Water and the weak bonds of biomolecules
Water and four kinds of weak bonds shape proteins, DNA and membranes: each bond is feeble, but thousands together hold a molecule in its form.
Key facts
- Water is a polar molecule: its oxygen is slightly negative and its hydrogens slightly positive, so water molecules form hydrogen bonds with each other and with polar solutes.
- Hydrogen bond: a hydrogen on N or O attracted to another N or O; weak alone, it holds the DNA double helix and the α-helix and β-sheet of proteins.
- Ionic (electrostatic) bonds or salt bridges join opposite charges, such as Lys⁺ with Asp⁻; water weakens them, so they matter most inside proteins.
- Hydrophobic effect: nonpolar groups cluster together to avoid water; it drives protein folding, membrane bilayers and the binding of many drugs.
- Van der Waals forces: very weak attractions between atoms in close contact; they add up when two surfaces fit closely (enzyme and substrate, antigen and antibody).
In detail
- A covalent C–C bond is roughly ten to a hundred times stronger than these weak bonds; weak bonds can form and break at body temperature, which is why binding is reversible.
- Water has a high specific heat and a high heat of vaporization: the body resists rapid temperature changes and loses heat by sweating.
- Water's high dielectric constant separates ions, which makes it a good solvent for salts and polar molecules.
- Amphipathic molecules (phospholipids, bile salts, fatty acids) have a polar head and a nonpolar tail; in water they form micelles and bilayers.
- Also called
- Noncovalent interactions, Hydrophobic effect
Clinical notes
- Heat, acids and detergents disrupt weak bonds: this is how proteins denature.
- Drugs bind their targets mainly through weak bonds, so binding is reversible and depends on a close fit.
- Bile salts form micelles thanks to the hydrophobic effect; without them dietary fat is poorly absorbed.
Test yourself
Think of the answer, then tap the question to see it.
Which weak interaction drives a protein to bury its nonpolar side chains?
The hydrophobic effect: nonpolar groups cluster away from water, which lets the surrounding water become less ordered.
Why are salt bridges weaker on the surface of a protein than inside it?
Water around the surface shields the charges; inside the protein there is little water to weaken the attraction.
Why does sweating cool the body?
Water has a high heat of vaporization, so evaporating sweat carries away a lot of heat.
Connections
Educational information, not medical advice. For any symptom, see a health professional.
Sources: water (CHEBI:15377) (ChEBI, EMBL-EBI, 2026); Biochemistry, Primary Protein Structure (StatPearls (NCBI Bookshelf), 2025); Physiology, Osmosis (StatPearls (NCBI Bookshelf), 2023).
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Representative model, for educational purposes only.