Neighbor Group Hydration Effects on Carboxylate Basicities in Partly Aqueous Solutions

Charge correlation effects on ionization of weak polyelectrolytes

Ionization curves of weak polyelectrolytes were obtained as a function of the charge coupling strength from Monte Carlo simulations. In contrast to many earlier studies, the present work treats counterions explicitly, thus allowing the investigation of charge correlation effects at strong couplings. For conditions representing typical weak polyelectrolytes in water near room temperature, ionization is suppressed because of interactions between nearby dissociated groups, as also seen in prior work. A novel finding here is that, for stronger couplings, relevant for non-aqueous environments in the absence of added salt, the opposite behavior is observed-ionization is enhanced relative to the behavior of the isolated groups due to ion-counterion correlation effects. The fraction of dissociated groups as a function of position along the chain also behaves non-monotonically. Dissociation is highest near the ends of the chains for aqueous polyelectrolytes and highest at the chain middle segments for non-aqueous environments. At intermediate coupling strengths, dissociable groups appear to behave in a nearly ideal fashion, even though chain dimensions still show strong expansion effects due to ionization. These findings provide physical insights on the impact of competition between acid/base chemical equilibrium and electrostatic attractions in ionizable systems.

Complete resolution of the microscopic protonation equilibria of N-methyl-d-aspartic acid and related compounds

Protonation equilibria of N-methyl-D-aspartate (NMDA, a specific glutamate receptor agonist) and its derivatives are characterized at the macroscopic and microscopic levels. (1)H NMR-pH and pH-potentiometric titrations were carried out to determine the macroconstants. Microconstants were obtained by appropriate combination of acidity and NMR parameters of the parent compound and its three synthetic derivatives. These derivatives were close models of the NMDA minor microspecies, allowing the calculation of all the 12 microconstants, the 8 microspecies concentrations and 3 site interactivity parameters. Reliability of the microconstants was assessed by three independent test methods. It was found that protonation of the secondary amino site decreases the beta- and alpha-carboxylate basicities almost exactly by one and two orders of magnitude, respectively, whereas protonation of one of the carboxylates lessens the basicity of the other one by a factor of 3. NMR-pH profiles, macro- and microscopic protonation schemes and species-specific distribution diagrams are presented.