INTERCHARGE DISTANCE OF FLEXIBLE ZWITTERIONIC MOLECULES IN SOLUTION

The conformation of flexible zwitterionic molecules in solutions of polar solvents is studied by means of a NMR chemical shift method. This method makes use of chemical shift changes of NMR lines, induced by the electrostatic field caused by the electrical charges of the zwitterion. The electrostatic field at the observed nucleus is calculated, taking into account the presence of a reaction field due to the polarization of solvent molecules around the solute ones. The intercharge distances of zwitterionic (trimethylammonio)alkanoates in aqueous and methanolic solutions are then deduced from 13C NMR chemical shift data as a function of the number of methylenes linking the cationic and anionic groups of the zwitterion. The comparison of experimental data with end to end distances predicted by the rotational isomerism state theory shows that the polymethylene chain has a more folded conformation than a free chain. Electrostatic attraction between the two zwitterion charges is thus important, although it could be expected that it could be offset by steric hindrance, i.e., by the bulkiness of terminal charged groups. © 1990 American Chemical Society.