Aliphatic Index Calculator

The index adds up the mole percentage of four residues under fixed weights and disregards the other sixteen entirely. Alanine carries a weight of one, valine 2.9, and isoleucine and leucine share a weight of 3.9. Every charged residue, every polar residue and phenylalanine, tryptophan and cysteine are invisible to it. The weights are coefficients from a regression in the original work, not physical constants, so they encode a relationship that was observed in folded globular proteins and nothing wider than that. That origin is worth remembering, because it fixes the population the number describes and it is the reason the interpretation stops where it does.

One immediate consequence is that the scale is bounded only by the composition that feeds it. A chain of pure alanine scores exactly 100 whatever its length, because its alanine mole percentage is 100 and the weight is 1. A thirty two residue chain made of eight alanines, eight valines, eight isoleucines and eight leucines scores 25 + 2.9 x 25 + 3.9 x 50, which is 292.5. Nothing about that figure is wrong; it shows that the familiar 60 to 100 range quoted for real proteins is a property of how those proteins tend to be composed, not a ceiling built into the index.

On a short chain the denominator matters more than the weights do. A single alanine in a four residue chain is 25 percent, so AGDE scores 25, a number that looks reassuringly like a protein index and is in fact just its alanine fraction. Put the same alanine into a chain of forty residues and it is 2.5 percent, so the index becomes 2.5. The peptide length calculator is what tells you which of those two regimes you are in before you read anything into the result, and on a short chain it is the one that decides how much the number means.

The formula is defined on mole percentage, which is the count divided by the length and multiplied by one hundred, not on the plain fraction. Recomputing it by hand with 0.25 where 25 belongs produces a result one hundred times too small, and the error is completely silent because nothing in the arithmetic fails: AGDE comes out at 0.25 rather than 25, which still looks like an index and is off by two orders of magnitude. The reason it survives review is that the wrong answer is the same shape as the right one: a small number in a field that normally holds a small number.

There is a quick check that catches it. For a chain containing alanine and none of the other three counted residues, the index must equal the alanine mole percentage exactly, so the answer is always between 0 and 100 in that case. Anything above 100 tells you the formula has been applied to fractions rather than percentages. The alternative route is to skip the percentages entirely: multiply each raw count by its weight, add the four terms, and divide once by the length at the end. One division is much harder to get wrong than two, and the counts come straight from the amino acid composition calculator .

There is a related case that looks like a failure and is not one. A chain with none of the four counted residues returns zero, which is a genuine arithmetic answer rather than a rejected input. It is also the least informative answer the page can give, because on a scale where real folded proteins sit around the middle of the hundreds, a zero describes a chain built entirely from the residues the formula was defined to ignore. Reporting it as a computed value rather than as an error is the right behaviour, and reading it as a low score rather than as an absence of relevant residues is the mistake.

The correlation behind the weights was made between composition and the melting behaviour of folded globular proteins. A short linear peptide has no fold and therefore no melting behaviour for the index to predict, so the stability reading has nothing to attach to. The number itself remains a faithful arithmetic summary of four mole percentages. Reading it as a stability figure for an unfolded chain is a category error rather than a small inaccuracy, and it is the easiest mistake to make because the word stability is printed right next to the result.

Beyond the fold, the index is silent about charge, and charge is what governs whether a chain stays in solution. Two peptides with identical alanine, valine, isoleucine and leucine percentages can differ in every ionisable residue they contain and score exactly the same. The hydrophobic residue fraction counts the non polar residues without weighting and therefore overlaps only partly, while the GRAVY guide sets out why a per residue weighted mean is the summary that actually responds to this kind of change. Charge is the variable that most often decides how a peptide behaves in solution, and an index that cannot see it is answering a narrower question than the one usually being asked.

Length is invisible in a way that catches people out. Two chains with identical composition score identically no matter how long they are, because the percentages are the same either way. That is a direct consequence of the definition, and it means the index is not a length descriptor. Comparing it across chains of very different sizes means comparing composition as well as size, or the comparison is not like for like, and a difference in the result may say nothing at all about the change you set out to make.

The result is dimensionless and it is not a temperature. There is no published conversion from an aliphatic index to a melting point in degrees, because the index is a regression coefficient applied to a composition rather than a physical quantity. Its useful form is a ranking: among chains of similar length and similar fold, a higher value sits at the end of the range that was associated with higher melting behaviour in the original data. Used any other way it stops carrying meaning, and a number quoted without the composition behind it cannot be checked by anybody else.

So the honest use is comparative and narrow. Compute it on two sequences, look at which of the four percentages actually changed, and treat the difference as a reason to go and look at something else rather than as a result in itself. If the question behind the number is how hydrophobic a chain is, the GRAVY score calculator answers it more directly; if it is how strongly a sequence might sit against a protein surface, the Boman index was built for that instead; and if it is simply how much material a chain contains, the molecular weight guide is the relevant arithmetic.