Amino acid chart.

Codes, classes and molecular weights.

Free amino acids; molecular weights in g/mol from standard atomic weights, each within 0.01 of PubChem. Leucine and isoleucine are isomers with the same formula and mass.

Hydropathy is the Kyte-Doolittle index: positive values lean hydrophobic, negative values hydrophilic. Isoleucine (4.5) is the most hydrophobic and Arginine (-4.5) the most hydrophilic. The calculator's GRAVY score is the average of these values over a sequence.

What a peptide is built from.

Inside a peptide, each amino acid has lost one water to a peptide bond, so the unit is a residue: the free amino acid minus H₂O. A peptide's mass is the sum of its residue masses plus one water (18.02) for the free ends. Average masses are what you weigh; monoisotopic masses are what a mass spectrometer reports for the lightest peak.

Add them up for any sequence with the peptide molecular weight calculator, which uses the same values.

The groups that carry a charge.

Seven side chains and the two ends of a chain can carry a charge, and a group's pKa is the pH at which it is half ionized. Published values depend on the method and the surroundings, so two sets are shown: the EMBOSS values the calculator uses for net charge and isoelectric point, and measured values.

Measured values are averages of 541 pKa values from 78 folded proteins, except arginine, whose 13.8 is the value of its guanidinium group from a 2015 potentiometry and NMR study, well above the figure of about 12 often cited in textbooks. In a real peptide, neighboring residues and structure shift every value.

D-amino acids, Aib and other residues.

Every standard amino acid except glycine is chiral, and proteins are built from the L forms. D-amino acids are their mirror images: same formula and mass, different shape, which changes how enzymes recognize them. Sequences mark them with a D- prefix, as in D-Ala. Some research peptides also use residues outside the 20 that have no one-letter code, so their masses are added by hand.

Adding these to the standard residues reproduces the published formulas. Ipamorelin, Aib-His-D-2-Nal-D-Phe-Lys-NH₂, comes to C₃₈H₄₉N₉O₅ (711.9 g/mol), and SS-31, D-Arg-Dmt-Lys-Phe-NH₂, to C₃₂H₄₉N₉O₅ (639.8 g/mol), both matching PubChem.

N-terminus to C-terminus.

Sequences are written from the N-terminus, the end with the free amino group, to the C-terminus, the end with the free carboxyl group, in one-letter or three-letter code. Caps sit at the ends: Ac- marks an acetylated N-terminus and -NH₂ an amidated C-terminus. TB-500, for example, is written Ac-LKKTETQ, and SS-31 is D-Arg-Dmt-Lys-Phe-NH₂. The TB-500 vs thymosin beta-4 guide shows how a seven-residue fragment maps onto its 43-residue parent, and N-terminus vs C-terminus covers the two ends in detail. For where peptides end and proteins begin, see peptide vs polypeptide vs protein.

Sequences behind the vials.

Each Helion compound page lists its sequence, formula and molecular weight, and each published lab report confirms identity by mass spectrometry. Browse the research library or open the COA Portal.

Specs and storage by compound.