Lodish H. - Molecular Cell Biology (5ed, Freeman, 2003) (794361), страница 16
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Each pair of nonbonding outerelectrons in an oxygen or nitrogen atom can accept a hydrogenatom in a hydrogen bond. The hydroxyl and the amino groups canalso form hydrogen bonds with water. (a) In liquid water, eachwater molecule apparently forms transient hydrogen bonds withhigh concentrations (Figure 2-6b). In general, molecules withpolar bonds that easily form hydrogen bonds with water canreadily dissolve in water; that is, they are hydrophilic.
Manybiological molecules contain, in addition to hydroxyl andamino groups, peptide and ester groups, which form hydrogen bonds with water (Figure 2-6c). X-ray crystallographycombined with computational analysis permits an accuratedepiction of the distribution of electrons in covalent bondsand the outermost unbonded electrons of atoms, as illustrated in Figure 2-7. These unbonded electrons can form hydrogen bonds with donor hydrogens.CH3HHOHOHHOHOOOHHHHHOHCPeptide group−waterMethylamine-waterOHOEster group−waterseveral others, creating a dynamic network of hydrogen-bondedmolecules.
(b) Water also can form hydrogen bonds withmethanol and methylamine, accounting for the high solubility ofthese compounds. (c) The peptide group and ester group, whichare present in many biomolecules, commonly participate inhydrogen bonds with water or polar groups in other molecules.HNCOVan der Waals Interactions AreCaused by Transient DipolesWhen any two atoms approach each other closely, they create a weak, nonspecific attractive force called a van derWaals interaction.
These nonspecific interactions result fromthe momentary random fluctuations in the distribution of theelectrons of any atom, which give rise to a transient unequaldistribution of electrons. If two noncovalently bonded atomsare close enough together, electrons of one atom will perturbthe electrons of the other. This perturbation generates a transient dipole in the second atom, and the two dipoles will attract each other weakly (Figure 2-8). Similarly, a polarcovalent bond in one molecule will attract an oppositely oriented dipole in another.Van der Waals interactions, involving either transientlyinduced or permanent electric dipoles, occur in all types ofmolecules, both polar and nonpolar.
In particular, van derWaals interactions are responsible for the cohesion betweenmolecules of nonpolar liquids and solids, such as heptane,CH3O(CH2)5OCH3, that cannot form hydrogen bonds orionic interactions with other molecules. The strength of vander Waals interactions decreases rapidly with increasing distance; thus these noncovalent bonds can form only whenCα▲ FIGURE 2-7 Distribution of bonding and outer nonbonding electrons in the peptide group. Shown here is oneamino acid within a protein called crambin.
The black linesdiagrammatically represent the covalent bonds between atoms.The red (negative) and blue (positive) lines represent contours ofcharge. The greater the number of contour lines, the higher thecharge. The high density of red contour lines between atomsrepresents the covalent bonds (shared electron pairs). The twosets of red contour lines emanating from the oxygen (O) and notfalling on a covalent bond (black line) represent the two pairsof nonbonded electrons on the oxygen that are available toparticipate in hydrogen bonding. The high density of blue contourlines near the hydrogen (H) bonded to nitrogen (N) represents apartial positive charge, indicating that this H can act as a donor inhydrogen bonding.
[From C. Jelsch et al., 2000, Proc. Nat’l. Acad. Sci.USA 97:3171. Courtesy of M. M. Teeter.]2.1 • Atomic Bonds and Molecular Interactionsδδ δCovalentradius(0.062 nm)δvan der Waalsradius(0.14 nm)▲ FIGURE 2-8 Two oxygen molecules in van der Waalscontact. In this space-filling model, red indicates negative chargeand blue indicates positive charge. Transient dipoles in theelectron clouds of all atoms give rise to weak attractive forces,called van der Waals interactions. Each type of atom has acharacteristic van der Waals radius at which van der Waalsinteractions with other atoms are optimal.
Because atoms repelone another if they are close enough together for their outerelectrons to overlap, the van der Waals radius is a measure ofthe size of the electron cloud surrounding an atom. The covalentradius indicated here is for the double bond of OUO; the singlebond covalent radius of oxygen is slightly longer.atoms are quite close to one another.
