Hold your hands palm to palm. They are mirror images, yet you cannot stack one perfectly on top of the other. Molecules can behave the same way. This property is called chirality.
An alpha carbon with four different groups attached is chiral. It can exist in two mirror-image forms, called L and D. The proteins made by living cells are built almost entirely from L amino acids. Glycine is the exception to chirality itself. Its side chain is a hydrogen, so its alpha carbon carries two identical groups and has no mirror form. D amino acids do occur in nature, for example in some bacterial cell walls, but they are uncommon in proteins.
Amino acids also carry charges. In water near neutral pH, the carboxyl group gives up a proton and becomes negative. The amino group picks up a proton and becomes positive. A molecule carrying both charges at once is called a zwitterion, from a German word meaning hybrid.
Whether a group holds or releases a proton depends on the pH of the solution and on the group's pKa. The pKa is the pH at which half of that group is protonated. When the pH is below the pKa, the group tends to keep its proton. When the pH is above the pKa, it tends to lose it.
Side chains with ionizable groups follow the same rule. This is why the charge of a peptide changes with pH, which affects how it dissolves, folds and binds.
References
- Berg JM, Tymoczko JL, Stryer L. Biochemistry, 5th ed., Section 3.1: Proteins Are Built from a Repertoire of 20 Amino Acids (2002)
- IUPAC-IUB Joint Commission on Biochemical Nomenclature, Nomenclature and Symbolism for Amino Acids and Peptides (1983)
Educational content about laboratory research. Not medical advice. All Orion compounds are for laboratory research use only.
