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.
-Given that entropy is negative (disorder decreases) in the reaction, which of the following statements is true regarding the formation of aqueous nitrotyrosine?

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.
-Given that entropy is negative (disorder decreases) in the reaction, which of the following statements is true regarding the formation of aqueous nitrotyrosine?

Quizplus · Accepted Answer

11ecba2d_119b_ca8c_a3bf_ad9073761b6a_MD0008_00 The change in Gibbs free energy ΔG of a process occurring at constant temperature and pressure is calculated from two state functions: the changes in enthalpy ΔH and entropy ΔS of a system. Gibbs free energy determines the process's spontaneity. A spontaneous process has a negative value for ΔG and is said to be thermodynamically favorable; a nonspontaneous process is the opposite. According to the equation ΔG = ΔH − TΔS the sign of the change in ΔS for a process determines whether the process becomes more or less thermodynamically favorable as the temperature increases. A negative ΔS causes ΔG to increase with temperature, whereas a positive ΔS causes ΔG to decrease with temperature. Table 1 shows that the changes in enthalpy and entropy for the reaction are both negative. Therefore, increasing the temperature leads to an increasingly greater (more positive) ΔG as shown by the above equation, making it less thermodynamically favorable at higher temperatures. (Choice A) The formation of nitrotyrosine is more thermodynamically favorable than the formation of ROI at 298.15 K because the ΔG = −85.9 kJ/mol for the formation of ROI (step 1), but ΔG = −85.9 + (−13.1) = −99.0 kJ/mol for the formation of nitrotyrosine (step 1 + step 2). (Choice B) The reaction becomes less thermodynamically favorable at higher temperatures because ΔS is negative for this reaction, causing an increase in ΔG as the temperature rises. High ΔG values correspond to thermodynamically unfavorable processes. (Choice C) The production of both ROI and nitrotyrosine are thermodynamically favorable at 298.15 K because they both have negative ΔG terms. Educational objective: The Gibbs free energy ΔG equation is used to determine the spontaneity of a system, where negative ΔG is spontaneous and positive ΔG is nonspontaneous. The sign of the change in entropy ΔS for a process determines whether it becomes more or less thermodynamically favorable (ie, spontaneous) as the temperature changes.

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