What do you mean by "decent binding results"? The fact that you get a line passing through the points does not mean the fit is fine. You must pay attention to the numbers.

From the images we can hardly see anything.

Now I can see the titrations much better...

The heats are very small and the signal (thermal power) in the thermogram is very low. This could lead to misleading conclusions. And now I can say that the results from the fitting are arguable, even though the fitting lines pass through the points (and just "through" the points, not "accompanying" the points).

When you apply the "one set of sites" model you get a Kd of 1 pM, which is quite high, and when you apply the "sequential model" you get two Kd's, one is 14 nM and the other 0.8 nM, which are still quite high and might indicate a large positive binding cooperativity. However, the data are not reliable enough to trust those estimations.

I would try other conditions (temperature, buffer, pH, ionic strength...) to see if the interaction shows a larger enthalpic contribution (either more exothermic or endothermic) and the fitting can be done in a better way.

Thanks Adrian for your valuable input.

I will try out your suggestions.

Yes I agree with Adrian - the heats are very small per injection, making it difficult to do the fitting. If your ITC has a 200 uL cell I recommend having at least 2.5 micro calories of heat change for for the 2nd injection (this is the integrated area, not the height of the peak). at a minimum 1 ucal for the 2nd injection. Try to change conditions like temperature and buffer if you cannot inecrease concentrations.

There is also not much difference in the heat changes at the beginning and the end of the titration. What does the control titration look like (ligand in syringe titrated into buffer)

Looking more closely at the heat changes for each injection, there is an initial endothermic heat change, followed by a small exothermic heat change. For the 3rd and 4th injections, the exothermic "peak" is bigger. With such small heat changes I do not want to over-interpret the results, do you see this with the control titration? The integration process will take both the endo- and exothermic heat changes and add them.

How many binding sites are you expecting? If your ligand concentration in the iTC syringe is too high, relative to the concentration in the cell, all of the binding will occur in the 1st few injections, and you may not have enough data points to fit, especially of the Kd is small, or there is more than one binding event.