On chemical reactions, which rates depend on viscosity in reaction media by the type of critical phenomena
Authors:   G.A. Razuvaev Institute of Organometallic Chemistry RAS, Nizhny Novgorod, Russian Federation, Treushnikov V. , Semenov V. 
Published: November 2023

Issues related to the control of the rates of chemical reactions in living systems are dis-cussed. A common, though practically unknown, kinetic pattern for both plants and animals is the presence of dependences of their rates on the nature of the molecular movements of reagents in reaction media. Reactions in them can take place either in the kinetic mode or in the mode with limited mobility of the reagents, the transitions between which are due to a change in viscosity. The possibility to control chemical reactions due to these transitions is due to the fact that their rates in these two modes can differ by many orders of magnitude, namely, 101.5–108 times or more. The most typical mechanism for controlling the rates of chemical reactions in the case of polymer chemistry is frontal photopolymerization with an extremely small reaction front width (FFP) in highly viscous media. FFP leads to the formation of a defect-free transparent product, when the release of quasi-particles of free volume from a thin layer of the polymerizable composition is ensured. Such a mechanism for controlling the rates of chemical reactions is most typical in the case of insulin synthesis in the islet of the pancreas. Exceeding the glucose concentration in blood leads to a liquefaction of the gland and the course of reactions in it in a kinetic mode at a rather high rate. A drop of its concentration in blood leads to the hardening of the gland and the course of the reaction in a mode with limited mobility of the reagents at a negligibly low rate. Viscosity in the membrane matrix can be changed as a result of either the ratio of lipids with saturated and unsaturated fatty acids in it, or temperature change. The latter is excluded in the case of warm-blooded animals, but is not excluded in systems that are in thermal equilibrium with the environment. In higher plants, all reactions take place in a kinetic mode and can switch to the mode with limited mobility of reagents only when the temperature in the external environment decreases. This transition causes all reactions in them to stop, but there is an exception. Cooling to 5–6 °C leads to a decrease in the rates of all processes in the cell, including active transport of ions, but to the “revival” of desaturases in membranes, causing catalysis of the conversion of lipids with saturated fatty acids into unsaturated fatty acids until a return to the kinetic course of this reaction, due to an increase in lipids with unsaturated fatty acids in the membrane. Paradoxically, the initiation of this reaction is not due to the liquefaction of the membrane matrix, but, on the contrary, its hardening in the case of temperature decreasing to 5–6 °C. Theoretical possibility of the conversion of lipids with saturated fatty acids into unsaturated ones has been shown for the mode with limited reagent mobility. The “revival” of desaturases in membranes is similar to the process of muscle contraction in the sarcoplasm: the formation of an actin-myosin complex due to an increase in viscosity in it and the transition of this process to the mode with limited mobility of reagents. The only difference is that the viscosity in the membrane matrix increases as a result of a decrease in temperature in the external environment, while in the sarcoplasm it is a result of the flow of calcium ions into it from the external environment. Calcium ions lead to the formation of a three-dimensional network in it, and, consequently, to an increase in viscosity in the sarcoplasm. Muscle contraction in the sarcoplasm occurs spontaneously in a result of such a transition, without the introduction of any chemical energy into it from external sources. With the exception of reactions associated with the “revival” of desaturases in membranes and the syn-thesis of insulin in the islet of the pancreas, the nervous system is involved in their regulation. The execution of the functions intended for tissues is carried out after the arrival of nerve impulses to them, which allow the exchange of certain water-soluble compounds and ions between cells and the external environment for a limited time due to the destruction of the “order” in the orientation of lipids in the membrane.