TAUTOMERISM (the same Greek tauto + meros a part) — a special type of an isomerism characteristic of many organic compounds existing in the form of mix of two and more isomers (tautomeric forms) passing each other. At T. biomolecules communication between their structure and function is accurately traced. Classical example of T. the keto-enol tautomerism, or keto-enol balance can serve (see. Enola , Ketones ), in many respects defining an important role, to-ruyu play these connections in chemistry, biochemistry and medicine.
Existence of T. in 1862 A. M. Butlerov predicted, the term «tautomerism» was offered in 1885 to Laarom (To. Laar). T. it is necessary to distinguish from a resonance, the Crimea is explained by the properties of molecules which are not described on the basis of only one valence bond structures (skhm. Molecule, excited states of molecules). In case of a resonance (but not tautomerisms) all atomic nuclei, making a molecule, remain on former places. At T. separate isomers exist really. They come to light by means of spectral methods and in some cases can be separated from each other in the conditions of low temperatures. Several types of T are known., differing both character of the migrating group, and number of atoms, lengthways to-rykh there takes place this migration. In most cases T. it is caused by presence of easily dissociating proton. As the hydrogen atoms (N) connected with carbon (C), dissociate poorly, T. can arise only in the presence of next carbonyl (WITH-) or other «activating» group. As a rule, easily dissociate the protons connected with oxygen atoms (O) or nitrogen (N), predetermining a possibility of T. in amides (peptide bonds) and in cyclic molecules, napr, in Vit. B12 (see. Cyanocobalamine ), DOFA (see. Dioksifenilalanin ), a subject (see. Gemoglobin ), purine bases (see), the pirimidinovy bases (see), etc. In an organism there is a special group of enzymes — tautomeraz, to-rye catalyze interconversion of tautomeric forms; high activity oxaloacetate-tautomeraza (KF 188.8.131.52) from animal fabrics has:
This enzyme provides formation of physiologically important, but less stable enol form oxalacetic to - you. Possibly, T. occurs also in molecules of enzymes. So, in chymotrypsin (see), trypsin (see) and the subtilizena assume existence of «system of charge transfer», with the help a cut the proton synchronously moves from the rest of serine to an imidazole and further — to asparaginic to - those, and enzyme is activated and gains ability to enter the catalytic act. This process is called a tautomeric catalysis. T. explain origins of a number of mutations (see. Mutation ). Each nitrogen base in polymeric molecule DNA is in a certain tautomeric form (see. Deoxyribonucleic acid ). At the same time several bases can be in less stable form. So, the tautomer of adenine (iminoform) cannot form complementary couple with thymine, the «lawful» partner any more, but is capable to form hydrogen bindings with tsitoziny (fig.).
As a result of replication of a gene (see. Replication ) the incorrect copy will turn out, in a cut one «letter» of a genetic code will be replaced (see). The tautomeric equilibrium defined for completely protonated tautomeric forms does not depend on the size pH, but moves at change of temperature, the nature of solvent and is very sensitive to linkng of tautomer with protein or other molecules.
See also Isomerism .
Bibliography: Ingold K. Theoretical fundamentals of organic chemistry, the lane with English, M., 1973; M e c of l of e r D. Biokhimiya, Chemical changes in living cell, the lane with English, t. 1 — 3, M., 1980.
E. V. Ramensky