Note that when two atoms form a covalent bond, the centers of the two atoms (the two atomic nuclei) are much closer together than the sum of the two van der Waals radii. Thus, 0.4 nm 0.15 nm 0.13 nm
Any molecules that can form hydrogen bonds to each other can alternatively form hydrogen bonds to water molecules. Because of this competition with water molecules, the hydrogen bonds formed between two molecules dissolved in water are relatively weak.
HYDROPHOBIC FORCES Water forces hydrophobic groups together, because doing so minimizes their disruptive effects on the hydrogen-bonded water network. Hydrophobic groups held
H together in this way are sometimes said to be held together by “hydrophobic bonds,” even though the apparent attraction is actually caused by a repulsion from the water.
Attractive forces occur both between fully charged groups (ionic bond) and between the partially charged groups on polar molecules.
The force of attraction between the two charges, ˜+ and ˜–, falls off rapidly as the distance between the charges increases.
In the absence of water, electrostatic forces are very strong. They are responsible for the strength of such minerals as marble and agate, and for crystal formation in common table salt, NaCl.
a crystal of salt, NaCl
Charged groups are shielded by their interactions with water molecules. Electrostatic attractions are therefore quite weak in water.
Similarly, ions in solution can cluster around charged groups and further weaken these attractions.
Despite being weakened by water and salt, electrostatic attractions are very important in biological systems. For example, an enzyme that binds a positively charged substrate will often have a negatively charged amino acid side chain at the appropriate place.
PaNel 2–4: an Outline of Some of the Types of Sugars Commonly Found in Cells
Monosaccharides usually have the general formula (CH2O) n, where n can be 3, 4, 5, 6, 7, or 8, and have two or more hydroxyl groups. They either contain an aldehyde group ( ) and are called aldoses, or a ketone group ( ) and are called ketoses. MONOSACCHARIDESC O C O H
In aqueous solution, the aldehyde or ketone group of a sugar Many monosaccharides differ only in the spatial arrangementmolecule tends to react with a hydroxyl group of the same of atoms—that is, they are isomers. For example, glucose,molecule, thereby closing the molecule into a ring.
galactose, and mannose have the same formula (C6H12O6) but differ in the arrangement of groups around one or two carbon atoms.
chemical properties of the sugars. But they are recognized by enzymes and other proteins and therefore can have major 5CH2OH has a number. biological effects.