BIOCHEMISTRY - L. Stryer - 1984

VOLUME 1

PART I. CONFORMATION AND DYNAMICS

CHAPTER 6. INTRODUCTION TO ENZYMOLOGY

6.25. The Presence of Water Weakens Polar Interactions

The polarity of molecules and their ability to form Hydrogen Bonds make Water a highly reactive compound. This is manifested in the fact that water weakens Electrostatic Interactions and hydrogen bonds that arise between Other Compounds. In these polar interactions, water acts as a very strong competitor. Let us consider, for example, the EFFECT OF WATER on The formation of a Hydrogen bond between carbonyl and amide groups (Fig. 6.29). Water hydrogen atoms are capable of replacing the NH group as a donor, and the water oxygen atom can act as a hydrogen acceptor for the CO group oxygen. Consequently, the formation of a strong hydrogen bond between CO and NH groups is possible only in the absence of water.

Class="center">Fig. 6.29. Water acts as a competitor in the formation of hydrogen bonds

The strength of electrostatic interactions in water is reduced by a factor of 80 compared to a vacuum (80 is the Dielectric Constant of water). The high dielectric constant of water reflects properties such as polarity and The ability to form an oriented solvation shell around ions, which weakens the electrostatic interaction between one ion and another (Fig. 6.30). Such oriented solvation shells create their own electric field, opposite in sign to the field created by the ion. As a result, the ability of ions surrounded by solvation shells to undergo electrostatic interactions is noticeably diminished. Water is characterized by an exceptionally high dielectric constant.

Fig. 6.30. Water weakens the electrostatic interaction of charged groups

6.26. Hydrophobic Interactions; Nonpolar Groups Tend to Associate in an Aqueous Environment

Everyone knows how small oil droplets in water merge into a single large drop. A similar process occurs at the atomic level, namely, nonpolar molecules or groups cluster together in an aqueous medium. This association process is driven by hydrophobic interactions. Metaphorically speaking, water squeezes nonpolar compounds together.

Hydrophobic interactions play a crucial role in macromolecular folding, the binding of substrates to Enzymes, and many other molecular processes. Let us examine their underlying basis. Suppose a molecule of a nonpolar compound, such as hexane, is introduced into water. A cavity is thereby formed in the water, which momentarily disrupts the hydrogen bonds between water molecules. Subsequently, the displaced water molecules reorient themselves and form the maximum possible number of new hydrogen bonds. However, this is achieved only at a certain "cost": the opportunities for forming favorable hydrogen bonds in the water lattice around the hexane molecules are significantly fewer than in pure water. As a result, the water molecules surrounding the hexane molecule are arranged in a much more ordered fashion than in the rest of the solution, which means that the Entropy of the solution decreases. Now let us see what happens if two hexane molecules find themselves in water. Will they reside in two small cavities (Fig. 6.31, A) or in a single large cavity (Fig. 6.31, B)? Experience shows that two hexane molecules associate and occupy one large cavity. This association occurs as a result of the release of individual oriented water molecules that previously surrounded the separated hexane molecules. Therefore, The basis of hydrophobic interaction is an increase in entropy driven by the gain in degrees of freedom of the released water molecules. Thus, nonpolar molecules in water associate with one another not due to a high mutual affinity, but primarily because of the existence of strong bonds between water molecules.

Fig. 6.31. Schematic representation of two hexane molecules in a small volume of water. A - Hexane molecules occupy separate cavities in the water Structure. B - Hexane molecules occupy a single shared cavity, which is energetically more favorable