IMAT Biology Lesson 1.1 | Biochemistry | Weak Interactions

Summary

The lesson introduces weak, non-covalent interactions, their role in living systems, and the four main types discussed. Although each interaction is weak on its own, many acting together can have a strong cumulative effect, helping biological molecules maintain their structures.

Chemistry background

• Atoms have electrons in shells. The outermost shell is the valence shell, and its electrons are called valence electrons.
• The lesson uses the octet rule as a basic model: atoms tend toward having eight electrons in their outer shell, a condition associated with greater stability.
• Sodium has one valence electron, while chlorine has seven. Sodium can transfer its electron to chlorine, forming oppositely charged particles that attract.
• Covalent bonds involve sharing electrons and are described as strong interactions. The lesson focuses instead on non-covalent interactions, which do not involve this kind of electron sharing.

Four weak interactions

1. Ionic interactions

   • Electrostatic attractions between oppositely charged atoms or molecules; like charges repel.
   • The sodium–chlorine example illustrates how electron transfer can produce positive and negative ions.
   • Ionic interactions are strongest in a vacuum and generally weaker in water, because water molecules interact with the charges and reduce their attraction.
   • In biological molecules, positively charged lysine and negatively charged glutamate are examples of groups that can attract.

2. Hydrogen bonds

   • Form when hydrogen is covalently attached to an electronegative atom, creating partial charges that can attract nearby partial charges.
   • The lesson identifies oxygen, nitrogen, and sulfur as common atoms involved in biological hydrogen bonding. It also mentions the familiar fluorine–oxygen–nitrogen mnemonic.
   • In DNA, hydrogen bonds join complementary bases: adenine pairs with thymine, and cytosine pairs with guanine. The lesson notes that adenine and thymine form two hydrogen bonds.
   • Numerous hydrogen bonds together help stabilize the DNA double helix.

3. Van der Waals interactions

   • Weak attractions between uncharged atoms or molecules that arise from permanent, induced, or temporary dipoles.
   • These interactions occur when particles are at an appropriate separation, described by the van der Waals radius. Particles that are too far apart interact little, while those that are too close repel.
   • The lesson describes three dipole-based forms:
     • Dipole–dipole: Attraction between existing partial-charge regions on polar molecules.
     • Dipole–induced dipole: A permanent dipole shifts the electron distribution of a nearby molecule, inducing a dipole in it.
     • Induced dipole–induced dipole: Temporary shifts in electron distribution create dipoles that induce corresponding dipoles in neighboring particles.

4. Hydrophobic interactions

   • Occur when nonpolar molecules are in water. The nonpolar molecules tend to cluster, reducing the amount of their surface exposed to water.
   • This is described as resulting from the exclusion of water rather than a direct attraction like the charge-based interactions.
   • Hydrophobic interactions contribute to a protein’s three-dimensional structure by helping nonpolar regions avoid contact with water.

The lesson closes by previewing a later discussion of organic molecules in organisms and their functions.

Speaker and source

• Presenter: Andre, identified in the subtitles as the presenter for Med School EU. The video metadata identifies him as Andriy Tryguba.