Abundant in Biomolecules

Atom and Outer Usual Number

Electrons of Covalent Bonds

stereoisomers. Many molecules in cells contain at least one asymmetric carbon atom, often called a chiral carbon atom. The different stereoisomers of a molecule usually have completely different biological activities because the arrangement of atoms within their structures differs, yielding their unique abilities to interact and chemically react with other molecules.

Carbon can also bond to three other atoms in which all atoms are in a common plane. In this case, the carbon atom forms two typical single bonds with two atoms and a double bond (two shared electron pairs) with the third atom (Figure 2-2b). In the absence of other constraints, atoms joined by a single bond generally can rotate freely about the bond axis, while those connected by a double bond cannot. The rigid planarity imposed by double bonds has enormous significance for the shapes and flexibility of large biological molecules such as proteins and nucleic acids.

The number of covalent bonds formed by other common atoms is shown in Table 2-1. A hydrogen atom forms only one bond. An atom of oxygen usually forms only two cova-lent bonds, but has two additional pairs of electrons that can participate in noncovalent interactions. Sulfur forms two co-valent bonds in hydrogen sulfide (H2S), but also can accommodate six covalent bonds, as in sulfuric acid (H2SO4) and its sulfate derivatives. Nitrogen and phosphorus each have five electrons to share. In ammonia (NH3), the nitrogen atom forms three covalent bonds; the pair of electrons around the atom not involved in a covalent bond can take part in non-covalent interactions. In the ammonium ion (NH4+), nitrogen forms four covalent bonds, which have a tetrahedral geometry. Phosphorus commonly forms five covalent bonds, as in phosphoric acid (H3PO4) and its phosphate derivatives, which form the backbone of nucleic acids. Phosphate groups attached to proteins play a key role in regulating the activity of many proteins (Chapter 3), and the central molecule in cellular energetics, ATP, contains three phosphate groups (see Section 2.4).

Bond Geometry H

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