Different units in the same Simbiology model

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I have a Simbiology PK model and the unit of all species (drug) is in microgram/milliliter. However, I would like to include a new reaction considering the drug and receptor binding behavior. The equation for this reaction is reversible as below.
Drug + Receptor <-> Complex: k_on*Drug*Receptor - k_off*Ccomplex
However, the unit of the k_on is 1/Molarity/Second and the unit of the k_off is 1/second and the unit for the Drug, Receptor and Complex should be in molarity. Is there any way I could keep my species unit still in microgram/milliliter but only have their unit become in molarity when they participate in this reaction? or is there any trick I could apply to achieve this thought?
Thank you very much.

Accepted Answer

Arthur Goldsipe
Arthur Goldsipe on 2 Aug 2021
I would recommend against mixing mass-based units and mole-based units for species participating in the same reaction. In such cases, it would be unclear which of these units would apply to the reaction rate. I personally would probably use molarity for all the species. If you also want to know the drug concentration in microgram/milliter, you can calculate that separately as a different output from your simulations. One option introduced in R2020a is to add an "observable" to your model for this. If you need a solution that also works in older versions of SimBiology, you could do something similar with a repeated assignment rule.

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