Quantify host binding when guests compete strongly directly. See occupancy shifts across realistic concentrations fast. Download reports, compare runs, and support your conclusions today.
| Scenario | Ht (mM) | G1t (mM) | G2t (mM) | Ka1 (1/mM) | Ka2 (1/mM) | Typical interpretation |
|---|---|---|---|---|---|---|
| Balanced competition | 1.00 | 2.00 | 2.00 | 1500 | 1500 | Similar binding; exchange near 50%. |
| Guest 2 favored | 1.00 | 2.00 | 2.50 | 1500 | 3500 | Guest 2 dominates bound fraction. |
| Guest 1 excess | 1.00 | 6.00 | 1.00 | 2500 | 2500 | Higher G1 shifts occupancy to guest 1. |
This calculator models a host that can bind two competing guests at equilibrium: H + G1 ⇌ HG1 and H + G2 ⇌ HG2.
Substituting mass balance yields closed expressions for the complexes using the free host:
The calculator solves [H] + [HG1] + [HG2] = Ht numerically, then reports: Exchange% = 100 · [HG2] / ([HG1] + [HG2]).
Guest exchange describes how a host distributes binding between two guests present together. The calculator reports the bound fraction that belongs to guest 2, computed as HG2 divided by HG1 plus HG2. When Ka2 exceeds Ka1, the same totals can yield a strong shift toward guest 2 even without large concentration changes. This is useful when ranking candidates or mapping selectivity in screening panels.
In many experiments, host is not negligible compared with guests, so simple proportional assumptions fail. Mass balance forces the system to respect limited host sites, making occupancies nonlinear with respect to totals. As Ht increases at fixed Ka values, the same guest totals can produce higher absolute complex concentrations, while the exchange percent may remain similar if both complexes scale together. At low Ht, the higher affinity guest often captures most binding even at modest excess.
Free host and free guests help plan titrations, detect saturation, and choose measurable windows. A low free host fraction indicates near saturation, where additional guest mainly shifts distribution rather than increasing total binding. If free guest remains high, you may be in a regime where changing totals offers limited sensitivity to Ka differences. For spectroscopy or calorimetry, targeting mid range occupancy can improve signal linearity and fit stability.
If you provide a relaxation rate k and a time t, the calculator gives an estimated approach toward equilibrium. This supports time course thinking, such as sampling shortly after adding guest 2. The estimate uses a first order relaxation, so it is best used for screening and planning, not for mechanistic rate extraction. When k is small, short t values can explain why measured exchange lags behind equilibrium predictions.
For clean reporting, keep units consistent between concentrations and association constants, and document the chosen initial state assumption. Exported CSV and PDF outputs are formatted for lab notebooks, QA reviews, and reproducible sharing. When comparing runs, vary one driver at a time: Ka ratios, guest totals, or host total, so your exchange trends remain interpretable. Include temperature, buffer, and ionic strength externally, because they influence Ka values and comparability across datasets. Record mixing order, incubation time, and measurement method for full scientific traceability.
It is the share of bound host that is complexed with guest 2: 100 × HG2 ÷ (HG1 + HG2). It ignores unbound host.
Yes. Convert using Ka = 1 ÷ Kd, with matching units. For example, if Kd is in mM, Ka is in 1/mM.
Because mass balance couples free and bound species. When host and guest totals are comparable, free concentrations shift, producing nonlinear occupancies.
Select the unit you use for concentrations. Ensure Ka values use the reciprocal of that unit, so calculations remain consistent.
Use it when you sample before equilibrium, such as timed exchange assays. It provides a simple first order relaxation estimate toward equilibrium.
Check that occupancies sum near 100%, complexes do not exceed Ht, and increasing Ka2 or G2t raises the guest 2 bound share in a sensible direction.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.