Samples were adjusted to a pH or pD range 7.4 ± 0.3, unless otherwise specified. Sample pH was determined using either a WPA CD720 meter and a CMW711 semi-micro single junction probe, or a Sartorius Docu-pH meter and a VWR symphony semi-micro combination double junction probe (part 14231-178). Microlitre aliquots of typically 0.1 or 1.0 M HCl or NaOH were added to solutions to alter their pH (for deuterated solutions, DCl or NaOD were used). After each addition of acid or base, the solution was briefly vortexed, and then its pH or pD rechecked .