Figure 1. Ferric ions enhance CCK₈PO₄ absorbance.

At pH 4.0 (A,C), addition of aliquots of ferric chloride to 40 μM CCK₈PO₄ (▲) in 10 mM Na⁺ acetate, 100 mM NaCl, 0.005% Tween 20, 0.4% DMSO, 298 K resulted in an increase in the absorption at 275 nm up to a molar ratio of 1.8. Addition of aliquots of FeCl₃ to 40 μM CCK₄ (■), CCK₈ (▼), or CCK₈SO₄ (◆) did not cause any change in absorption. At pH 6.5 (B,D) in 10 mM Na⁺ PIPES, 100mM NaCl, 0.005% Tween 20, 0.4% DMSO, an increase in absorption at 275 nm was observed on addition of ferric chloride to CCK₈, CCK₈SO₄, or CCK₈PO₄, but not CCK₄. Data are expressed as a percentage of the absorbance of that peptide without ferric ions. Points are means of at least three separate experiments; bars represent the SEM. Lines represent the best fit to one (CCK₈) or two (CCK₈SO₄, CCK₈PO₄) site models with the program BioEqs; the appropriate K_d values are given in Table 1.