Average Molecular Weight Calculator

Start from the monomers and the rule becomes obvious. A chain of n residues begins as n free amino acids, and joining them into a linear chain makes exactly n minus 1 peptide bonds, because a chain with n members has n minus 1 links. Each bond condenses a carboxyl group with an amine group and releases one water. So n monomers have lost n minus 1 waters, and the finished peptide contains exactly one more water worth of hydrogen and oxygen than the sum of its residue masses does.

The residue masses tabulated for the twenty standard amino acids are therefore not free amino acid masses. Each one already has 18.01528 subtracted from it, once per residue, so summing n residue masses has implicitly removed n waters. The finished molecule only lost n minus 1, so adding one water back makes the two counts agree. Reach the same answer from a table of free amino acid masses instead and you subtract n minus 1 waters once and stop, which is a genuinely independent route to the same figure and worth being able to run as a check.

The size of the error this rule prevents is worth quantifying, because it is not a rounding matter. A twenty one residue peptide has twenty bonds, so adding a water per residue rather than per chain inflates the answer by twenty waters, which is 360.3 Da on a molecule of roughly 2300 Da. That is close to sixteen percent, which is large enough to look like a different compound rather than like a slip, and small enough to survive a casual check on the order of magnitude. A cheap bound catches it either way: divide the residue sum by the length, and the mean has to fall between the lightest residue at 57.05 and the heaviest at 186.21.

Every element in a peptide is a mixture of isotopes rather than a single atom. Carbon is overwhelmingly carbon 12, but a small fraction of any sample is carbon 13, nitrogen 14 comes with a little nitrogen 15, and so on for every element present. The average convention replaces each element with a single abundance weighted mean over its isotopes, and those abundances are tabulated and periodically revised by the body that maintains atomic weights. The result is the mass of the mixture, which is why it belongs in a division and not in a spectrum.

That distinction is what makes the average the correct divisor. Weigh 5 mg of a peptide and you have weighed a sample distributed across many isotopologues in natural proportions, so dividing that mass by the average molecular weight counts the molecules correctly. Divide by the monoisotopic mass instead and the count comes out low by exactly the fraction the isotopic offset represents. The two conventions are set against each other in monoisotopic mass against average molecular weight , and the monoisotopic mass calculator computes the other one. It is also why the average figure travels unchanged through every downstream calculation on this site, and why a published value quoted to one decimal place can be checked against a derived mass without first being adjusted for isotopes.

The offset is not a fixed amount per residue, which matters if the aim is to predict a comparison in advance. Set each average atomic weight beside its monoisotopic mass and the differences per atom come out at roughly 0.011 Da for carbon, 0.004 for nitrogen, 0.004 for oxygen, 0.0002 for hydrogen and 0.09 for sulfur. Carbon dominates simply because there is so much of it, so a chain of 120 carbons carries about 1.3 Da of offset on its own, and one methionine residue carries as much offset as very nearly four alanine residues.

The result describes a free peptide with an unmodified chain and free termini, and it is exact for that description. Real material frequently is not that. A peptide trifluoroacetate carries a counter ion, a peptide acetate carries a different one, a monohydrate carries bound water, and each of those shifts the mass of the vial contents away from the free base value a sequence produces. A supplier who quotes a molecular weight for a salt form has quoted the mass of the salt rather than of the peptide, and both numbers are internally correct.

Isotope enrichment is the case that neither convention handles. A peptide made from carbon 13 or nitrogen 15 labelled building blocks has a mass whose isotopic composition has to be stated explicitly, because both tables assume natural abundance. For labelled material the mass comes from the labelling scheme rather than from either calculator here, and no sequence can supply it. Bound solvent and counter ions behave the same way, in the sense that they are facts about the sample and not about the molecule. Where any of them applies, the honest procedure is to state which form the figure describes and then apply the corresponding shift separately, rather than expecting one derived number to cover every presentation of the same peptide.

The four decimal places on the result are arithmetic reproducibility, not measurement precision. A specification quoted to one decimal place resolves 0.05 Da, and a balance is coarser still. Round the derived mass to what the source can support before comparing the two, which is what the molar mass cross check calculator exists for, and where a large gap needs a chemical explanation rather than a rounding one, the modification mass calculator is the place to test candidate shifts. Comparing a derived mass against a rounded published figure is only meaningful once both have been reduced to the same number of significant figures, and that reduction is a judgement about the source rather than about the calculation.

Almost everything done with a vial starts by applying this figure. From a mass in milligrams, dividing by it in grams per mole is the first step towards a mole count, a molarity or a molar extinction coefficient, and the mass to moles calculator performs that division while the molarity calculator adds the volume step on top. Working in that order, sequence first and moles second, means the one number looked up is used consistently instead of being retyped from memory at each stage. The molarity step then needs a volume, and the volume is the input most often mistyped, so it is worth establishing the two habits separately: derive the molar mass once, then treat it as a constant for every later calculation that needs it.

It is also the figure to test a specification against, and the size of a disagreement is more informative than its sign alone. A gap of a few tenths of a dalton on a short chain is the isotope convention. A gap of tens of daltons is chemistry that a modification table accounts for. A gap in the hundreds means the two entries describe different molecules, or that one of them is simply wrong. The molecular formula calculator reaches a mass by counting atoms rather than summing tabulated residues, which makes it a check on the residue sum and not a restatement of it, and the longer treatment of the same arithmetic sits in peptide molecular weight from sequence .