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.
-Which of the following reaction coordinate diagrams best represents the two-step reaction involving tyrosine to nitrotyrosine in a lipid environment?

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.
-Which of the following reaction coordinate diagrams best represents the two-step reaction involving tyrosine to nitrotyrosine in a lipid environment?

Quizplus · Accepted Answer

11ecba2d_1199_ceb9_a3bf_61acff06c322_MD0008_00 The reaction coordinate diagram shows how the energy of a system changes during a chemical reaction. The graph consists of free energy on the y-axis and the reaction progress on the x-axis, with reactants on the left and products on the right. It illustrates both the Gibbs free energy and the activation energy of each reaction depicted. The difference in energy between the starting and ending points is the change in Gibbs free energy, ΔG. If the reaction's overall ΔG is negative, the products will have a lower Gibbs free energy than the reactants and the reaction is spontaneous (exergonic). If the reaction's overall ΔG is positive, the products will have a higher Gibbs free energy than the reactants and the reaction is nonspontaneous (endergonic). The activation energy of a reaction is the difference in energy between the reactants (or intermediates) and the transition state, illustrated as the maxima on the curve. A multistep reaction contains the same number of peaks as there are steps. Based on the passage, the nitration of tyrosine in a lipid environment is a two-step reaction. The first step has a ΔG of −91.8 kJ/mol with a large activation energy, and the second step has a ΔG of −14.1 kJ/mol with a smaller activation energy. These features must all be reflected in the reaction diagram. (Choice A) The Gibbs free energy ΔG of step 1 is shown as positive but should be negative, according to Table 1. (Choice B) The activation energy for step 2 is shown as larger than the activation energy for step 1 but should be smaller, according to Figure 1. (Choice C) The Gibbs free energy ΔG for step 2 is positive but should be negative, according to Table 1. Educational objective: The reaction coordinate diagram is a representation of the change in energy of a system during a chemical reaction. The relative energies of the reactants, transition states, intermediates, and products are illustrated as their respective heights on the diagram.

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