However, if atoms gettoo close together, they become repelled by the negativecharges of their electrons. When the van der Waals attractionbetween two atoms exactly balances the repulsion betweentheir two electron clouds, the atoms are said to be in van derWaals contact. The strength of the van der Waals interactionis about 1 kcal/mol, weaker than typical hydrogen bonds andonly slightly higher than the average thermal energy of molecules at 25 ºC. Thus multiple van der Waals interactions, avan der Waals interaction in conjunction with other noncovalent interactions, or both are required to significantly influence intermolecular contacts.35Nonpolar molecules or nonpolar portions of moleculestend to aggregate in water owing to a phenomenon called thehydrophobic effect.
Because water molecules cannot formhydrogen bonds with nonpolar substances, they tend to form“cages” of relatively rigid hydrogen-bonded pentagons andhexagons around nonpolar molecules (Figure 2-9, left). Thisstate is energetically unfavorable because it decreases therandomness (entropy) of the population of water molecules.(The role of entropy in chemical systems is discussed in alater section.) If nonpolar molecules in an aqueous environment aggregate with their hydrophobic surfaces facing eachother, there is a reduction in the hydrophobic surface areaexposed to water (Figure 2-9, right).
As a consequence, lesswater is needed to form the cages surrounding the nonpolarmolecules, and entropy increases (an energetically more favorable state) relative to the unaggregated state. In a sense,then, water squeezes the nonpolar molecules into spontaneously forming aggregates. Rather than constituting an attractive force such as in hydrogen bonds, the hydrophobiceffect results from an avoidance of an unstable state (extensive water cages around individual nonpolar molecules).Nonpolar molecules can also associate, albeit weakly,through van der Waals interactions. The net result of the hydrophobic and van der Waals interactions is a very powerful tendency for hydrophobic molecules to interact with oneanother, not with water.
Simply put, like dissolves like. Polarmolecules dissolve in polar solvents such as water; nonpolarmolecules dissolve in nonpolar solvents such as hexane.NonpolarsubstanceHighly orderedwater moleculesWaters released into bulksolutionHydrophobicaggregationThe Hydrophobic Effect Causes NonpolarMolecules to Adhere to One AnotherBecause nonpolar molecules do not contain charged groups,possess a dipole moment, or become hydrated, they are insoluble or almost insoluble in water; that is, they are hydrophobic. The covalent bonds between two carbon atomsand between carbon and hydrogen atoms are the most common nonpolar bonds in biological systems.
Hydrocarbons—molecules made up only of carbon and hydrogen—arevirtually insoluble in water. Large triacylglycerols (or triglycerides), which comprise animal fats and vegetable oils, alsoare insoluble in water. As we see later, the major portion ofthese molecules consists of long hydrocarbon chains. Afterbeing shaken in water, triacylglycerols form a separate phase.A familiar example is the separation of oil from the waterbased vinegar in an oil-and-vinegar salad dressing.Unaggregated state:Water population highly orderedLower entropy; energeticallyunfavorableAggregated state:Water population less orderedHigher entropy; energeticallymore favorable▲ FIGURE 2-9 Schematic depiction of the hydrophobiceffect.
Cages of water molecules that form around nonpolarmolecules in solution are more ordered than water molecules inthe surrounding bulk liquid. Aggregation of nonpolar moleculesreduces the number of water molecules involved in highlyordered cages, resulting in a higher-entropy, more energeticallyfavorable state (right ) compared with the unaggregated state (left).2.2 • Chemical Building Blocks of CellsIn an aqueous environment, nonpolar molecules or nonpolar portions of larger molecules are driven together bythe hydrophobic effect, thereby reducing the extent of theirdirect contact with water molecules (see Figure 2-9).Chemical Building Blocks of Cells2.2■37The three most abundant biological macromolecules—proteins, nucleic acids, and polysaccharides—are all polymers composed of multiple covalently linked identical ornearly identical small molecules, or monomers (Figure 2-11).The covalent bonds between monomer molecules usually areformed by dehydration reactions in which a water moleculeis lost:Molecular complementarity is the lock-and-key fit between molecules whose shapes, charges, and other physical properties are complementary.
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