I was just browsing through the biochemistry and biophysics tabs and noticed a question that was commented as being "off topic" and better posted in the Chemistry SE.

How does one derive a KD from an equilibrium titration experiment? I am definitely making a mistake somewhere

Yes, this is a pretty basic thermo question that could have belonged in Chemistry.SE, but I thought it was reasonable to ask here. Also, this is not a great question in terms of quality, so I'm not mortified at it not getting an answer. However, as an example it got me thinking.

The concept is really fundamental to understanding biochemistry and biophysics. The only reason this doesn't have a "a strong biological component" (according to the commenter) is because its a theoretical model question, but theoretical models are a big part of biology! If this person asked about measuring the O2 binding energies of haemoglobin vs myoglobin, that is a classic biochemistry question, and uses the exact same kind of analytical framework. Surely that question would not be off-topic here. The only real difference is that one is pure theoretical and the other is slightly more concrete.

One last example - questions about the continuum elastic model of a membrane. Questions about the theoretical underpinning of the model might belong in Physics.SE, as they discuss elastic moduli, stress, strain and all that good physics stuff. But a question about applying that model to an actual membrane would certainly belong in Biology.SE. Its unlikely anyone on Physics.SE would know or care about the properties of different membrane compositions, or care about the interactions between membranes and peptides or transmembrane proteins. It therefore seems weird to force questions about the underlying model elsewhere.

I think its a worthwhile question as to how we treat purely theoretical biochemistry or biophysics questions. Biology is generally a big tent, accepting methodologies and frameworks from all sorts of other fields, and I think discussion of those models belongs here too.

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    $\begingroup$ This is not theoretical biology. Check out the journal to find out about that field. This is a question about the physician-chemical principles underlying interaction of biological molecules. The question wasn’t really whether it was on topic, but that it is described in many text books and in many websites. $\endgroup$
    – David
    Jul 21, 2022 at 20:11
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    $\begingroup$ I made a statement and gave a reference to back it up (The Journal of Theoretical Biology was intended). Please do not make personal remarks. And I don't care whether you have a Nobel Prize, this question and your answer are not what is generally considered Theoretical Biology. I also wonder quite why you should feel obliged to raise on Meta one individual's comment on a question. My own criticism would be that the poster was asking how to derive a constant from his experimental observations, and would have expected him to check and quote sources. $\endgroup$
    – David
    Jul 30, 2022 at 11:26
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    $\begingroup$ I wrote the comment. I agree with Bryan's assessment below. Regardless of whether there is a protein-binding tag at Chem.SE, the question is entirely answerable using principles of general chemistry (as you demonstrated in your answer), and would have been a better fit at Chem.SE. It still fits here, somewhat, but on the subject continuum between Bio and Chem, I thought it was closer to their purview. Some people disagreed, and that's fine. That's what Meta is for. $\endgroup$
    – MattDMo
    Jul 30, 2022 at 18:44

1 Answer 1


I don't think we do a very good job of handling these cases consistently. See a couple related questions on the borders of when something is "biology" vs not:

Where should I ask questions related to biology but need explanation in chemistry also?

Is a basic question about entropy off-topic here?

Certainly Biology depends a lot on physics and chemistry, particularly in the disciplines closest to those field like biochemistry and biophysics.

To me, the purpose of a policy directing chemistry questions back to Chemistry.SE (and similar for Physics) is to avoid content duplication. We really want there to be one place in the SE network that has a question/answer on a given topic, so that one place can get all the attention to make it the most optimal place to find an answer. However, there is no good mechanism in the SE software for marking something as a "cross-site" duplicate.

The guideline I think we've generally settled on is that questions that require any biology to understand the question or answer are certainly on-topic here. Questions that may be useful for biology but are entirely understandable in terms of chemistry belong better on Chemistry.SE, whether or not they are theoretical.

To me, this seems like a bit of a borderline case; certainly the question is phrased as a biology question, but as a commenter points out, the solution is entirely about chemistry, and I am certain there are questions on Chemistry.SE that already address KD as a concept. For a typical educational pipeline, it will be assumed that students reaching a question like this in a biology (or biochemistry) course will have already learned about the concept in a general chemistry setting.

For the particular example question linked here, the answer you've posted is entirely chemistry: there is no biology used at all in the answer, because no biology is needed for it. That seems like an argument that the question should have been closed and moved to chemistry. However, it's attracted positive votes here as well in addition to some votes to migrate.

I feel like the other possible questions you mention here, about membrane physics and hemoglobin/oxygen binding, typically involve a lot more specifics of the biology side of things than this other example does, even if it's as simple as understanding how different binding affinities in different pH contributes to oxygen delivery.

We end up with arguments on the other side of things, too, between this site and Psych&Neuro or MedicalSciences. Ultimately, all these fields can be placed on some continuum, but that doesn't mean that some organizational boundaries between them are nonsense.


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