Passage
Heme is found in several proteins, including hemoglobin in blood and myoglobin in muscle. It has a four-coordinate iron-bound porphyrin structure where ferrous iron (Fe²⁺) is bound to four nitrogen atoms through coordinate covalent bonds. The most common heme found in the body is heme b in hemoglobin (Figure 1) .
Figure 1 Structure of heme b found in hemoglobinAlong with the four nitrogen ligands in the porphyrin structure, His F8 also binds to Fe²⁺ in the axial position, leaving the other axial position available to bind oxygen. In this setting, iron's atomic d orbitals are no longer degenerate. The energetic difference between the d orbitals depends on the ligands.In deoxygenated heme, iron sits out of the plane formed by the nitrogen ligands in the porphyrin because its d orbitals make it slightly too large to fit in the porphyrin ring. However, when O₂ binds to heme, iron's electrons re-arrange themselves. The d orbitals become smaller, and iron sits within the plane of the nitrogen atoms.When iron is scarce in the body, Zn²⁺ will coordinate to the ligands in the protoporphyrin structure through a nonenzymatic process, forming Zn-protoporphyrin IX (ZPP) . ZPP and protoporphyrin IX (PPIX) have distinct emission spectra when excited in the blue range whereas heme does not. Hematofluorometry can be used as a simple, inexpensive indicator for iron deficiency in whole blood by placing a blood sample in a fluorometer and measuring the amount of ZPP relative to the amount of heme. Figure 2 illustrates the fluorescence emission spectra for ZPP and protoporphyrin IX measured from a blood sample when ZPP is excited at 425 nm and PPIX is excited at 407 nm.
Figure 2 ZPP and PPIX fluorescence emission spectra
Adapted from Hennig G, Gruber C, Vogeser M, et al. Dual-wavelength excitation for fluorescence-based quantification of zinc protoporphyrin IX and protoporphyrin IX in whole blood. J Biophotonics. 2014;7(7) :514-24.
-Comparing the peak excitation wavelengths stated in the passage and the highest-intensity fluorescence peaks in Figure 2, which of the following is true about ZPP detection in blood samples?

Question

Passage
Heme is found in several proteins, including hemoglobin in blood and myoglobin in muscle. It has a four-coordinate iron-bound porphyrin structure where ferrous iron (Fe²⁺) is bound to four nitrogen atoms through coordinate covalent bonds. The most common heme found in the body is heme b in hemoglobin (Figure 1) .

Figure 1 Structure of heme b found in hemoglobinAlong with the four nitrogen ligands in the porphyrin structure, His F8 also binds to Fe²⁺ in the axial position, leaving the other axial position available to bind oxygen. In this setting, iron's atomic d orbitals are no longer degenerate. The energetic difference between the d orbitals depends on the ligands.In deoxygenated heme, iron sits out of the plane formed by the nitrogen ligands in the porphyrin because its d orbitals make it slightly too large to fit in the porphyrin ring. However, when O₂ binds to heme, iron's electrons re-arrange themselves. The d orbitals become smaller, and iron sits within the plane of the nitrogen atoms.When iron is scarce in the body, Zn²⁺ will coordinate to the ligands in the protoporphyrin structure through a nonenzymatic process, forming Zn-protoporphyrin IX (ZPP) . ZPP and protoporphyrin IX (PPIX) have distinct emission spectra when excited in the blue range whereas heme does not. Hematofluorometry can be used as a simple, inexpensive indicator for iron deficiency in whole blood by placing a blood sample in a fluorometer and measuring the amount of ZPP relative to the amount of heme. Figure 2 illustrates the fluorescence emission spectra for ZPP and protoporphyrin IX measured from a blood sample when ZPP is excited at 425 nm and PPIX is excited at 407 nm.

Figure 2 ZPP and PPIX fluorescence emission spectra
Adapted from Hennig G, Gruber C, Vogeser M, et al. Dual-wavelength excitation for fluorescence-based quantification of zinc protoporphyrin IX and protoporphyrin IX in whole blood. J Biophotonics. 2014;7(7) :514-24.
-Comparing the peak excitation wavelengths stated in the passage and the highest-intensity fluorescence peaks in Figure 2, which of the following is true about ZPP detection in blood samples?

Quizplus · Accepted Answer

11ecba2d_11ad_7e0e_a3bf_db3170deae1d_MD0008_00 Because a photon's energy is inversely proportional to its wavelength, smaller wavelengths correspond to higher energies. Some molecules can absorb the energy of a photon in a process called excitation, causing an electron to move from the ground state to a higher energy (excited) state. The change in the electron's energy is equal to the energy of the absorbed photon. The electron can then relax back to the ground state when the molecule emits light, heat, or both. When some of the energy is lost as heat and the rest is lost as light, the emitted photon will have less energy than the absorbed photon, and therefore will have a longer wavelength. This process is known as fluorescence. As stated in the last sentence of the passage, ZPP has an excitation maximum of 425 nm and an emission maximum of 595 nm whereas PPIX is excited at 407 nm and emits a photon at 625 nm. Because ZPP can be excited by a longer wavelength than PPIX, ZPP requires less energy to excite its electrons from the ground state. (Choice A) The peaks of the emission spectra are at different wavelengths, so ZPP and PPIX do not have peak photon emissions of the same energy. (Choice C) Because ZPP and PPIX were excited using different wavelengths (425 nm and 407 nm), the photons exciting each molecule do not have equal energy. (Choice D) The energy of the absorbed photon is the same energy that excites electrons. Because ZPP and PPIX have different excitation wavelengths, they each absorb a different amount of energy, which indicates electronic transitions of different energy in each molecule. Educational objective: Electromagnetic radiation at a certain wavelength has photons of a particular energy. Energy is inversely proportional to wavelength.

Passage Heme Is Found in Several Proteins, Including Hemoglobin in Blood