H. Lodish - Molecular Cell Biology (5ed, Freeman, 2003) (796244), страница 16
Текст из файла (страница 16)
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.
Multiple noncovalent interactions can form between complementary molecules,causing them to bind tightly (see Figure 2-10), but not between molecules that are not complementary.■HOX1OOH HOX2OOH n HOX1OX2OOH H2OThe high degree of binding specificity that results frommolecular complementarity is one of the features that distinguish biochemistry from typical solution chemistry.■POLYMERSMONOMERSH2NHOCCProteins are linear polymers containing ten to severalthousand amino acids linked by peptide bonds. Nucleic acidsHOHHHHOHHOHHOHHONCCNCCNCCNCCR1RR2R3HNOCCOHR5OHR4H2OPolypeptideAmino acidB4OPOOB2B3O35OSugar 1OO3HO3HOPO5HO3OPO53OOO544OHO1HOOHOHMonosaccharideOHHO1HOOH2O1OHOHOOH4OHOHOOHOHOH O54OH OOOONucleic acidNucleotidePPHOB1BaseOPolysaccharide1HOOHOHH2OPolar groupHydrophilichead groupPhosphateGlycerolCO COHydrophobicfatty acyltailsPhospholipid bilayerGlycerophospholipid▲ FIGURE 2-11 Covalent and noncovalent linkage ofmonomers to form biopolymers and membranes.
Overview ofthe cell’s chemical building blocks and the macrostructures formedfrom them. (Top) The three major types of biologicalmacromolecules are each assembled by the polymerization ofmultiple small molecules (monomers) of a particular type:proteins from amino acids (Chapter 3), nucleic acids fromnucleotides (Chapter 4), and polysaccharides frommonosaccharides (sugars). The monomers are covalently linkedinto polymers by coupled reactions whose net result iscondensation through the dehydration reaction shown.
(Bottom)In contrast, phospholipid monomers noncovalently assemble intobilayer structure, which forms the basis of all cellular membranes(Chapter 5).38CHAPTER 2 • Chemical Foundationsare linear polymers containing hundreds to millions of nucleotides linked by phosphodiester bonds. Polysaccharidesare linear or branched polymers of monosaccharides (sugars)such as glucose linked by glycosidic bonds.A similar approach is used to form various large structures in which the repeating components associate by noncovalent interactions.
For instance, the fibers of thecytoskeleton are composed of many repeating protein molecules. And, as we discuss below, phospholipids assemblenoncovalently to form a two-layered (bilayer) structure thatis the basis of all cellular membranes (see Figure 2-11). Thusa repeating theme in biology is the construction of large molecules and structures by the covalent or noncovalent association of many similar or identical smaller molecules.Amino Acids Differing Only in TheirSide Chains Compose ProteinsThe monomeric building blocks of proteins are 20 aminoacids, all of which have a characteristic structure consistingof a central carbon atom (C) bonded to four differentchemical groups: an amino (NH2) group, a carboxyl(COOH) group, a hydrogen (H) atom, and one variablegroup, called a side chain, or R group. Because the carbonin all amino acids except glycine is asymmetric, these molecules can exist in two mirror-image forms called by convention the D (dextro) and the L (levo) isomers (Figure 2-12).The two isomers cannot be interconverted (one made identical with the other) without breaking and then re-forminga chemical bond in one of them.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
