Bonds that belong in the Hall of fame.

Abstract

Life could not exist without water and its unusual properties. These properties prevent lakes from freezing from the bottom and allow plants to defy gravity and raise it to their leaves. Water’s special properties are also critical to the function of the eye. They are conferred by the hydrogen bond. Water is a funny little molecule possessing twofold rotational symmetry in both its physical structure and electrical dipole. It comprises an oxygen atom that has two unpaired electrons covalently bonded to two hydrogen atoms that are located 105 degrees from each other. Electrons tend to be closer to the oxygen atom than the hydrogen atom, giving water its polarized charge. The attraction of the electropositive hydrogen ends of one water molecule to the electronegative oxygen ends of another water molecule produce hydrogen bonds. Hydrogen bonds are weak, 1/10 as strong as covalent bonds. Hydrogen bonds provide water with a structure and substance as the molecules form a loose crystal structure, or quasi-polymeric system. This creates high surface tension allowing water to bead up when placed on a glass slide or slither into a canaliculus by capillary action. This property allows the aqueous layer of the tear film to have a smooth uniform thickness, and for the compressibility of the tear film so the aqueous layer remains stable after a blink. It also allows for an adequate volume of tears to contain necessary vital gases (oxygen and carbon dioxide exchange), buffering salts, enzymes, and protective proteins. Its polarization allows for these substances to go into solution. To produce free water molecules that can evaporate, the hydrogen bonds of liquid water must first be broken. This takes a significant energy input (high specific heat). This requirement fortunately prevents the rapid evaporation of tears. The aqueous layer is sandwiched between the mucin and oily layers of the tear film. Membrane-bound mucins form the immediate protective coat of the cornea. The aqueous layer then creates a smooth corneal surface by filling in the gaps and adhering to the hydrophilic mucins with hydrogen bonds strong enough to allow for stability but weak enough to allow for fluid exchange. The outer oily layer of the tear film is non-polarized and therefore hydrophobic, allowing it to remain discrete and float on, rather than mix with, the aqueous layer. This enhances the refractive power of the eyes because oil has a higher index of refraction. The oil coating helps prevent aqueous evaporation. The Meibomian glands secrete the oil at the periphery of the tear film and it is these oily reservoirs that are picked up by the eyelid and spread over the tear film with a blink. The reservoirs themselves produce the hydrophobic barrier that limits the aqueous layer from running off. The hydrogen bond makes water a magical substance, allowing us to see and life to exist.