Protonation State

Protonation state describes which atoms in a molecule or binding site carry protons under a given chemical environment. It affects charge, hydrogen bonding, docking geometry, and interaction scoring.

A molecule can be viewed as a state-dependent object:

$$m(pH)=(G,q(pH),H(pH))$$

$G$ is the molecular graph, $q(pH)$ is the charge assignment, and $H(pH)$ is the hydrogen/protonation pattern at a stated pH or protocol.

Why It Matters

For 2D property models, ignoring protonation may be acceptable for some endpoints. For structure-based drug discovery, protonation can change docking poses, interaction fingerprints, and score interpretation.

Protocol Boundary

Protonation is not assigned by a generic graph alone. A public note should state the protocol:

$${\mathrm{prot}}_{\pi }(m)\to (m_{\mathrm{prot}},q,H)$$

where $\pi$ includes pH, tool, rule set, receptor context if used, and whether hydrogens are explicit.

For structure-based modeling, ligand and receptor preparation must be compatible:

$${\text{state}}_{\mathrm{complex}}=(\text{ligand protonation},\text{receptor protonation},\text{pH/protocol})$$

Do not imply that one tool or default state is universally correct.

Checks

• Is the pH or protonation protocol stated?
• Are ligand and receptor protonation handled consistently?
• Are charged states preserved through featurization?
• Are generated or standardized molecules re-protonated before 3D use?
• Is the note public-safe and free of private target preparation details?
• Is protonation assigned before conformer generation and docking?
• Does the featurizer encode formal charge and hydrogens as required by the task?

Failure Modes

• A neutralized 2D molecule is used for splitting, but charged forms are used for docking.
• Formal charge is removed or ignored in graph features.
• Ligand and receptor are prepared with incompatible protonation assumptions.
• A model trained on one protonation protocol is evaluated on another without recording the shift.
• Public notes include private target-preparation details instead of generic protocol fields.

Related

• Molecular standardization
• Molecular featurization contract
• Tautomer
• Receptor and ligand preparation
• Protein-ligand interaction
• Scoring function