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The essence of biological processes the basis of the uniformity of living systems is in its most fundamental sense
molecular interactions; in other words, the chemistry that takes place between molecules. Biochemistry is the chemistry
that takes place within living systems. To truly understand biochemistry, we need to understand chemical bonding. We
review here the types of chemical bonds that are important for biochemicals and their transformations.
The strongest bonds that are present in biochemicals are covalent bonds, such as the bonds that hold the atoms together
within the individual bases shown in Figure 1.3. A covalent bond is formed by the sharing of a pair of electrons between
adjacent atoms. A typical carbon-carbon (C-C) covalent bond has a bond length of 1.54 Å and bond energy of 85 kcal
mol-1 (356 kJ mol-1). Because this energy is relatively high, considerable energy must be expended to break covalent
bonds. More than one electron pair can be shared between two atoms to form a multiple covalent bond. For example,
three of the bases in Figure 1.4 include carbon-oxygen (C=O) double bonds. These bonds are even stronger than C-C
single bonds, with energies near 175 kcal mol-1 (732 kJ mol-1).
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For some molecules, more than one pattern of covalent bonding can be written. For example, benzene can be written in
two equivalent ways called resonance structures. Benzene’s true structure is a composite of its two resonance structures.
A molecule that can be written as several resonance structures of approximately equal energies has greater stability than
does a molecule without multiple resonance structures. Thus, because of its resonance structures, benzene is unusually
Chemical reactions entail the breaking and forming of covalent bonds. The flow of electrons in the course of a reaction
can be depicted by curved arrows, a method of representation called “arrow pushing.” Each arrow represents an electron