Chirality

A carbon bearing four different substituents is a stereocentre, and its two non-superimposable mirror-image arrangements are enantiomers. Every amino acid except glycine has one at the alpha carbon, designated L or D. Enantiomers share formula, molecular weight and essentially every scalar physical property, differing only in the direction they rotate plane-polarised light, but a receptor is itself chiral and distinguishes them completely, which is why ribosomal synthesis uses only L residues.

In peptide chemistry the practical issue is epimerisation during synthesis rather than whole-molecule inversion. Activating a residue's carboxyl for coupling makes its alpha proton more acidic and a fraction of the chain can invert at that position; cysteine and histidine are the notorious offenders, and racemisation rises with base strength and coupling time. The product is a diastereomer impurity carrying exactly the parent mass that often appears on reversed-phase chromatography as a shoulder rather than a resolved peak.

This is why a mass and a purity number are not jointly sufficient for identity. Mass spectrometry is blind to stereochemistry, and a chromatogram reported as 99 percent pure can conceal several percent of epimer beneath the main peak. Demonstrating stereochemical integrity needs chiral amino acid analysis after hydrolysis or a method validated to resolve the specific diastereomer, and neither appears on a commodity certificate.

The other misunderstanding concerns what a D residue does. Substituting one at a labile position is a well-established stabilising strategy, but inverting every centre gives the whole-molecule enantiomer, which binds the mirror image of its target and is generally inactive rather than equivalent. Retro-inverso designs, reversing sequence as well as stereochemistry, work for some short linear epitopes and fail for others.