Water is a unique substance whose special properties are intimately tied to the processes of life. Life originally evolved in a watery environment, and most of an organism’s cellular chemistry and metabolism occur inside the water-solvated contents of the cell. Water solvates or "wets" the cell and the molecules in it, plays a key role as a reactant or product in an innumerable number of biochemical reactions, and mediates the interactions between molecules in and out of the cell. Many of water’s important properties derive from the molecule's polar nature, which derives from the asymmetric arrangement of its polar covalent bonds between hydrogen and oxygen.
In BIS2A, the ubiquitous role of water in nearly all biological processes is easy to overlook by getting caught up in the details of specific processes, proteins, the roles of nucleic acids, and in your excitement for molecular machines (it'll happen). It turns out, however, that water plays key roles in all of those processes and we will need to stay continuously aware of the role that water is playing if we are to develop a more functional understanding. Be on the lookout and also pay attention when your instructor points this out.
In a liquid state, individual water molecules interact with one another through a network of dynamic hydrogen bonds that are being constantly forming and breaking. Water also interacts with other molecules that have charged functional groups and/or functional groups with hydrogen bond donors or acceptors. A substance with sufficient polar or charged character may dissolve or be highly miscible in water and is referred to as being hydrophilic (hydro- = “water”; -philic = “loving”). Molecules with more nonpolar characters such as oils and fats do not interact well with water and separate from it rather than dissolve in it. We call these nonpolar compounds hydrophobic (hydro- = “water”; -phobic = “fearing”). We will consider some of the energetic components of these types of reactions in other another chapter.
Figure 1. In a liquid state water forms a dynamic network of hydrogen bonds between individual molecules. Shown are one donor-acceptor pair.
Attribution: Marc T. Facciotti (original work)
Water's solvent properties
Since water is a polar molecule with slightly positive and slightly negative charges, ions and polar molecules can readily dissolve in it. Therefore, we refer to water as a solvent of other polar molecules and ionic compounds. Charges (or partial charges) associated with these molecules (the solutes) will interact electrostatically with water’s partial charges. Polar bonds with the potential to donate or accept hydrogen bonds will form hydrogen bonds with water. Water molecules that interact directly with individual solute molecules will have their motions slightly constrained as will other nearby molecules. We refer to the layer or partially constrained waters surrounding a solute particle as a hydration layer, hydration shell or sphere of hydration.
When ionic salts are added to water, the individual ions interact with the polar regions of the water molecules, and the ionic bonds are likely disrupted in the process called dissociation. Dissociation occurs when atoms or groups of atoms break off from molecules and form ions. Consider table salt (NaCl, or sodium chloride). A dry block of NaCl is held together by ionic bonds and is difficult to dissociate. When NaCl crystals are added to water, however, the molecules of NaCl dissociate into Na+ and Cl– ions, and spheres of hydration form around the ions. The positively charged sodium ion is surrounded by the partially negative charge of the water molecule’s oxygen. The negatively charged chloride ion is surrounded by the partially positive charge of the hydrogen on the water molecule. One may imagine a model in which the ionic bonds in the crystal are "traded" for many smaller scale ionic bonds with the polar groups on water molecules.
Figure 2. When table salt (NaCl) is mixed in water, spheres of hydration are formed around the ions. This figure depicts a sodium ion (dark blue sphere) and a chloride ion (light blue sphere) solvated in a "sea" of water. Note how the dipoles of the water molecules surrounding the ions are aligned such that complementary charges/partial charges are associating with one another (i.e., the partial positive charges on the water molecules align with the negative chloride ion whereas the partial negative charges on the oxygen of water align with the positively charged sodium ion).
Attribution: Ting Wang - UC Davis (original work modified by Marc T. Facciotti)
Possible NB Discussion Point
A pharmaceutical company wants to develop a new antibiotic that is more water soluble than an existing antibiotic. Their strategy will be to add various functional groups to the existing antibiotic and then test the water solubility of the resulting antibiotic. The scientists are trying to decide which functional group(s) to try first. Which one(s) would you recommend and why?
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