Amino Acid Composition Calculator

Composition is not a property of peptide behaviour. It is an inventory, and its value comes from being the material every other descriptor on a peptide page is built out of. A mean hydropathy divides a weighted sum by the residue count. An extinction coefficient is a weighted sum over tryptophans, tyrosines and cystine pairs. A molecular weight is a sum of residue masses. In each of those the count from this page is either the divisor or one of the summands, so an error in the composition does not stay local. It propagates into every number computed after it.

Doing the count before anything else is therefore a discipline rather than a formality, because it is the cheapest number on the page to verify and the most widely reused. A transcription slip almost always shows up here. A substitution of one residue for another leaves the length untouched while raising one count and lowering another, which a length check cannot see and a composition table makes obvious. A deletion does change the length, so length catches it first, but the composition then identifies which count is short and by how many, and that is usually enough to locate the position.

The percentages on this page are mole fractions written as percentages. They are not mass fractions, and the two disagree residue by residue by an amount that can be large. Converting is not a matter of scaling the whole table. Each percentage has to be multiplied by the tabulated mass of the residue it belongs to, the results summed, and the total divided through. That weighted step is exactly what the average molecular weight calculator performs, so the two pages together give a mole composition and the mass each part of it carries.

A chain of GWWY shows the gap cleanly. Three of the four positions are aromatic residues, so each one is 75 percent of the chain by count, and each carries a residue mass well above the average for a peptide. Those three positions therefore account for a noticeably larger share of the mass than 75 percent. Nothing is inconsistent here: the two numbers answer two different questions, and a reader who wants the mass breakdown has to convert rather than assume. Any estimate that starts from counts moves by a different amount depending on which of the two it started from.

The twenty residues are sorted into exactly one class each, which is what makes the class percentages easy to compute and awkward to combine. The aromaticity calculator and the hydrophobic, polar and charged fraction pages all read the same classification and differ only in which classes they sum. The cost is that a single residue can answer more than one of those questions at once. Tyrosine is aromatic because its ring system absorbs ultraviolet light, polar because its phenolic hydroxyl hydrogen bonds with water, and non polar across most of its surface. One classification, three defensible questions, three overlapping answers.

It is worth treating a class as a statement about connectivity rather than about behaviour, because it is one. Cysteine is listed as polar because a free thiol hydrogen bonds, and a cysteine that has formed a disulfide has no such group left. Histidine is listed as basic because it can accept a proton, and at a pH well above its own pKa it is mostly unprotonated and mostly neutral. Neither label is wrong and neither is much use once the environment is not the one the classification was built for, which is a good reason to reach for the net charge calculator when the question is about charge rather than about membership.

One tempting move is to invert this page: take a molecular weight and ask which composition produced it. That fails for two independent reasons, and either one on its own is fatal. Leucine and isoleucine carry identical tabulated residue masses on both the average and the monoisotopic scale, so even a perfect mass leaves an unbreakable choice between them. Separately, any given multiset of residues can be written in any order at all, and order leaves no trace in a mass. The mass fixes neither which residues are present nor where they sit.

The mass to sequence estimator makes that ambiguity visible rather than concealing it, and it will not name a composition for you. Composition is reliable in the forward direction only: you have the sequence, and the question is what it is made of. From there, molecular weight from a sequence covers how the counts and the residue masses combine, the molecular formula calculator carries the same composition forward into element counts by removing one water for every peptide bond, and the extinction coefficient calculator uses three residue counts plus a bond count you have to supply yourself.