Biochemistry

intermolecular. A single hydrogen bond is rather weak and fragile, but the formation of even several such bonds acting cooperatively is a stabilizing factor. Hydrophobic interactions. Compounds that carry charges, as well as those capable of forming hydrogen bonds, are polar or hydrophilic (from the Greek, hydor - water and philia - love). Due to the fact that the electronegative charge of carbon is close to hydrogen, carbon is not capable of forming hydrogen bonds. C - H bonds are nonpolar, therefore hydrocarbon chains are hydrophobic (from the Greek hydor - water and phobos - fear). Water molecules, trying to form hydrogen bonds between themselves, push out hydrophobic groups and molecules, forcing them to transform into associates. This process is spontaneous. Hydrophobic associates are of paramount importance for the "architecture" of biomembranes, nucleic acids and proteins. In proteins, hydrophobic interactions arise due to the desire of non-polar hydrophobic side chains of a number of amino acids (Vai, Ala, Lei, He, Fen, Tyr, Three) to avoid contact with water. In this case, the polypeptide chain collapses into as a ball, within which hydrophobic groups approach each other, and polar groups interacting with water are located on the surface. Van der Waals forces. The forces of intermolecular interaction are named after the Dutch physicist J.D.Van der Waals, who first suggested in 1873 that they exist. By their nature, these bonds resemble hydrogen bonds, since they arise on the basis of the coulomb forces of electrostatic attraction. However, while constant dipoles of highly polar covalent bonds are involved in the formation of hydrogen bonds, then van der Waals forces arise when induced, unstable dipoles of weakly polar covalent bonds, for example, C - H. Despite the fact that the Van der Waals forces are very weak, they actually determine the association of many nonpolar groups in biomolecules. Ionic bonds. This type is based on the mutual electrostatic attraction of oppositely charged ions. An example of such an interaction is the relationship between the negatively charged carboxyl group -COO and the positively charged protonated amino group -NH 2 ' . The most important feature of weak chemical bonds is that their energy slightly exceeds the kinetic energy of thermal motion (2.5 kJ I mol). This small excess is enough to cause weak interactions both inside and between biomolecules. Such fragile secondary chemical bonds are very labile, which leads to their rapid occurrence and decay. Levels of organization of the protein molecule When considering the structure of a protein molecule, it is customary to distinguish four levels of organization: primary, secondary, tertiary and quaternary. 52