pKa
/ pee-KAY-ay /
Some acids let go of their hydrogen ion eagerly, others cling to it; pKa is a single number that tells you how reluctant a given acidic group is to release its H+. If pH answers 'how acidic is this solution?', pKa answers 'at what acidity does this particular group give up its proton?'
Every group that can hand off a hydrogen ion has a characteristic pKa — the pH at which exactly half of those groups have released their H+ and half are still holding on. Above that pH (more basic), the group is mostly deprotonated and often carries a negative charge; below it (more acidic), it stays protonated and neutral. So pKa is a tipping point. A handy rule, the Henderson-Hasselbalch idea, says when the surrounding pH equals the pKa, the group is fifty-fifty; each pH unit away tilts the ratio by tenfold. A low pKa means a strong acid that lets go easily; a high pKa means a weak one that holds tight.
In molecular biology, pKa explains why a molecule is charged or not at the cell's pH near 7.4. The carboxyl group of an amino acid (pKa around 2) is deprotonated and negative at body pH; an amino group (pKa around 9-10) is protonated and positive. Certain side chains, like histidine (pKa near 6), sit right at the edge, switching charge with tiny pH shifts — which is exactly why enzymes use histidine to shuttle protons during catalysis. Knowing pKa lets a biologist predict charge, and charge predicts how molecules attract, repel, and react.
At blood pH 7.4, an amino acid's carboxyl group (pKa around 2) is far above its pKa and so is negatively charged, while its amino group (pKa around 9) is below its pKa and so is positively charged — giving the molecule both a plus and a minus end at once.
pKa is the pH at which a group is half-deprotonated.
Do not confuse pKa with pH: pKa is a fixed property of a particular acidic group, while pH is the changing acidity of the surrounding solution. The group's charge depends on the difference between the two.