Water makes up about 70 percent of a cell's weight. Every peptide in a cell is surrounded by it, so water shapes how peptides behave.
Water is a polar molecule. Its oxygen atom pulls electrons more strongly than its two hydrogens do. This leaves the oxygen slightly negative and the hydrogens slightly positive. Neighbouring water molecules attract each other through these charges, forming hydrogen bonds.
Other molecules can be sorted by how they get along with water. Polar and charged groups form hydrogen bonds or charge attractions with water. They dissolve easily and are called hydrophilic, which means water loving. Groups made mostly of carbon and hydrogen are nonpolar. They cannot bond with water, so they are pushed together. They are called hydrophobic, which means water fearing.
Amino acids carry side chains of both kinds. When a chain folds, the hydrophobic side chains tend to cluster on the inside, away from water. The hydrophilic side chains tend to stay on the outside, facing water. This simple sorting is one of the main forces that gives a folded protein its shape.
The same rules govern binding. When two molecules meet, matching patches of charge, polarity and water-avoiding surfaces help them fit together. Change the water around them, for example by adding salt or changing the pH, and their behaviour can change too.
References
- Alberts B et al. Molecular Biology of the Cell, 4th ed., The Chemical Components of a Cell (2002)
- Berg JM, Tymoczko JL, Stryer L. Biochemistry, 5th ed., Section 3.1: Proteins Are Built from a Repertoire of 20 Amino Acids (2002)
Educational content about laboratory research. Not medical advice. All Orion compounds are for laboratory research use only.
