Boman Index Calculator

The method takes a published free energy of transfer from water into a hydrophobic environment for each of the twenty residues, adds those twenty numbers up as far as the chain extends, and divides by the length. The idea underneath it is sound and worth stating, because it is the only reason a table of coefficients can say anything at all: a residue sitting in a binding interface or in a membrane has left water, and the free energy cost of leaving water is part of what any interaction has to pay. Summing twenty per residue costs gives a single comparable number for any chain.

What that number is, is a screening score for one narrow purpose, which is putting sequences into an order before any experiment is run. It is not a measurement. Nothing was measured on your peptide, and no claim about binding, about potency or about any effect in a system follows from the value. The one thing it can legitimately do is sort a list of candidates so that the effort on the next step is spent at the promising end rather than the other way round. Ranking and measuring are different activities, and the difference is most easily lost once a number appears on a page wearing a unit.

The ranking is only ever as good as the table behind it. Every entry in that table is itself a fitted or measured quantity with its own conditions attached, and more than one published residue table exists. Two tools can therefore disagree about the same sequence while each is applying its own table correctly, and neither is broken. The productive way to treat a disagreement is to look up which table each tool used, which the amino acid composition calculator helps with by giving you the raw counts to recompute against either of them by hand.

The sign is the single easiest thing to get wrong here, because published tables do not all run the same way round. The residues that favour a hydrophobic environment sit at one end of the original table, with tryptophan and phenylalanine among them, while small and polar residues sit at or near zero and on the other side. Some sources quote that table directly and others quote its negative so that a larger number reads as more potential. Both conventions are in circulation, and the same underlying quantity appears with opposite signs in the literature and in different tools.

The practical rule is mechanical rather than chemical. Before comparing this page with a figure quoted somewhere else, establish which orientation that source uses, and then compare magnitudes. If a source quotes 0.4 and this calculator returns -0.4, the two are the same mean under opposite conventions and they agree exactly. Two numbers of equal magnitude and opposite sign are a match, not a contradiction, and treating them as a conflict is how a correct result gets thrown out. Neither number is wrong in that case; only the comparison was made in the wrong frame.

A second check catches unit slips rather than sign slips. Because the result is a mean of twenty fixed coefficients, it has to lie inside the range those coefficients span, and a number outside that band means a coefficient was entered wrongly. A wildly large result almost always means a decimal point moved, and the general habit of keeping figures in one consistent set of units, set out in the rounding and precision guide , is what catches that class of error early. None of this is peculiar to this index; it is simply more exposed here, because the inputs are bare numbers rather than quantities a parser has already checked.

Averaging is where nearly all of the weakness for a peptide comes from. On a four residue chain one residue is a quarter of the answer, so the output is very close to that single coefficient and the other three barely register. A chain of two tryptophans and two glycines returns the mean of those two values and nothing else, and it conveys no information about the fact that the tryptophans are adjacent to each other. Shortening the chain makes the index noisier rather than more precise, which is the opposite of what people expect from doing a calculation on fewer numbers. The peptide length calculator is worth running first so you know which regime you are in.

The mean also discards order completely. Any permutation of the same composition gives exactly the same result, so a block of hydrophobic residues and the same residues scattered along a chain are indistinguishable to it. For a folded protein that is a tolerable approximation, since the fold is held together by interactions the index also ignores. For a short linear peptide that contacts a surface, when and where each residue is presented is plausibly the entire question, and the index cannot see any of it. A charged residue that is permanently ionised and one that is only partly so at the working pH look identical here; the charge versus pH calculator is the page that separates them.

The distance between this score and any experimental result is the whole of what the score is not. It cannot tell you whether a sequence interacts with a particular target, how strongly, or under what conditions, and it carries no information at all about what a peptide does in a living system. What it can do is put a set of sequences into a defensible order so that a screening step has somewhere to begin. Read the output as a filter, and read anything concluded from it as one more hypothesis that still has to be tested.

If you want a genuinely independent view of the same chemistry rather than a second reading of the same table, the GRAVY score calculator weights every residue on a hydropathy scale taken from different observations, and the Kyte-Doolittle guide explains where that scale comes from. The hydrophobic residue fraction gives a third, purely unweighted view of the same chain. When three independent summaries disagree with one another, that disagreement is itself information, and it is a better use of attention than any one score on its own. A set of measures that all lean the same way is still three estimates of one assumption, not three confirmations of it.