Passage
In polluted urban environments, airborne proteins can undergo nitration reactions that increase their allergenic potential toward the human respiratory system. For example, upon exposure to ozone (O₃) and nitrogen dioxide (NO₂) under atmospheric conditions, the amino acid tyrosine is known to undergo a two-step irreversible reaction leading to the formation of nitrotyrosine. The first step involves the formation of a reactive oxygen intermediate (ROI) tyrosyl radical through oxidation of the phenolic site of tyrosine. Nitrotyrosine is then formed in the second step through addition of NO₂ by a radical-radical termination reaction. The elementary steps for this reaction are shown in Scheme 1.
Scheme 1 Elementary steps of nitration of tyrosineAirborne protein particles can exist in a variety of environments including the aqueous phase, gaseous phase, or as conglomerates with other biomolecules such as lipids. Because the thermodynamic properties and kinetics of the nitration reaction vary based on the chemical environment in which it occurs, recent studies have focused on understanding the reaction under different conditions.Density functional theory is a powerful computational quantum-mechanical modeling method that can determine the properties of many-electron systems by producing approximate solutions to the Schrödinger equation. Researchers used density functional theory to model the geometries and potential energy surfaces of the reactants, intermediates, and products of the nitration reaction in gaseous, aqueous, and lipid-rich environments. From this modeling, the thermodynamic and kinetic properties were calculated. Figure 1 shows the calculated activation energies E_a in kJ/mol, and Table 1 lists the associated thermochemical properties for both steps of the reaction in all three environments.
Figure 1 Calculated activation energies for each step of the studied reactions in three environmentsTable 1 Calculated Thermochemical Properties of the Studied Reactions (298.15 K)
Adapted from Sandhiya L, Kolandaivel P, Senthilkumar K. Oxidation and nitration of tyrosine by ozone and nitrogen dioxide: reaction mechanisms and biological and atmospheric implications. J Phys Chem B. 2014;118(13) :3479-90.
-Suppose a chemist decides to conduct and monitor the reaction for 10 min at 298.15 K in the three environments described in the passage. Which of the three reactions is most likely to form the greatest amount of product?

Question

Passage
In polluted urban environments, airborne proteins can undergo nitration reactions that increase their allergenic potential toward the human respiratory system. For example, upon exposure to ozone (O₃) and nitrogen dioxide (NO₂) under atmospheric conditions, the amino acid tyrosine is known to undergo a two-step irreversible reaction leading to the formation of nitrotyrosine. The first step involves the formation of a reactive oxygen intermediate (ROI) tyrosyl radical through oxidation of the phenolic site of tyrosine. Nitrotyrosine is then formed in the second step through addition of NO₂ by a radical-radical termination reaction. The elementary steps for this reaction are shown in Scheme 1.

Scheme 1 Elementary steps of nitration of tyrosineAirborne protein particles can exist in a variety of environments including the aqueous phase, gaseous phase, or as conglomerates with other biomolecules such as lipids. Because the thermodynamic properties and kinetics of the nitration reaction vary based on the chemical environment in which it occurs, recent studies have focused on understanding the reaction under different conditions.Density functional theory is a powerful computational quantum-mechanical modeling method that can determine the properties of many-electron systems by producing approximate solutions to the Schrödinger equation. Researchers used density functional theory to model the geometries and potential energy surfaces of the reactants, intermediates, and products of the nitration reaction in gaseous, aqueous, and lipid-rich environments. From this modeling, the thermodynamic and kinetic properties were calculated. Figure 1 shows the calculated activation energies E_a in kJ/mol, and Table 1 lists the associated thermochemical properties for both steps of the reaction in all three environments.

Figure 1 Calculated activation energies for each step of the studied reactions in three environmentsTable 1 Calculated Thermochemical Properties of the Studied Reactions (298.15 K)

Adapted from Sandhiya L, Kolandaivel P, Senthilkumar K. Oxidation and nitration of tyrosine by ozone and nitrogen dioxide: reaction mechanisms and biological and atmospheric implications. J Phys Chem B. 2014;118(13) :3479-90.
-Suppose a chemist decides to conduct and monitor the reaction for 10 min at 298.15 K in the three environments described in the passage. Which of the three reactions is most likely to form the greatest amount of product?

Quizplus · Accepted Answer

11ecba2d_119b_2e4b_a3bf_b3900829eeeb_MD0008_00 A reaction can be under either kinetic or thermodynamic control. When the conditions are such that a reaction is irreversible (eg, low temperature), it is said to be under kinetic control, and the most favorable product is the one whose pathway has the lowest activation energy. Unless the reactions have had time to complete, the fastest reaction will yield the most product. When a reaction occurs under conditions such that it is reversible (eg, high temperatures), it is said to be under thermodynamic control. Under these conditions, all products form readily and the most stable product will be favored (ie, the most negative ΔG), as it is least likely to undergo the reverse reaction. According to the passage, the reactions are all irreversible at 298.15 K, and therefore are under kinetic control. As a result, the reaction in aqueous solution will likely produce the greatest amount of product over 10 min because it has the lowest activation energy and will proceed the fastest. (Choice A) Although the product is most stable for the reaction in a lipid environment, this attribute is irrelevant for reactions under kinetic control as the reverse reaction is unfeasible. Furthermore, this reaction has a higher activation energy than the same reaction in an aqueous environment. (Choice C) The reaction pathway in a gaseous environment has a very large activation energy and is less favorable under kinetic control. (Choice D) Because the passage states the reactions are under irreversible conditions, it can be assumed they are under kinetic control. Knowledge of the activation energies for the three reactions is sufficient to determine the most favorable product. Educational objective: Irreversible reactions tend to be under kinetic control, which favors products whose pathways have the lowest activation energy. Reversible reactions tend to be under thermodynamic control, which favors the most stable product.

Passage In Polluted Urban Environments, Airborne Proteins Can Undergo Nitration Reactions