetism) and gives them high reactivity. Many free radicals exist in chemical reactions and biological systems (e.g., hydrogen atoms). When a covalent bond breaks, the paired electrons are equally shared by two atoms, a process known as homolytic fission. The energy required for covalent bond homolytic fission can be supplied by heat energy, electromagnetic radiation, etc. Many covalent bonds, such as -C-C-, -C-H-, and -C-O- bonds, require high temperatures of 450–600°C to break. Chemists have studied many free radical reactions in high-temperature gas-phase reactions and believe that combustion is a free radical process. A· represents the A radical, and B· represents the B radical. When a covalent bond in a water molecule undergoes homolytic fission, it generates a hydrogen radical (H·) and a hydroxyl radical (·OH). The opposite of homolytic fission is heterolytic fission. When a covalent bond undergoes heterolytic fission, one atom accepts a pair of electrons and gains a negative charge, while the other atom loses an electron and gains a positive charge. For example, the heterolytic fission of water generates H? and OH?, which are respectively called the hydrogen ion and hydroxide ion. Neither of them has unpaired electrons, so they are not free radicals.
Section 2 Oxygen and Its Derivatives
The naturally occurring oxygen molecule (as shown in Figure 1-1) is a free radical, with two unpaired electrons located in different π anti-bonding orbitals. The two electrons have the same spin quantum number (parallel spin). This is the stable state or ground state (ground state) of oxygen. When an oxygen molecule oxidizes another atom or molecule, it must accept two electrons, and these two electrons must have antiparallel spin to fill the empty spaces in the π orbitals. According to Pauli’s principle, two electrons in an atomic or molecular orbital must have opposite spins, so it is impossible to meet this standard, which limits the transfer of electrons to the oxygen molecule, making O? capable of accepting only one electron at a time (spin restriction). This means that oxygen can only react slowly with many non-radical substances. The reactive form of oxygen is singlet oxygen (1O?), which is formed when ground-state oxygen accepts energy. There are two types of singlet oxygen: the 1ΔgO? state is 93.7 kJ (22.4 kcal) higher in energy than ground-state oxygen, while the 1Σg? state is even more reactive, being 156.9 kJ (37.5 kcal) higher in energy than ground-state oxygen. The 1ΔgO? state is not a free radical because it does not have unpaired single electrons. Both types of singlet oxygen have no spin restriction, and their oxygen reactivity is greatly increased. Because the O—O bond in H?O? is relatively weak, it is easily broken by homolytic fission to form hydroxyl radicals (·OH).