8885dc02_47-747/25/0310:05 AM Page52mac76mac76:385 Part I Structure and Catalysis thermodynamic terms, formation of the solution occurs hydrophobic--they are unable to undergo energetically with a favorable free-energy change: AG=AH -TAS, favorable interactions with water molecules, and they where AH has a small positive value and T'As a large interfere with the hydrogen bonding among water mol positive value; thus AG is negative ecules. All molecules or ions in aqueous solution inter fere with the hydrogen bonding of some water mole Nonpolar Gases Are Poorly Soluble in Water cules in their immediate vicinity, but polar or charged solutes(such as Nacl) compensate for lost water-water The molecules of the biologically important gases CO2, hydrogen bonds by forming new solute-water interac O2, and N2 are nonpolar In O2 and N2, electrons are tions. The net change in enthalpy(AH) for dissolving shared equally by both atoms In CO2, each C-0 bond these solutes is generally small Hydrophobic solutes is polar, but the two dipoles are oppositely directed and however, offer no such compensation, and their addi cancel each other(Table 2-3). The movement of mole- tion to water may therefore result in a small gain of en- cules from the disordered gas phase into aqueous solu- thalpy; the breaking of hydrogen bonds between water tion constrains their motion and the motion of water molecules takes up energy from the system. Further molecules and therefore represents a decrease in en- more, dissolving hydrophobic compounds in water pro tropy. The nonpolar nature of these gases and the de duces a measurable decrease in entropy. Water mole- crease in entropy when they enter solution combine to cules in the immediate vicinity of a nonpolar solute are make them very poorly soluble in water (Table 2-3). constrained in their possible orientations as they form Some organisms have water-soluble carrier proteins a highly ordered cagelike shell around each solute mol- (hemoglobin and myoglobin, for example) that facilitate ecule. These water molecules are not as highly oriented nsport Carbon dioxide forms carbonic acid as those in clathrates, crystalline compounds of non (H2CO3) in aqueous solution and is transported as the polar solutes and water, but the effect is the same in HCO3 (bicarbonate)ion, either free--bicarbonate is both cases: the ordering of water molecules reduces en- very soluble in water(-100 g/L at 25C)-or bound to tropy. The number of ordered water molecules, and hemoglobin. Two other gases, NHa and H2s, also have therefore the magnitude of the entropy decrease, is pro- ological roles in some organisms; these gases are po- portional to the surface area of the hydrophobic solute r and dissolve readily in water. enclosed within the cage of water molecules. The free- energy change for dissolving a nonpolar solute in water Nonpolar Compounds Force Energetically Unfavorable is thus unfavorable: AG-AH-TAS, where AH has Changes in the Structure of Water a positive value, AS has a negative value, and AG is positive When water ed with benzene or hexane. two Amphipathic compounds contain regions that are phases form; neither liquid is soluble in the other Non- polar (or charged) and regions that are nonpolar ( table polar compounds such as benzene and hexane are 2-2). When an amphipathic compound is mixed with TABlE 2-3 Solubilities of some gases in Water Solubility in water(g/L) N≡三N 0.018(40°C) Carbon dioxide 0.97(45°C) O=C=0 0(10°C) Hydrogen sulfide 1860(40°C) he arrows represent electric dipoles there is a partial negative charge(8 )at the head of the arrow, a partial positive charge (6: not shown here)at the tail. TNote that polar molecules dissolve far better even at low temperatures than do nonpolar molecules at relatively high temperaturesthermodynamic terms, formation of the solution occurs with a favorable free-energy change: G  H T S, where H has a small positive value and T S a large positive value; thus G is negative. Nonpolar Gases Are Poorly Soluble in Water The molecules of the biologically important gases CO2, O2, and N2 are nonpolar. In O2 and N2, electrons are shared equally by both atoms. In CO2, each CUO bond is polar, but the two dipoles are oppositely directed and cancel each other (Table 2–3). The movement of mole￾cules from the disordered gas phase into aqueous solu￾tion constrains their motion and the motion of water molecules and therefore represents a decrease in en￾tropy. The nonpolar nature of these gases and the de￾crease in entropy when they enter solution combine to make them very poorly soluble in water (Table 2–3). Some organisms have water-soluble carrier proteins (hemoglobin and myoglobin, for example) that facilitate the transport of O2. Carbon dioxide forms carbonic acid (H2CO3) in aqueous solution and is transported as the HCO3 (bicarbonate) ion, either free—bicarbonate is very soluble in water (~100 g/L at 25 C)—or bound to hemoglobin. Two other gases, NH3 and H2S, also have biological roles in some organisms; these gases are po￾lar and dissolve readily in water. Nonpolar Compounds Force Energetically Unfavorable Changes in the Structure of Water When water is mixed with benzene or hexane, two phases form; neither liquid is soluble in the other. Non￾polar compounds such as benzene and hexane are hydrophobic—they are unable to undergo energetically favorable interactions with water molecules, and they interfere with the hydrogen bonding among water mol￾ecules. All molecules or ions in aqueous solution inter￾fere with the hydrogen bonding of some water mole￾cules in their immediate vicinity, but polar or charged solutes (such as NaCl) compensate for lost water-water hydrogen bonds by forming new solute-water interac￾tions. The net change in enthalpy (H) for dissolving these solutes is generally small. Hydrophobic solutes, however, offer no such compensation, and their addi￾tion to water may therefore result in a small gain of en￾thalpy; the breaking of hydrogen bonds between water molecules takes up energy from the system. Further￾more, dissolving hydrophobic compounds in water pro￾duces a measurable decrease in entropy. Water mole￾cules in the immediate vicinity of a nonpolar solute are constrained in their possible orientations as they form a highly ordered cagelike shell around each solute mol￾ecule. These water molecules are not as highly oriented as those in clathrates, crystalline compounds of non￾polar solutes and water, but the effect is the same in both cases: the ordering of water molecules reduces en￾tropy. The number of ordered water molecules, and therefore the magnitude of the entropy decrease, is pro￾portional to the surface area of the hydrophobic solute enclosed within the cage of water molecules. The free￾energy change for dissolving a nonpolar solute in water is thus unfavorable: G  H T S, where H has a positive value, S has a negative value, and G is positive. Amphipathic compounds contain regions that are polar (or charged) and regions that are nonpolar (Table 2–2). When an amphipathic compound is mixed with 52 Part I Structure and Catalysis TABLE 2–3 Solubilities of Some Gases in Water Solubility Gas Structure* Polarity in water (g/L)† Nitrogen NmN Nonpolar 0.018 (40 °C) Oxygen OPO Nonpolar 0.035 (50 °C) Carbon dioxide Nonpolar 0.97 (45 °C) Ammonia Polar 900 (10 °C) Hydrogen sulfide Polar 1,860 (40 °C) H G S D H  H GN A H D H  OPCPO   *The arrows represent electric dipoles; there is a partial negative charge (
) at the head of the arrow, a partial positive charge (

; not shown here) at the tail. † Note that polar molecules dissolve far better even at low temperatures than do nonpolar molecules at relatively high temperatures. 8885d_c02_47-74 7/25/03 10:05 AM Page 52 mac76 mac76:385_reb: