Biological Structures Are Formed by Hydrogen Bonds
Surface tension of water is dramatic and unusual. The water molecules unable to interact with the diffuse air at an air-water interface reach a low energy configuration featuring the maximum number of interactions (hydrogen bonds) between neighboring water molecules. The structured water is more stable and resists stretching much better than the fluid water just a few molecules deeper into the liquid. It is this structured water that holds together the molecular aggregates that make a cell.
Electrons Orbiting Hydrogen Have Higher Energies than Valence Electrons of Oxygen or Nitrogen
Oxygen atoms have eight protons in their nuclei, two inner electrons and six valence electrons. Two more electrons can fit into the outer, valence shell, which is the lowest energy configuration for the electrons. Similarly, nitrogen can fit three more electrons into its valence shell.
Hydrogen has but a single proton and single electron. The lowest energy configuration for valence electrons around hydrogen is with a full shell of two electrons.
Hydrogens Share Bonding Electrons Unequally with O and N
Hydrogen atoms share their valence electrons unequally with oxygen or nitrogen in covalent bonds. Thus the electrons participating in these bonding orbitals are located closer to the higher positive charge of the oxygen or nitrogen nucleus than to the single proton of the hydrogen. The hydrogen, consequently, has a partial positive charge and the pairs of valence electrons have regions of partial negative charge.
Hydrogen Bond Is Slightly Ionic, but Mostly a Shared Pair of Valence Electrons
Water that consists of a single oxygen covalently bonded to two hydrogens, can make hydrogen bonds to four other water molecules. The partial positive charge of each hydrogen is attracted (when the molecules collide) to the partial negative charge on a pair of valence electrons of another water molecule. This temporary, initial interaction facilitates the reorientation into the rigid alignment required for a hydrogen bond, with the pair of oxygen valence electrons between the hydrogen and oxygen nuclei.
Hydrogen Bond Unstable to Kinetic Motion
Water molecules are in constant movement. Aggregates of hydrogen-bonded molecules, microscopic icebergs, are momentarily stable at room temperature. This means that the energy of motion, kinetic energy, that reflects the temperature of the water, is approximately equal to the energy required to break hydrogen bonds. Thus, the energy of the hydrogen bond is just 1–2 kcal/mole or approximately one 20th of the energy of a covalent bond.
Hydrogen Bonds Hold Biomolecules Together: DNA
Hydrogen bonds connect the AT and GC base pairs in DNA, and these hydrogen bonds are stable, because the stacked base pairs provide a central core that excludes water. Thus, individual hydrogen bonds may break, but they quickly reform, because the rest of the molecule is still held together.
Protein
The secondary structures of protein, i.e. alpha helices and beta sheets, are held in rigid configurations by hydrogen bonds between neighboring amino acids along the primary amino acid sequence of the protein. Thus, the rigidity needed for the function of proteins, such as enzymes, is provided by repetitive hydrogen bonds within the helices and sheets of the protein.
Carbohydrates, Polysaccharides
Structural hydrogen bonds are also found in the most abundant biological macromolecule on earth, the polysaccharide crystal cellulose. Cellulose fibers are formed of long chains of glucose molecules that form hydrogen bonds with adjacent chains in a molecular crystal that is difficult for enzymes to attach and therefor persists as the remains of the cell walls of plants.
Complexes and Organelles
Hydrogen bonding is essential in the structuring of water that holds proteins together in complexes, such as ribosomes, and holds lipids together to make membranes. Thus, water and its unique ability to form hydrogen bonds that can be made and broken at biological temperatures, is an essential feature of living organisms.
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