Bioisostere Structure Editing and Property Comparison
Start from aspirin and use the existing molecular editor to investigate a deliberate structural change. The live engine recalculates the edited graph’s descriptors, while comparison tools can retain a reference. This is manual structure/property exploration; the app does not generate bioisosteres or predict that a replacement preserves biological activity.
The shipped aspirin example supplies a compact aromatic scaffold with an ester and carboxylic acid. The reference comparison uses salicylic acid, ChEMBL424. Removing the acetyl group changes the structure and property profile; this pair is a deacetylation comparison, not evidence of biological equivalence or a validated bioisosteric replacement.
Define a matched comparison before changing the graph
A useful comparison starts with a specific question: for example, what happens to the calculated polar surface estimate when a named group is changed? Save the reference identity and make the smallest edit that addresses that question. If several groups change at once, the resulting descriptor difference remains valid arithmetic but cannot isolate the contribution of one modification.
The aspirin reference is deliberately small enough to inspect atom by atom. Ensure that valence, bond order and charge remain chemically intended after an edit. The canonical representation can help reveal an accidental change that is difficult to see in a drawing. Give the modified structure a distinct label rather than leaving the original drug name attached to a different molecule.
Separate property optimisation from activity retention
An analogue may move a computed property in a desired direction while losing a binding interaction or gaining a liability. Whole-molecule descriptors do not reveal the spatial arrangement of every donor and acceptor in a bound conformation. A scaffold hop also changes the geometry connecting substituents, so similar formula-based properties cannot establish similar target engagement.
Use the comparison to document a hypothesis and select measurements, not to claim a successful replacement. Retain the original and modified structures, descriptor method and any measured endpoints with their units. If the aim is improved exposure, add suitable solubility, permeability and stability data. If the aim is retained potency, test the relevant assay under matched conditions. The editor provides a consistent numerical view of the proposal, while those experiments test its consequences.
Equations and variables
Difference between two descriptor values
ΔP = Pmodified − Preference
- Pmodified
- The same descriptor calculated for the modified representation
- Preference
- That descriptor for the retained reference representation
- ΔP
- Arithmetic difference; not a prediction of biological effect
Topological polar surface area
TPSA = Σ Ai
- Ai
- Polar fragment contribution assigned by the descriptor method (Ų)
- TPSA
- Topological estimate from the represented molecular graph (Ų)
Scope and limitations
- No automatic bioisostere generation, scaffold-hopping search or potency prediction is implemented.
- A descriptor difference requires two actual structures calculated with the same method and form policy. The aspirin/salicylic-acid pair is a structural reference comparison, not a matched experimental potency study.
- Replacing a group can alter stereochemistry, conformation, ionisation, stability and reactivity alongside the displayed descriptors.
Sources and provenance
- RDKit descriptor implementation and versioned reference checksBSD-3-Clause
Method implementation; computed molecular properties are estimates.
- ChEMBL 37 parent structures in the shipped compound libraryCC BY-SA 3.0; share-alike
Existing library capture: 28 September 2026. Preserve attribution when redistributing its rows.
- Wildman and Crippen: atomic contributions to molecular propertiesCited method; no reproduced article data
- Ertl, Rohde and Selzer: fast calculation of molecular polar surface areaCited method; no reproduced article data
Technical questions
Does a similar TPSA make two groups bioisosteric?
No. A scalar descriptor can match while shape, hydrogen-bond geometry, ionisation, reactivity or binding interactions differ. A bioisosteric replacement is a context-dependent medicinal-chemistry hypothesis that needs appropriate structural and experimental support.
Can the workspace automatically suggest replacements?
The current tool offers manual editing and property comparison. It does not rank generated analogues or infer a suitable replacement from a target structure. Keep a proposed edit labelled as a design idea until the necessary evidence is available.
How should I interpret a negative delta logP?
It means the selected algorithm estimates a lower neutral-species partition coefficient for the modified representation. It does not establish improved aqueous solubility or exposure. Check ionisation, solid-state behaviour and the conditions of any experimental measurements used for comparison.
Connected workflows
- TPSA and Crippen logP calculation
Read the definitions of descriptors being compared.
- Chemical structure and canonical SMILES
Check that an edit produced the intended molecular graph.
- 4PL IC50 curve fitting
Test an activity hypothesis with an appropriately qualified response curve.
- Morgan Fingerprint and Tanimoto Similarity Comparison Tool
Compare whole-molecule Morgan fingerprint similarity against single-group edits.
- Matched Molecular Pair (MMP) Substituent Shift Calculator
Read the descriptor shift of a single-point matched-molecular-pair edit.