Tripeptides were dissolved in sodium phosphate (0.1M, pH 11.8) at double the desired concentration, and then the resulting solutions were diluted with an equal volume of mildly acidic sodium phosphate (0.1M, pH 5.6) to reach a final pH 7.0 ± 0.1 and a final concentration of 25 mM for each tripeptide, unless otherwise stated in the text.