2.9 : Noncovalent Attractions in Biomolecules

Covalent bonds form when pairs of electrons are shared between interacting atoms. These strong bonds require a large amount of energy to break.

While covalent bonds are intramolecular forces that connect atoms into molecules, intermolecular and noncovalent attractions stabilize groups of atoms between molecules or within different parts of a larger molecule.

Though individually weak, many noncovalent interactions together can keep molecules associated for an extended time.

Four major types of noncovalent attractions occur in biological systems: ionic interactions, hydrogen bonds, Van der Waals forces, and hydrophobic interactions.

Ionic interactions occur between oppositely charged ions.

The DNA backbone has many negatively charged phosphate groups close to each other. To stabilize the molecule, cations such as magnesium interact with the phosphates groups, neutralizing the net charge on the DNA.

A hydrogen bond forms when a hydrogen atom that is covalently bonded to a highly electronegative atom, such as oxygen or nitrogen, interacts with the lone pair of electrons on another electronegative atom.

In DNA, the complementary strands are paired together by hydrogen bonds. A base from one strand shares its covalently bonded hydrogens with nitrogen or oxygen atoms on another base on the opposite strand.

Van der Waals interactions occur when two molecules approach each other closely.

These nonspecific attractive forces result from temporary dipoles generated by the rapid movement of electrons throughout the molecule. However, when the molecules get too close, electrostatic repulsion overturns Van der Waals interactions.

Lastly, hydrophobic interactions are a forced association of hydrophobic groups due to repulsion from the water molecules.

In an aqueous environment, the hydrophobic parts of lipid molecules cannot easily disrupt the hydrogen bonding between water molecules.

The interaction between the water molecules is stronger than the interaction between water and the lipid molecules. So, the hydrophobic parts of the lipids associate, causing the lipids to aggregate.

Noncovalent attractions are associations within and between molecules that influence the shape and structural stability of complexes. These interactions differ from covalent bonding in that they do not involve sharing of electrons.

Four types of noncovalent interactions are hydrogen bonds, van der Waals forces, ionic bonds, and hydrophobic interactions.

Hydrogen bonding results from the electrostatic attraction of a hydrogen atom covalently bonded to a strong-electronegative atom like oxygen, nitrogen, or fluorine by another electronegative atom. The hydrogen atom develops a partial positive charge as the electronegative atom to which it is bonded draws the electron cloud near it. As a result, a weak interaction occurs between the δ⁺ charge of the hydrogen and the δ^–charge on the neighboring electronegative atom. This type of interaction forms regularly between water molecules. Independent hydrogen bonds easily break; however, they occur in large numbers in water and organic polymers, creating a significant force in combination.

A second type of interaction called van der Waals is driven by temporary attractions between electron-rich and electron-poor regions of two or more atoms (or molecules) that are near each other. These interactions can contribute to the three-dimensional structures of proteins essential for their function.

Another type of interaction is ionic bonding that occurs between oppositely charged ions. In biological systems, ionic interactions arising from oppositely charged ions can also help stabilize biomolecules’ structure. Metal ions such as magnesium interact with negatively charged biomolecules such as DNA. The magnesium ion binds to the negative phosphate groups, thereby neutralizing the charge and helping to pack the long DNA polymer into solenoid or toroid structures.

Lastly, the hydrophobic effect is a noncovalent interaction in which hydrophobic molecules aggregate to minimize contact with water in an aqueous environment. As a consequence, the hydrophobic regions of a polypeptide become buried within the structure during protein folding.

Tags

Noncovalent Attractions Biomolecules Covalent Bonds Intramolecular Forces Intermolecular Attractions Noncovalent Interactions Ionic Interactions Hydrogen Bonds Van Der Waals Forces Hydrophobic Interactions DNA Backbone Negatively Charged Phosphate Groups Cations Hydrogen Bond Formation Complementary Strands Base Pairing Van Der Waals Interactions Attractive Forces

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