Glues from living things

Section 4 of 4

Waves pound the rocks of a seashore all day, and mussels hold on. Their glue sets under water and grips wet rock, something very few glues made by people can do.

On a wet surface, a layer of water molecules sits in the way, hydrogen bonded to the rock. A glue has to push that water aside before its own molecules can get close. A mussel anchors itself with a beard of tough fibres called the byssus, and each fibre ends in a small pad of glue made of proteins.

The proteins at the face of each pad are rich in DOPA, an amino acid with two -⁠OH groups on neighbouring corners of a benzene ring. In one of them, about 3 in every 10 amino acids are DOPA. The mussel makes the protein with tyrosine, which has one -⁠OH on its ring, and an enzyme then adds the second -⁠OH, the same step that turns tyrosine into the Parkinson's medicine levodopa.

Levodopa (L-DOPA), C₉H₁₁NO₄An amino acid, the L hand of DOPA. A benzene ring with two -OH groups on neighbouring corners, joined through a -CH₂- group to a carbon that carries an -NH₂ group, a -COOH group and a hydrogen atom.OHOHOHONH2
DOPA, C₉H₁₁NO₄, the amino acid in mussel glue, in the L hand found in proteins. In the protein chain its -⁠NH₂ and -⁠COOH groups form peptide bonds, and the two -⁠OH groups on the ring face outwards.

The pair of -⁠OH groups is what grips. Together they push water off the rock and make hydrogen bonds with its oxygen atoms, and they also hold on to the iron and other metal atoms in the rock.

DOPA in mussel glue grips wet rock with hydrogen bonds Top: a wavy protein chain. Two DOPA side chains hang down from it, each a benzene ring with two -OH groups on neighbouring corners at its lower side. Bottom: a band labelled wet rock, with a row of oxygen atoms along its top edge. Each -OH hydrogen of DOPA points down at an oxygen of the rock, joined to it by a dashed hydrogen bond. Between and beside the DOPA rings, water molecules still sit on the rock. OHOHOHOHOOOOOOO H₂O H₂O DOPA protein chain wet rock
DOPA side chains of a mussel protein against wet rock. Each -⁠OH hydrogen makes a hydrogen bond (dashed) with an oxygen of the rock, where water molecules still cover the rest of the surface.

Some of the DOPA does a second job. An enzyme and oxygen turn its two -⁠OH groups into two C=O groups, the change that browns a cut apple. The new group bonds to neighbouring protein chains, so the glue cross-links and holds itself together: adhesion and cohesion from the one amino acid.

Glues from plants and animals

People glued things with natural materials for thousands of years before plastics, and several of these glues are still in use.

Some glues from living things
GlueMade fromUsed for
mussel glueproteins rich in DOPAholding on to wet rock
spider gluesticky proteins in droplets on the webcatching insects
hide gluecollagen, boiled out of animal skin and boneviolins and furniture
starch pasteflour cooked in waterbookbinding and paper repairs
birch tarbirch bark heated with little airfixing stone blades to handles

Hide glue is protein, held together by hydrogen bonds, so heat and moisture loosen it again. Violin makers use it for that reason: a violin can be opened for repair without damaging the wood. Neanderthals in what is now Italy were making birch tar about 200,000 years ago, one of the oldest known uses of glue.

Quick check

Which part of DOPA lets mussel glue grip wet rock?

That's the end of lesson 32

In this lesson you found out:

  • How glue holds on to a surface
  • Why superglue sets in seconds
  • How two-part epoxy glue hardens
  • Glues made by living things
Start Lesson 33: Fibres and fabrics All lessons