MACS Matchmaker

Heterogeneous Analyte — Parallel Reactions

When independent sample characterization suggests a stable mixture, the parallel-reaction model can represent two candidate analyte components — for example monomer and aggregate, or two conformers — with different kinetic phases. This simulator mirrors the model used by the evaluation: the fitted response is the sum of two effective Langmuir 1:1 components.

How to interpret the model

Both response components use the nominal injected concentration. Each fitted Rmax,i is therefore an effective amplitude that can absorb component abundance, molecular-mass response, activity, and accessible capacity. It is not a direct measurement of the molar fraction of that component.

The two terms do not explicitly compete for a finite pool of free ligand. Treat the model as a low-occupancy or empirical approximation. At high surface occupancy, shared-site competition and exchange require a coupled competitive-binding model and the fitted rate constants can become biased.

Parallel reactions
A1A2++LLA1·LA2·LL denotes the same ligand class in both parallel terms
R(t)=R1(t)+R2(t),dRidt=kon,i C (Rmax,i−Ri)−koff,i RiR(t) = R_1(t) + R_2(t),\quad \frac{dR_i}{dt} = k_{on,i}\, C\, (R_{max,i} - R_i) - k_{off,i}\, R_i

Component 1 can represent a candidate dominant population and component 2 a candidate minor aggregate, oligomer, or conformer. Their effective responses add, so a small slow-dissociating term can dominate the late tail.

Assumptions
  • Sample composition stays constant across the dilution series.
  • Each component follows a reversible, pseudo-first-order Langmuir 1:1 response term.
  • Effective component amplitudes absorb abundance and response weighting.
  • Shared-site competition, exchange, mass transport, and rebinding are not modelled explicitly.
Parameters
Component 1 (dominant population)

kon,1 (M⁻¹ s⁻¹)

koff,1 (s⁻¹)

Component 2 (minor population)

kon,2 (M⁻¹ s⁻¹)

koff,2 (s⁻¹)

Nominal concentration series (nM)
Kinetic mode
Single-cycle: concentrations are injected sequentially on the same surface, no regeneration.

Noise σ (normalized, dimensionless)

Derived

KD,1 = 10 nM

KD,2 = 2 nM

Req,1 (highest C) = 0.79 (dimensionless)

Req,2 (highest C) = 0.2 (dimensionless)

Diagnostic cues
  • A minor component with a slow koff can create a pronounced late dissociation tail.
  • Set Rmax,2 to zero to recover the Langmuir 1:1 model.
  • This fit cannot by itself prove analyte heterogeneity: surface heterogeneity, avidity, rebinding, and aggregation can produce similar traces.
  • Compare untreated and size-exclusion-purified sample, or repeat at lower surface density, before assigning the two components.
Download traces (CSV)