Figure : Titrations of tripeptide amino acids (Glu, Asp, His) in water. The top and bottom rows show titrations performed with the modified AA CHARMM36 and CG Martini 33 force fields, respectively. In all simulations neutrality was maintained by including ten buffer particles in combination with the charge constraint. Dots show the fraction of frames in which the residue is deprotonated, and the dashed lines represent the fits to the Henderson-Hasselbalch equation. For His, blue color represents the macroscopic pK a , while yellow and red represent the microscopic pK a values for HSD (proton on N δ ) and HSE (proton on N ), respectively. In the Martini 2.0 model, HSD and HSE are indistinguishable and hence only the macroscopic titration curve is shown. From the fits the pK a values were estimated and listed in Table .