885c0247-747/25/0310:05 AM Page53mac76mac76:385e Chapter 2 Water Hydro 8° g pids in H2° molecule forces molecules to become Hydrophobic g "Flickering clusters"of H2O Highly ordered HoO molecules form cages"around the hydrophobic alkyl chains (a) .@@ Clusters of lipid molecules FIGURE 2-7 Amphipathic compounds in aqueous solution (a)Long. Only lipid portion chain fatty acids have very hydrophobic alkyl chains, each of which is surrounded by a layer of highly ordered water molecules. (b)By ° cluster force the ordering of water clustering together in micelles, the fatty acid molecules expose the smallest possible hydrophobic surface area to the water,and fewer 06. are ordered, and water molecules are required in the shell of ordered water. The energy gained by freeing immobilized water molecules stabilizes the micelle. water, the polar, hydrophilic region interacts favorably Micelles with the solvent and tends to dissolve, but the nono- All hydrophobic lar, hydrophobic region tends to avoid contact with the groups are water(Fig. 2-7a). The nonpolar regions of the mole- sequestered from water: ordered cules cluster together to present the smallest hy- shell of H2O drophobic area to the aqueous solvent, and the polarre- gions are arranged to maximize their interaction with ntropy is further the solvent (Fig. 2-7b). These stable structures of am- phipathic compounds in water, called micelles, may contain hundreds or thousands of molecules. The forces that hold the nonpolar regions of the molecules together are called hydrophobic interactions. The strength of hydrophobic interactions is not due to any intrinsic at- traction between nonpolar moieties. Rather, it results from the system's achieving greatest thermodynamic polar regions. Hydrophobic interactions among lipids stability by minimizing the number of ordered water and between lipids and proteins, are the most impor molecules required to surround hydrophobic portions e tant determinants of structure in biological membranes the solute molecules Hydrophobic interactions between nonpolar amino Many biomolecules are amphipathic, proteins, pig- acids also stabilize the three-dimensional structures of ments, certain vitamins, and the sterols and phospho- proteins lipids of membranes all have polar and nonpolar surface Hydrogen bonding between water and polar solutes regions. Structures composed of these molecules are also causes some ordering of water molecules, but the tabilized by hydrophobic interactions among the non- effect is less significant than with nonpolar solutes. PartDispersion of lipids in H2O Clusters of lipid molecules Micelles (b) (a) “Flickering clusters” of H2O molecules in bulk phase Highly ordered H2O molecules form “cages” around the hydrophobic alkyl chains Hydrophilic “head group” O O C H C H H H O Each lipid molecule forces surrounding H2O molecules to become highly ordered. Only lipid portions at the edge of the cluster force the ordering of water. Fewer H2O molecules are ordered, and entropy is increased. All hydrophobic groups are sequestered from water; ordered shell of H2O molecules is minimized, and entropy is further increased. – Hydrophobic alkyl group water, the polar, hydrophilic region interacts favorably with the solvent and tends to dissolve, but the nonpo￾lar, hydrophobic region tends to avoid contact with the water (Fig. 2–7a). The nonpolar regions of the mole￾cules cluster together to present the smallest hy￾drophobic area to the aqueous solvent, and the polar re￾gions are arranged to maximize their interaction with the solvent (Fig. 2–7b). These stable structures of am￾phipathic compounds in water, called micelles, may contain hundreds or thousands of molecules. The forces that hold the nonpolar regions of the molecules together are called hydrophobic interactions. The strength of hydrophobic interactions is not due to any intrinsic at￾traction between nonpolar moieties. Rather, it results from the system’s achieving greatest thermodynamic stability by minimizing the number of ordered water molecules required to surround hydrophobic portions of the solute molecules. Many biomolecules are amphipathic; proteins, pig￾ments, certain vitamins, and the sterols and phospho￾lipids of membranes all have polar and nonpolar surface regions. Structures composed of these molecules are stabilized by hydrophobic interactions among the non￾polar regions. Hydrophobic interactions among lipids, and between lipids and proteins, are the most impor￾tant determinants of structure in biological membranes. Hydrophobic interactions between nonpolar amino acids also stabilize the three-dimensional structures of proteins. Hydrogen bonding between water and polar solutes also causes some ordering of water molecules, but the effect is less significant than with nonpolar solutes. Part Chapter 2 Water 53 FIGURE 2–7 Amphipathic compounds in aqueous solution. (a) Long￾chain fatty acids have very hydrophobic alkyl chains, each of which is surrounded by a layer of highly ordered water molecules. (b) By clustering together in micelles, the fatty acid molecules expose the smallest possible hydrophobic surface area to the water, and fewer water molecules are required in the shell of ordered water. The energy gained by freeing immobilized water molecules stabilizes the micelle. 8885d_c02_47-74 7/25/03 10:05 AM Page 53 mac76 mac76:385_reb: