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# 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.

Updated Nov 12, 2026

## 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.

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## Connections

## Related 5

- [Protein structure and folding](https://durgical.com/en/biochemistry/topics/protein-structure-and-folding/)
- [Membrane structure](https://durgical.com/en/biochemistry/topics/membrane-structure/)
- [DNA structure and chromatin](https://durgical.com/en/biochemistry/topics/dna-structure-and-chromatin/)
- [pH, buffers and the Henderson–Hasselbalch equation](https://durgical.com/en/biochemistry/topics/ph-buffers-and-the-henderson-hasselbalch-equation/)
- [Amino acids: structure, charge and pKa](https://durgical.com/en/biochemistry/topics/amino-acids-structure-charge-and-pka/)

Educational information, not medical advice. For any symptom, see a health professional.

Sources: [water (CHEBI:15377)](https://www.ebi.ac.uk/chebi/searchId.do?chebiId=CHEBI:15377) (ChEBI, EMBL-EBI, 2026); [Biochemistry, Primary Protein Structure](https://www.ncbi.nlm.nih.gov/books/NBK564343/) (StatPearls (NCBI Bookshelf), 2025); [Physiology, Osmosis](https://www.ncbi.nlm.nih.gov/books/NBK557609/) (StatPearls (NCBI Bookshelf), 2023).

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