Passage Oxytocin Is a Naturally Occurring Peptide Hormone That Is Used

Passage
Oxytocin is a naturally occurring peptide hormone that is used clinically to prevent postpartum hemorrhage, one of the leading causes of maternal mortality globally.  Because oxytocin is heat and light sensitive, it requires refrigeration at temperatures below 8 °C.  However, in underdeveloped regions with hot climates (such as sub-Saharan Africa) , refrigeration is often unavailable for pharmaceutical storage and transport.In an effort to improve the availability of oxytocin in regions with limited refrigeration, researchers conducted two experiments to investigate the use of buffers and divalent alkaline-earth metal salt additives as a means to produce an aqueous oxytocin formulation that is stable when stored at temperatures up to 50 °C.Experiment 1To evaluate effects from buffer concentration, two sets of solutions were prepared containing 0.3 mg/mL oxytocin monoacetate salt in 5, 10, and 50 mM citrate buffer solution made from a mixture of citric acid (a weak acid) and sodium citrate (a salt of the conjugate base) .  An unbuffered aqueous solution served as a control sample.  Using 1.0 M NaOH, the buffered solutions were adjusted to pH 4.5 (the pH at which oxytocin is more stable) and then stored at 5 °C (set 1) and 50 °C (set 2) for 4 weeks in a dark area.  After storage, the amount of undegraded oxytocin in each sample was determined (Figure 1) using reverse-phase, high-performance liquid chromatography (RP-HPLC) .
Figure 1  Oxytocin recovery in a citrate buffer (pH 4.5) and water (pH 6.8) Experiment 2To examine the effect of divalent alkaline-earth metal cation additives, Experiment 1 was repeated, but the solutions were modified to contain 0.3 mg/mL of oxytocin with 5, 10, or 50 mM of a divalent cation (from a chloride salt) dissolved in 10 mM citrate buffer.  The results are shown in Figure 2.
Figure 2  Oxytocin in 10 mM citrate buffer with added Ca²⁺, Mg²⁺, or Zn²⁺ salts
-Based on the passage, which of the following buffers could be used instead of the citrate buffer in Experiments 1 and 2?

A) Acetate (pK_a 4.7)
B) Phosphate (pK_a 6.5)
C) Tris (pK_a 8.1)
D) Carbonate (pK_a 9.7)

Correct Answer:

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