Can One Molecule Be Positive and Negative at Once? Amino Acids Do It Constantly

Certain molecules contain both a group that readily gives up a hydrogen ion and a group that readily accepts one. In water the transfer happens internally, so the acidic group loses its hydrogen and becomes negative while the basic group gains one and becomes positive. The molecule has the same atoms as before and now carries two full charges in different places, summing to zero. The name is German and means hybrid ion, which describes the situation exactly.

Separated charges change the physical properties completely:

• •Melting points far higher than similar uncharged molecules
• •Many decompose before melting at all
• •High solubility in water and poor solubility in oils
• •Strong attraction between neighbouring molecules
• •Crystals that are hard and salt-like rather than waxy
• •Behaviour in an electric field that depends on acidity

Every amino acid exists in this form at ordinary acidity and the consequences run through biology. The acidic group at one end loses its hydrogen, the basic group at the other gains one, and the resulting doubly charged neutral molecule is what actually exists in a cell. That explains why amino acids are crystalline solids rather than oily liquids, why they dissolve readily in water and not in fats, and why the acidity of a solution changes their behaviour so sharply. The particular acidity at which the charges exactly balance is characteristic of each one.

The requirement is simply an acidic and a basic group on the same molecule, and that combination is commoner than it first appears. All twenty amino acids used in proteins qualify. So do the short chains formed when a few are joined, and so does a protein as a whole in a more complicated way, since it carries many such groups along its length. Certain detergents are built this way deliberately, because a molecule with balanced charges is gentle on skin and on proteins, which is why they appear in shampoos and in the buffers used to handle delicate biological samples.

The acidity at which a molecule carries no net charge is a fingerprint and laboratories use it constantly. Applying an electric field to a mixture in a gel with a gradient of acidity causes each molecule to migrate until it reaches the point where its charges balance, at which it stops, so the mixture separates into sharp bands by that property alone. The technique separates proteins that differ by a single amino acid. The same principle explains why proteins are least soluble and most likely to precipitate at that particular acidity, which is used for purification and which is why milk curdles when soured.

A molecule with both an acidic and a basic group transfers a hydrogen internally, so it carries a positive and a negative charge at once while being neutral overall. That gives high melting points, water solubility and salt-like crystals. Every amino acid exists this way in a cell, and the acidity at which the charges balance is a fingerprint used to separate proteins.