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Depending on the active compound's specific method of action, sunscreens are broadly classified into two groups: organic and inorganic. Organic sunscreens typically contain aromatic compounds that absorb ultraviolet (UV) radiation. The absorbed energy from the UV radiation causes these compounds to enter electronically excited states which then relax back to the ground state and harmlessly re-emit the absorbed radiation as heat. Alternatively, inorganic sunscreens contain metallic nanoparticles that serve to reflect, scatter, and absorb incident radiation. The mechanism for UV protection of each group is shown in Figure 1.
Figure 1 Method of action for organic and inorganic sunscreensThe effectiveness of sunscreens is traditionally evaluated based on their ability to prevent reddening of the skin, as indicated by their sun protection factor (SPF) . However, studies have shown that additional deleterious effects, such as the suppression of T cell-mediated immune responses, can occur at lower UV doses than erythema, indicating the need for new metrics to judge sunscreen efficacy. For this reason, researchers conducted a study to compare the protective ability of six sunscreens against photo-isomerization of the photoreceptor trans-urocanic acid (UCA) to its cis form, a process believed to be linked to UV-induced immunosuppression. Table 1 lists the sunscreens tested and their relevant characteristics.Table 1 Results from Tested Sunscreens
In vivo experiments were conducted on two groups of human volunteers. Participants in the acute exposure group received a single UV dose of 100 mJ/cm² whereas participants in the chronic exposure group received the same dosage once daily over four consecutive days. Each sunscreen was applied to a different spot on the volunteers' back prior to irradiation. A portion of skin without sunscreen was also irradiated to serve as a control. After irradiation, the sunscreens were removed and the skin was wiped with filter papers. Filters were washed with potassium hydroxide, and the aqueous solution was analyzed for UCA using high-pressure liquid chromatography. Figure 2 shows the mean percentages of cis-UCA formed upon irradiation for each exposure type and sunscreen tested.
Figure 2 Percentage of UCA in cis configuration upon UV exposure with various sunscreens
Adapted from Van der molen RG, Out-luiting C, Driller H, Claas FH, Koerten HK, Mommaas AM. Broad-spectrum sunscreens offer protection against urocanic acid photoisomerization by artificial ultraviolet radiation in human skin. J Invest Dermatol. 2000;115(3) :421-6.
-Which of the following is true concerning the energy difference ΔE between the ground state and the photo-excited state that occurs most often for the active compound in sunscreen B10?

Question

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Depending on the active compound's specific method of action, sunscreens are broadly classified into two groups: organic and inorganic. Organic sunscreens typically contain aromatic compounds that absorb ultraviolet (UV) radiation. The absorbed energy from the UV radiation causes these compounds to enter electronically excited states which then relax back to the ground state and harmlessly re-emit the absorbed radiation as heat. Alternatively, inorganic sunscreens contain metallic nanoparticles that serve to reflect, scatter, and absorb incident radiation. The mechanism for UV protection of each group is shown in Figure 1.

Figure 1 Method of action for organic and inorganic sunscreensThe effectiveness of sunscreens is traditionally evaluated based on their ability to prevent reddening of the skin, as indicated by their sun protection factor (SPF) . However, studies have shown that additional deleterious effects, such as the suppression of T cell-mediated immune responses, can occur at lower UV doses than erythema, indicating the need for new metrics to judge sunscreen efficacy. For this reason, researchers conducted a study to compare the protective ability of six sunscreens against photo-isomerization of the photoreceptor trans-urocanic acid (UCA) to its cis form, a process believed to be linked to UV-induced immunosuppression. Table 1 lists the sunscreens tested and their relevant characteristics.Table 1 Results from Tested Sunscreens

In vivo experiments were conducted on two groups of human volunteers. Participants in the acute exposure group received a single UV dose of 100 mJ/cm² whereas participants in the chronic exposure group received the same dosage once daily over four consecutive days. Each sunscreen was applied to a different spot on the volunteers' back prior to irradiation. A portion of skin without sunscreen was also irradiated to serve as a control. After irradiation, the sunscreens were removed and the skin was wiped with filter papers. Filters were washed with potassium hydroxide, and the aqueous solution was analyzed for UCA using high-pressure liquid chromatography. Figure 2 shows the mean percentages of cis-UCA formed upon irradiation for each exposure type and sunscreen tested.

Figure 2 Percentage of UCA in cis configuration upon UV exposure with various sunscreens
Adapted from Van der molen RG, Out-luiting C, Driller H, Claas FH, Koerten HK, Mommaas AM. Broad-spectrum sunscreens offer protection against urocanic acid photoisomerization by artificial ultraviolet radiation in human skin. J Invest Dermatol. 2000;115(3) :421-6.
-Which of the following is true concerning the energy difference ΔE between the ground state and the photo-excited state that occurs most often for the active compound in sunscreen B10?

Quizplus · Accepted Answer

11ecba2d_1198_21fd_a3bf_3934b26f5b33_MD0008_00 The energy of a photon E_photon is determined according to 11ecba2d_1198_21fe_a3bf_2332cdffed62_MD0008_00 where h is Planck's constant, c is the speed of light, and λ is the wavelength of the photon. Table 1 shows that the peak protection of sunscreen B10 is at 297 nm, meaning the active compound most commonly absorbs photons of that wavelength. Because an atom or molecule can only absorb energy exactly equal to the difference between two energy levels, the difference in energy ΔE between the ground state and the most commonly occurring photo-excited state for the active compound in sunscreen B10 is 11ecba2d_1198_21ff_a3bf_e1ca686ddf8b_MD0008_00 (Choice A) The energy of a photon is inversely proportional to its wavelength, so hc must be divided, not multiplied, by 297 nm. (Choice B) Subtracting the energy of a 300-nm photon from that of a 250-nm photon would give the energy difference between the photons (requires that hc is divided, not multiplied, by the wavelength difference), but this is not the energy difference between the ground and excited states of a molecule. (Choice C) Although B10 can absorb photons with wavelengths between 250 nm and 300 nm, each absorption event occurs for only one photon at a time. The molecule would have to simultaneously absorb a photon at 250 nm and a photon at 300 nm for this answer to be correct. Educational objective: The energy of a photon is given by 11ecba2d_1198_2200_a3bf_af85f1b3a2f4_MD0008_00 For a molecule or atom to transition from ground to an excited state, it must absorb a single photon with energy exactly equal to the energy difference between the two states.

Passage Depending on the Active Compound's Specific Method of Action, Sunscreens