Global fitting: the key for a robust analysis

Download use case: Global Fitting


The Indian parable of “The six blind men and the elephant” tells the story of six blind men who touch an elephant in the hope of learning what it is like. As each one can only feel a different part of the animal the individual conclusions obtained are in disagreement and none of them provides a real view of the full elephant. “only by sharing what each of you knows can you possibly reach a true understanding”; that´s the moral behind this nice story.


Fig 1: The six blind men and the elephant: only a global analysis of the overall data provides a true understanding.

The binding assay(s) achieved to characterize a molecular interaction often provides not just one, but several binding curves from which the affinity constant is obtained.
Sometimes, an individual fit of these curves yield a set of binding constants that are significantly different from them; this result can be very confusing because, in principle, these binding curves are a representation of the same binding event and should converge to provide the analogous information. Often, the explanation for this behaviour is that the different curves indeed provide only partial and/or different information of the interaction, not enough to unequivocally determine the binding affinity through individual analysis.

“It´s like feeling only a separate part of the elephant”

This is a typical scenario when facing the study of complex binding events that involve more than one equilibrium and several binding curves are obtained, i.e. from different frequencies of the spectra in a titration experiment, from data registered using different techniques (ITC, NMR, Optical Spectroscopies…) and/or from experiments performed at different concentrations of the species participating in the binding event.

Analogous to the parable of the six men and the elephant, the way to get a true understanding of the binding event consist of the global analysis of the different curves.

Fig 2: The binding curve obtained from 2D NMR titrations.

Being aware of the relevance of global analysis, in AFFINImeter we count with the possibility to perform Global fitting of multiple data to tailored binding models where one or more fitting parameters are shared between isotherms. The number and identity of the parameters shared are selected by the user.
Moreover, two or more parameters can be related through mathematical relationships designed by the user. All these features make our global fitting tool the most potent among others to perform a robust analysis of binding data of complex interactions.

About the disuses of  Isothermal Titration Calorimetry in drug discovery research

Isothermal Titration Calorimetry (ITC) is the gold standard for the calculation of affinity in molecular interactions. Many times, researchers claim that the high consumption of sample does not offset the use of ITC for Kd calculation.
Conversely, ITC hides many surprises in the acquisition data that can provide more information in a single experiment that other techniques that are more expensive and more complicated to use.

Download the PDF file of Implementation of kinITC into AFFINImeter


1. ITC collects data from the interaction as a function of time that can be analyzed to obtain kinetic information (kon and koff values). It can cover a very similar range as Surface Plasmon Resonance in a “label-free” and “in-solution” manner (Fig 1).

2. ITC can also provide valuable information about the mechanism of interaction. The high sensitivity of the ITC sensor makes it sensitive to more intriguing interactions as conformational changes, cooperativity…

Using a global fitting approach for the analysis of the isotherms and a model builder to create tailored binding models, the different mechanisms of interaction can be confirmed and characterized.

Find attached a couple of publications describing the application of this new method for ITC data analysis:

Download the PDF file of Implementation of kinITC into AFFINImeter


AFFINImeter binding models for Nuclear Magnetic Resonance

AFFINImeter is already well known in ITC binding data analysis for providing the possibility to use tailored binding models created by the user. The models are generated with the tool “model builder” that includes a letter code “M-A-B” to describe titrate (M), titrant (A) and if necessary, the presence of a third species (B) (Figure 1).


Fig.1 Example of a competitive binding model created in AFFINImeter where the titrant in syringe “A” binds to the titrate in cell “M” to form a 1:1 complex “MA” and a second ligand “B” mixed in the cell with “M” forms the complex “MB” and thus competes with “A”.


Following the same approach, the binding models available for the software AFFINImeter for Nuclear Magnetic Resonance are generated with the model builder and based on the “MAB” code. But there are significant differences between ITC and NMR data analysis when the time comes to select a binding model from AFFINImeter, which have an origin in the inherent characteristics of each technique and in the different experimental design. In chemical shift perturbation (CSP) NMR titration experiments, the observed parameter used to monitor the progress of the binding event is the chemical shift of titrate resonance signals. Hence, the models used for NMR data analysis require the presence of compound “M” (titrate) as it is the species from which changes associated with the binding process are monitored. Conversely, in ITC the observed parameter is the heat change upon interaction and this parameter is not necessarily linked to a particular species “M”, “A” or “B”.

An illustrative example is the evaluation of a monomer-dimer self-association process using NMR or ITC. In NMR, the standard experimental setup would consist in the incremental dilution of the compound sample at high concentration in the NMR tube, to monitor dimer dissociation (Figure 2a). In ITC the standard experimental setup would consist in a titration of the compound sample at high concentration in the syringe (species “A” according to the AFFINImeter code) into the calorimetric cell filled up with solvent (Figure 2b).


Fig.2 Representation of experimental setup for a) NMR dilution experiment and b) ITC dilution experiments. The corresponding schemes of AFFINImeter binding models for data analysis are shown.



Would you like to know more about AFFINImeter for Nuclear Magnetic Resonance? Press the button below:



The Model Builder is a versatile tool to translate binding interactions into mathematical models

The Model Builder is one of the novel features of AFFINImeter.  Through the model builder AFFINImeter offers an unlimited amount of thermodynamic models for ITC data analysis. The overall binding equilibria within the species involved in the experiments is easily drawn by the user directly in chemical language. Then AFFINImeter translates the resulting reaction scheme into robust  binding  models to be used to isotherm ITC simulation or to perform Isothermal Titration Calorimetry curve fitting.

The model builder is a versatile tool, it allows to design models involving up to three different species (i.e. the case of two ligands that compete with each other to bind a macromolecule) and has the advantage to selectively place them in the  syringe cell and/or in the calorimetric cell. 

Binding Reaction Scheme of a competitive interaction
Reaction Scheme of a Competitive Binding Interaction

It also allows the design of models for dissociation, ranging from simple homodimers to higher-order oligomers. During the model construction no mathematical equations are required, once the whole set of binding interactions is defined by the user in the reaction builder, AFFINImeter internally generates the system of equations that define the reaction scheme proposed. The new model (reaction scheme and equations) is saved  internally by AFFINImeter and listed in the user’s database so that can be utilized anytime so simulate or fit data.

AFFINImeter Reaction Builder
Competitive binding model builded with the AFFINImeter tool

AFFINImeter-ITC offers an exclusive unlimited amount of personalized model families including

  • Unrestricted Competitive Sequential Binding with no limitation in the stoichiometry of the binding model.
  • Competitive Multiple and Independent Sets of Identical and Independent Sites.
  • Dissociation of any chemical species including homogeneous n-mers, heterogeneous complexes and even micelles.

With this extensive offer of model families the user will be able to perform the thermodynamic characterization of a vast variety of biological and physicochemical processes from ITC measurements. A few examples of classical and new applications of ITC experiments that you can analyze with AFFINImeter are:

  • Protein-ligand or host-guest complex formation with unlimited stoichiometries
  • Competition of different molecules to occupy a given binding site even for high order complexes
  • Binding between ligands and polymers or large macromolecules with any number of (independent) sets and/or sites
  • Dissociation/aggregation of supramolecular heterogeneous species including protein oligomers
  • Structural and thermodynamic information of large aggregates, including micelles: aggregation number, enthalpy of formation, Gibbs energy and dilution heat of monomers and aggregates