The Effect of Using DMSO as a Cosolvent for Ligand Binding Studies

Sammanfattning: DMSO sees common application as a solvent for studies of synthetic molecules in biological systems, in fields such as drug design. Despite this, the possibility and nature of solvent interactions with both synthetic ligand and their target macromolecules is widely unexamined. DMSO could potentially have significant effect on binding characteristics due to conformational changes of protein structure. In this thesis, I have studied the effects of DMSO on the binding of a synthetic ligand to the carbohydrate recognition domain of galectin-3C using isothermal titration calorimetry (ITC) and NMR relaxation dispersion experiments. ITC reports on the thermodynamics of binding, whereas NMR relaxation dispersion reports on the kinetics of binding. ITC results displayed an increase in calculated KD from 5.588 μM to 13.02 μM with the addition of 10 % (v/v) DMSO, as well as an increase in free energy from −30.28 kJ/mol to −28.1665 kJ/mol. NMR relaxation dispersion studies showed that the chemical exchange rate of binding site residues decreased as well. Kinetic on- and off-rates were determined from the chemical exchange rate. Notably the on-rate was affected by DMSO ~10% more than the off-rate, indicating that the reduced binding affinity is primarily caused by a reduced frequency of protein-ligand encounters.