From Big Medical Encyclopedia

CONFORMATION — one of geometrically various conditions of a polyatomic molecule caused by rotation of any atom or groups of atoms around one or several covalent bonds without rupture of these bonds that essentially distinguishes conformational states from the phenomenon isomerism (see). In enzymatic reactions at education enzyme - a substrate complex there are simultaneous mutually induced conformational changes and substrate and enzyme so that the reacting groups of substrate and enzyme appeared in the space relations, optimum for binding. To., which are accepted by a molecule in some conditions, considerably define a row physical. and chemical properties of substance, and To. molecules in a transient state of the reactionary act exerts impact on speed and the direction of reactions, especially enzymatic, and reactions of transfer through biol, membranes that has big obshchebiol. value.

Many biologically active agents (e.g., antibiotics) in the course of functioning in a cell or change the To., or change To. molecules targets, about a cut they interact. The cyclic peptide antibiotics like valinomitsin changing the To can be examples of connections of the first type. at interaction with the ions postponed through biol, a membrane. As examples of connections of the second type (or jointly the first and second) such antineoplastic antibiotics as Actinomycinum or such connections possessing antimicrobic action as acriflavine and etidiumbromid for which molecule target is DNA can serve. At interaction with these connections K. molecules DNA significantly changes.

The forms of one connection differing only To. their molecules, are called konformer. Owing to properties of symmetry of covalent bonds (i.e. bonds between atoms in molecules in which electric charges) rotation of any atom or group of atoms perhaps only around single bonds do not appear and it is impossible around double and acetylene bonds. Distinguishable To. are possible in that case when the rotating groups of atoms have no cylindrical symmetry to an axis of rotation. The elementary inorganic molecule, for a cut are possible distinguishable To., hydrogen peroxide (I) is, and organic — ethane (II and III):

The turn around the central communication of rather nek-ry initial position is possible on any corner. Therefore for these molecules perhaps infinite number various To. At rotation the distance between the atoms which are not connected directly in a molecule changes (e.g., between hydrogen atoms in a molecule of hydrogen peroxide). Interaction between such atoms (attraction or pushing away) is function of distance between them. Therefore various To. one molecule are energetically not equivalent. For lack of external influences the molecule aims to accept To., answering to one of minima of free energy. Therefore from infinite number possible To. only those are of interest the few rather stable To., who answer minima of free energy, and also those which answer maxima of free energy and, therefore, there correspond to a power barrier of interconversions stable K.

Sushchestvuyet two main ways of the image To. on paper: perspective (see, e.g., formulas I and II) and projective. The first is not regulated by strict rules and for the use and understanding demands a certain training of space imagination. The second includes a projection of the considered molecule or its fragment to the plane, perpendicular bonds, and the atom, next to the observer, at this communication is represented by a point, and more remote — a circle. Bonds of other atoms or groups with these two are projected on the plane of the drawing in the form of radial straight lines. The stablest To. molecules of ethane in such projection — see a formula III.

Bibliography: Iliyel E., etc. Conformational analysis, the lane with English, M., 1969; Ovchinnikov Yu. A., And in and N about in V. T. and Sh to r about A. M. Membrano-aktivnye complexons, page 120, M., 1974; Hundred d-d and r J. Stereokhimiya's t of carbohydrates, the lane with English, M., 1975.

A. F. Bochkov.