Amino Acid Structure: The 20 Residues, One Reference Chart

Drawn once, it covers all twenty:

H₂N — Cα(H)(R) — COOH

Three features of that scaffold do permanent work:

Chirality. With four different groups attached (in every amino acid except glycine), the α-carbon is a stereocenter — the molecule exists as two non-superimposable mirror images, L and D. Natural synthesis produces L-amino acids essentially exclusively; D-residues appear in peptides only by design, installed to resist enzymatic degradation, as in ipamorelin's D-2-Nal and D-Phe.

Two ionizable ends. The amine can hold a proton (–NH₃⁺) and the carboxyl can release one (–COO⁻). At neutral pH an amino acid carries both charges at once — a zwitterion — which is why free amino acids behave like salts: crystalline, water-soluble, high-melting.

Built-in linkability. An amine on one end and a carboxyl on the other is exactly what condensation into chains requires — the peptide bond forms between the carboxyl of one and the amine of the next, and the side chains are left projecting from the finished backbone like charms from a bracelet.

Grouped by side-chain class. Residue mass is the average mass an amino acid contributes inside a chain (its free mass minus the 18.02 Da of water lost in bond formation) — the number peptide molecular weights are actually built from.

Two rows repay a second look. Leucine and isoleucine share a mass to four decimal places — sequence isomers that no mass measurement can separate, the standing example of why identity-by-mass has limits. And histidine's near-neutral pKa plus metal affinity is the chemistry behind GHK-Cu: the His in Gly-His-Lys is the residue that grips the copper ion.

Nonpolar (hydrophobic). Gly, Ala, Val, Leu, Ile, Pro, Met. These avoid water — driving chains to bury them, sticking to the C18 surface in reverse-phase HPLC (hydrophobic residues elute later), and drawing peptide molecules toward each other in aggregation.

Aromatic. Phe, Tyr, Trp. Bulky rings; Tyr and Trp absorb UV at 280 nm strongly enough that a sequence containing them can be quantified spectrophotometrically — one of the methods behind net peptide content.

Polar, uncharged. Ser, Thr, Cys, Asn, Gln. Hydrogen-bonders that keep peptides soluble; the amide pair (Asn, Gln) is the site of the deamidation degradation pathway, and Cys pairs into structure-locking disulfides.

Acidic. Asp, Glu. Negatively charged at neutral pH — pulling a peptide's net charge down and its isoelectric point toward acid.

Basic. Lys, Arg, His. Positively charged — and the practical reason peptide vials contain counterions: each protonated Lys/Arg pairs with a trifluoroacetate from synthesis and purification, so basic-rich sequences carry the highest counterion mass loads.

Net charge is simply the ledger of the last two classes (plus the termini) — the property that decides electrophoretic and chromatographic behavior and where a sequence's aggregation-prone isoelectric point sits.

The three-letter codes (Gly, Ala, Ser…) are near-mnemonic abbreviations used when sequences are written residue-by-residue — BPC-157's opening reads Gly-Glu-Pro-Pro-Pro. The one-letter codes exist because long sequences make three letters unwieldy; most are the initial (G, A, S, L…), and the collisions are historical workarounds worth memorizing once: K = lysine, R = arginine, W = tryptophan, Q = glutamine, N = asparagine, D = aspartate, E = glutamate, F = phenylalanine, Y = tyrosine. In one-letter form, tesamorelin's 44 residues fit on a single line.