Weak Interactions in Aqueous Systems: -Water Forms Hydrogen Bonds with Polar Solutes
Hydrogen bonds are not unique to water. They readily form between an electronegative atom (the hydrogen acceptor, usually oxygen or nitrogen with a lone pair of electrons) and a hydrogen atom covalently bonded to another electronegative atom (the hydrogen donor) in the same or another molecule (Fig. 2–3). Hydrogen atoms covalently bonded to carbon atoms do not participate in hydrogen bonding, because carbon is only
fIGURE 2–3 Common hydrogen bonds in biological systems. The hydrogen acceptor is usually oxygen or nitrogen; the hydrogen donor is another electronegative atom.
slightly more electronegative than hydrogen and thus the COH bond is only very weakly polar. The distinction explains why butanol (CH3(CH2)2CH2OH) has a relatively high boiling point of 117 C, whereas butane (CH3(CH2)2CH3) has a boiling point of only 0.5 C. Butanol has a polar hydroxyl group and thus can form in termolecular hydrogen bonds. Uncharged but polar bio molecules such as sugars dissolve readily in water because of the stabilizing effect of hydrogen bonds between the hydroxyl groups or carbonyl oxygen of the sugar and the polar water molecules. Alcohols, aldehydes, ketones, and compounds containing NOH bonds all form hydrogen bonds with water molecules (Fig. 2–4) and tend to be soluble in water. Hydrogen bonds are strongest when the bonded molecules are oriented to maximize electrostatic inter action, which occurs when the hydrogen atom and the two atoms that share it are in a straight line—that is, when the acceptor atom is in line with the covalent bond between the donor atom and H (Fig. 2–5). Hydrogen bonds are thus highly directional and capable of hold
FIGURE 2–5 Directionality of the hydrogen bond. The attraction between the partial electric charges (see Fig. 2–1) is greatest when the three atoms involved (in this case O, H, and O) lie in a straight line. When the hydrogen-bonded moieties are structurally constrained (as when they are parts of a single protein molecule, for example), this ideal geometry may not be possible and the resulting hydrogen bond is weaker. FIGURE 2–5 Directionality of the hydrogen bond. The attraction between the partial electric charges (see Fig. 2–1) is greatest when the three atoms involved (in this case O, H, and O) lie in a straight line. When the hydrogen-bonded moieties are structurally constrained (as when they are parts of a single protein molecule, for example), this ideal geometry may not be possible and the resulting hydrogen bond is weaker. ing two hydrogen-bonded molecules or groups in a specific geometric arrangement. As we shall see later, this property of hydrogen bonds confers very precise three-dimensional structures on protein and nucleic acid molecules, which have many intramolecular hydrogen bonds.
