Many biological compounds contain a carbon Amines and amides are two important examples ofbonded to an oxygen. For example, compounds containing a carbon linked to a nitrogen.
alcohol H Amines in water combine with an H+ ion to become The –OH is called a positively charged.
hydroxyl group.
Amides are formed by combining an acid and an
The C—O is called aamine. Unlike amines, amides are uncharged in water.
carbonyl group.
ketone C An example is the peptide bond that joins amino acids in a protein.
carboxylic acid O The –COOH is called a
C carboxyl group. In water H this loses an H+ ion to
OH Nitrogen also occurs in several ring compounds, including become –COO .
important constituents of nucleic acids: purines and pyrimidines.
esters Esters are formed by a condensation reaction between an acid and an alcohol.
SULFHYDRYL GROUP The C SH is called a sulfhydryl group. In the amino acid cysteine, the sulfhydryl group may exist in the reduced form, C SH or more rarely in an oxidized, cross-bridging form, CSSC
PHOSPHATES Inorganic phosphate is a stable ion formed from Phosphate esters can form between a phosphate and a free hydroxyl group. phosphoric acid, H3PO4. It is also written as Pi . Phosphate groups are often attached to proteins in this way.
also _ __ written as HOPO C OHHOPO C OPO H2O
The combination of a phosphate and a carboxyl group, or two or more phosphate groups, gives an acid anhydride. Because compounds of this kind are easily hydrolysed in the cell, they are sometimes said to contain a “high-energy” bond.
_ also written as __ also written as molecules such as ATP
PaNel 2–2: Water and Its Influence on the behavior of biological Molecules
Two atoms, connected by a covalent bond, may exert different attractions for Molecules of water join together transiently the electrons of the bond. In such cases the bond is polar, with one end in a hydrogen-bonded lattice. Even at 37oC, slightly negatively charged (˜–) and the other slightly positively charged (˜+). 15% of the water molecules are joined to four others in a short-lived assembly known as a “fickering cluster.”
Although a water molecule has an overall neutral charge (having the same number of electrons and protons), the electrons are asymmetrically distributed, which makes the molecule polar. The oxygen nucleus draws electrons away The cohesive nature of water is from the hydrogen nuclei, leaving these nuclei with a small net positive charge. responsible for many of its unusual The excess of electron density on the oxygen atom creates weakly negative properties, such as high surface tension, regions at the other two corners of an imaginary tetrahedron. specifc heat, and heat of vaporization.
Because they are polarized, two ˜+ adjacent H2O molecules can form H H hydrogen bond a linkage known as a hydrogen H 0.17 nm bond. Hydrogen bonds have only about 1/20 the strength O H O 2˜+ of a covalent bond.
H H 0.10 nmHydrogen bonds are strongest when hydrogen bond covalent bondthe three atoms lie in a straight line. ˜+
Substances that dissolve readily in water are termed hydrophilic. They are Molecules that contain a preponderance composed of ions or polar molecules that attract water molecules through of nonpolar bonds are usually insoluble in electrical charge effects. Water molecules surround each ion or polar molecule water and are termed hydrophobic. This is on the surface of a solid substance and carry it into solution. true, especially, of hydrocarbons, which
H contain many C–H bonds. Water molecules are not attracted to such molecules and so
O have little tendency to surround them and H OH carry them into solution.
Ionic substances such as sodium chloride H dissolve because water molecules are