Non-covalent bonds

The destruction of covalent bonds takes up huge amounts of energy. The breakdown of an O2 molecule into two oxygen atoms needs ~460 kJ mol-1. Thus, nowhere in "living" biochemistry are covalent bonds actually destroyed; if one is broken, another one is created. Nonetheless, many biochemical functions are using so-called weak/secondary/non-covalent bonds.

Weak bonds are created and destroyed much more easily than covalent ones. The typical range of energy needed to destroy such a weak bond is 4-30 kJ mol-1. Thus, the formation of weak bonds is energetically favorable, but these bonds are also easily broken by kinetic (thermal) energy (the normal movement of molecules). Biochemical interactions are often temporary (e.g., a substrate has to leave an enzyme quickly after being processed), for which the weakness of these bonds is essential. Also, biochemical specificity (e.g., enzyme-substrate-recognition) is achieved through weak bonds, utilizing two of their major properties:

- Since individual weak bonds are, well, weak, several of them have to occur in a specific pattern at the same time in roughly the same place.

- The short range of weak bonds.

There are three basic types of weak bonds, and a fourth "pseudo-bond":

Ionic bonds

Ionic bonds are electrostatic attractions between permanently charged groups. Ionic bonds are not directed. Example:

X-CO2- ..... H3+N-Y
~ 20 kJ mol-1

Hydrogen bonds

Hydrogen bonds are also established by electrostatic attraction, though not between permanently charged groups, but rather between atoms temporarily charged by a dipole moment, resulting from the different electronegativity of atoms within a group. Hydrogen bonds are even weaker than ionic bonds, and they are highly directional, usually along a straight line. The most common hydrogen bonds in biochemistry are:

X-OH ..... O-Y
X-OH ..... N-Y
X-NH ..... O-Y
X-NH ..... N-Y

Hydrogen bonds equal an energy between 12-29 kJ mol-1

Van der Waals attractions

Van der Waals attractions are established between electron density-induced dipoles. They form when the outer electron shells of two atoms almost (but not quite) touch. The distance of the atoms is very important for these weak interactions. If the atoms are too far apart, the interactions are too weak to establish; if the atoms are too close to each other, their electron shells will repell each other. Van der Waals attractions are highly unspecific; they can occur between virtually any two atoms. Their energy is between 4-8 kJ mol-1.

Hydrophobic forces

Hydrophobic forces are not actually bonds, so this list has four items, but still just three bond types. In a way hydrophobic forces are the negation of the hydrogen bonds of a polar solute, usually water, enclosing a nonpolar molecule. For a polar solute like water, it is energetically unfavourable to "waste" a possible hydrogen bond by exposing it towards a nonpolar molecule. Thus, water will arrange itself around any nonpolar molecule in such a way that no hydrogen bonds point towards that molecule. This results in a higher order, compared to "freely" moving water, which leads to a lower entropy level and is thus energetically unfavourable. If there is more than one nonpolar molecule in the solute, it is favourable for the nonpolar molecules to aggregate in one place, reducing their surrounding, ordered "shell" of water to a minimal surface. Also, in large molecules, such as proteins, the hydrophobic (nonpolar) parts of the molecule will tend to turn towards the inside, while the polar parts will tend to turn towards the surface of the molecule.


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