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
-Which of the following experimental changes will decrease the percent recovery of oxytocin in Experiment 1?

Question

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
-Which of the following experimental changes will decrease the percent recovery of oxytocin in Experiment 1?

Quizplus · Accepted Answer

11ecba2d_1244_194a_a3bf_83b242f0c39d_MD0008_00 A buffered solution consists of a mixture of a weak acid (HA) and a salt of its conjugate base (A⁻) or a mixture of a weak base and its conjugate acid. Buffered solutions resist changes in pH by neutralizing H⁺ or OH⁻ ions when small amounts of a strong acid or strong base are added. The equilibrium expression for the dissociation of a weak acid is: 11ecba2d_1244_194b_a3bf_5f30880b8acd_MD0008_00 where K_a is the acid dissociation constant and [H⁺], [A⁻], and [HA] are the molar concentrations of the chemical species at equilibrium. Rearranging this expression to solve for [H⁺] and taking the negative logarithm of both sides of the equation yields the relationship between pH, [HA], and [A⁻]: 11ecba2d_1244_194c_a3bf_e9eb75ae91d2_MD0008_00 11ecba2d_1244_405d_a3bf_9baec9577270_MD0008_00 11ecba2d_1244_405e_a3bf_616dacac88ad_MD0008_00 Accordingly, the pH of a buffered solution depends on the concentration ratio of acid to conjugate base. Adding more acid will decrease the pH and adding more base will increase the pH. In Experiment 1, the citrate buffer is made of citric acid [HA] and sodium citrate [A⁻]. Adding more sodium citrate will increase the pH of the solution. The passage states that oxytocin is more stable at a pH of 4.5, and Figure 1 shows that at the higher pH of 6.8 (water), less oxytocin was recovered. (Choice B) The experiment measures the percent recovery of oxytocin. Decreasing the initial amount of oxytocin in solution will lower the amount present in solution but will not change the percentage recovered. (Choice C) Cooling the samples during the experiment will not decrease the percent recovery because the results in Experiments 1 and 2 show that oxytocin is more stable at cooler temperatures. (Choice D) Increasing the buffer concentration will increase buffer capacity to resist changes in pH. Based on the results in Experiment 1, this will not decrease oxytocin recovery. Educational objective: A buffered solution consists of a weak acid and a salt of its conjugate base or a weak base and its conjugate acid. Buffers resist changes in pH by neutralizing added H⁺ and OH⁻ ions. The pH of a buffered solution depends on the ratio of weak acid and its conjugate base.

Passage Oxytocin Is a Naturally Occurring Peptide Hormone That Is Used