1. What is pKa?

The pKa value is the negative base-10 logarithm of the acid dissociation constant (Ka) of a solution. It indicates the strength of an acid or base in solution and helps predict protonation states at different pH values.

2. How Does ChemAxon Predict pKa?

ChemAxon uses advanced machine learning algorithms trained on experimental data to predict pKa values based on molecular structure. The predictions account for:

• Molecular structure (from SMILES notation)
• Temperature effects
• Ionic strength of the solution
• Electronic and steric effects

3. Importance of pKa Calculation

Applications: pKa values are crucial for understanding drug solubility, permeability, formulation stability, and predicting absorption, distribution, metabolism, and excretion (ADME) properties in drug discovery.

4. Using the Calculator

Tips:

• Enter valid SMILES notation for your compound
• Specify temperature (default 25°C)
• Set ionic strength (default 0.1M)
• Results show predicted acidic and basic pKa values

5. Frequently Asked Questions (FAQ)

Q1: How accurate are ChemAxon's pKa predictions?
A: Typically within ±0.5-1.0 units of experimental values for most drug-like molecules, though accuracy varies by chemical class.

Q2: What's the difference between acidic and basic pKa?
A: Acidic pKa refers to proton donation (COOH → COO⁻ + H⁺), while basic pKa refers to proton acceptance (NH₃⁺ → NH₂ + H⁺).

Q3: Why does temperature affect pKa?
A: pKa is temperature-dependent because dissociation equilibria are influenced by thermal energy changes.

Q4: Can I get micro-pKa values?
A: This calculator provides macro-pKa values. For microspecies distribution, use ChemAxon's MarvinSketch with the Microspecies Calculator plugin.

Q5: What molecules work best with this calculator?
A: Works best with small to medium-sized organic molecules (MW < 1000) containing typical acidic/basic functional groups.