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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.
-Because step 2 of the reaction in aqueous phase is rate-limiting, the formation of ROI from tyrosine and ozone in step 1 can be assumed to reach a state of equilibrium. Tyrosine(aq) + O3(aq) ⇌ ROI(aq) + HO3(aq) Which of the following will occur if the temperature of this system is raised from 298.15 K to 350 K?

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.
-Because step 2 of the reaction in aqueous phase is rate-limiting, the formation of ROI from tyrosine and ozone in step 1 can be assumed to reach a state of equilibrium.

Tyrosine(aq) + O3(aq) ⇌ ROI(aq) + HO3(aq) Which of the following will occur if the temperature of this system is raised from 298.15 K to 350 K?

Quizplus · Accepted Answer

11ecba2d_119d_2a22_a3bf_13cca45bd41b_MD0008_00 Equilibrium is reached in a reaction when the rate at which products are formed is equal to the rate at which reactants are formed. Per Le Châtelier principle, if a system in equilibrium is disturbed by a change in temperature, pressure, or a component's concentration, the system will counteract the disturbance by driving the reaction in a given direction such that equilibrium is restored. Changes in pressure and concentration achieve this without altering the equilibrium constant K_eq, whereas a change in temperature leads to a change in K_eq. The effect of temperature on an equilibrium reaction can be illustrated by treating heat as a chemical reagent and applying Le Châtelier principle. Step 1 of the nitration of tyrosine in an aqueous environment is an exothermic reaction, and heat can be viewed as a product of the reaction. Raising the temperature causes a shift toward the reactants as heat is consumed to rebalance the equilibrium reaction. The shift toward reactants leads to an increase in reactant concentration, and therefore a decrease in K_eq. (Choices A, C and D) Because the reaction is exothermic, raising the temperature causes the reaction to be driven toward the reactants, increasing their concentration and causing a decrease in K_eq rather than an increase or remaining the same. Educational objective: Equilibrium is reached in a reaction when the rate at which products are formed is equal to the rate at which reactants are formed. The equilibrium constant K_eq is defined as the ratio of the reactants and products when a reaction is in equilibrium, and only changes in temperature can alter K_eq. Le Châtelier principle states that a system responds to disturbances in equilibrium by shifting the reaction to counter the effect of the disturbance.

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