NMR chemical shift references for binding constant determination in aqueous solutions

For deuterium oxide solutions of various compounds including cyclic and acyclic saccharides, oligoglycines, and sodium chloride., the proton chemical shifts of these solutes and the tetramethylammonium ion (TMA), referred to an external standard, change linearly with the molarity of solute. This slope is almost independent of the kind of protons of solute and TMA and is proportional to the product of the molar volume of the solute and the difference in volume magnetic susceptibility between deuterium oxide and the solute. The equilibrium formation constant and structure of ion-pair of TMA and the benzenesulfonate ion (BS) are evaluated from chemical shifts referred to an internal standard. At least some of the four methyl groups of TMA in the ion-pair complex are located above the benzene ring of BS. This structure indicates that TMA and BS form their ion pair by hydrophobic and electrostatic attraction. These results serve not only for choosing appropriate internal references and internal or external standard methods for chemical shift measurements in aqueous solutions but also for correcting the chemical shift referred to the external standard.