Where the word peptide stops meaning anything

A peptide is a chain of amino acids joined by peptide bonds. Past roughly fifty residues the convention is to call it a protein instead, and the line is a convention rather than a chemical fact: nothing changes at residue fifty-one.

Why the length matters commercially

Short chains are made by solid-phase synthesis. Long ones increasingly are not, because yield falls with every cycle and past a certain length recombinant expression in cells becomes the cheaper route. That changes the impurity profile, the certificate and the price.

The things that are not peptides

Two items in this catalog are filed under research compounds rather than peptides, and it is not a technicality. NAD+ is a dinucleotide. 5-Amino-1MQ is a small-molecule methylquinolinium. Neither is a chain of amino acids and neither is made the way the rest of the shelf is made.

Most shelves in this market file both under peptides. It is a small thing that tells you whether the person writing the catalog knows what they are selling.

Blends are several peptides, not one

A blend vial holds two or more separate peptides co-lyophilized. It is one vial and one lot number, but the certificate should report each component separately, because a purity figure for a mixture is not a meaningful number.

Common questions

What is the difference between a peptide and a protein?

It is length, by convention. A peptide is a chain of amino acids joined by peptide bonds, and past roughly fifty residues the convention is to call it a protein instead. The line is a convention and not a chemical fact: nothing changes at residue fifty-one.

Is NAD+ a peptide?

No. NAD+ is a dinucleotide, and 5-Amino-1MQ, which most shelves also file under peptides, is a small-molecule methylquinolinium. Neither is a chain of amino acids and neither is made the way peptides are made, so this catalog files both under research compounds.

Why are long chains made differently from short ones?

It comes down to yield. Short chains are made by solid-phase synthesis, but yield falls with every added residue, and past a certain length recombinant expression in cells becomes the cheaper route. That change of method changes the impurity profile, the certificate and the price.

Why do two peptides of the same length cost different amounts?

It is because modifications change the molecule and the price. An acetylated N-terminus, an amidated C-terminus, a fatty diacid on a lysine or a lactam bridge each add steps that can fail, which is why two chains of the same length can differ several-fold in cost.