Co-crystallization of protein/ligands complexes is frequently problematic, even on established biological systems. Much time, efforts and samples can be spared using preliminary biophysical methods in order to asses a correct formation of the complex in conditions that are optimal for co-crystallization. Among these approaches, Isothermal Titration Calorimetry (ITC) is well-suited for analysis of nucleic acids interactions. A major advantage of ITC over other equivalent biophysical approaches is that no labeling of the sample is needed. In addition, ITC is not subjected to molecular weight limitations and can be used virtually in any buffers. As a consequence, it can be directly used for crystallization experiments, allowing the formation of the complex in a more controlled way and with the additional advantage of providing thermodynamic and binding information ‘for free’.
Here we will show through various examples how to implement ITC as a guide to improve co-crystallization of protein-ligand complexes.