One day hanging out in the supermarket, although Xiaobian is not good at cooking, I was still attracted by a variety of thick cut steaks and leg of lamb, that kind of fresh red makes people, and you see the salmon next door, red, watermelon red, warm orange, light pink... A large piece of bells and whistles red, the freshness of the color transmission makes people really have an appetite. No, suddenly a question came to mind, why is fresh meat red?
Figure 1 Beef slice photography [1] (Enjoy the meat slices, sugar-free is zero calories)
Speaking of color, let's start with the visible light spectrum that Xiaobian is good at. People can perceive color because the optic cells are stimulated by electromagnetic waves in the visible light range. The color of the object is determined by its reflective spectrum, which is related to the physical properties of the substance itself and the surface structure.
Fig. 2 Electromagnetic waves[2]
Let's start with the most basic element characteristic colors. In the case of the well-known flame color experiment, the flame will appear characteristic colors when burning some metal elements or their compounds. The principle is that during combustion, the electrons in the atom absorb energy, will jump from the lower energy orbital to the higher energy orbital, and over time, the electron will return to the lower energy orbital, this process will release photons.
Fig. 3 Flame color reaction of metal elements[3]
Fig. 4 Electron transition from excited state to ground-state radiated photon[4]
The wavelength (λ) of the released photons and the difference in atomic orbital energy levels (E) are E = hν = hc/λ, where h, ν, and c are Planck's constant, frequency, and speed of light, respectively. The energy levels of electron orbitals in different elements differ, so elements have a series of characteristic spectral lines. If the wavelength of the photon falls within the visible range, it can be perceived by the human eye. Taking metal Na as an example, its characteristic spectrum in the visible light band comes from the transition of electrons between the 3p orbit and the 3s orbit, and due to its fine structure, there are double lines in the spectrum, with wavelengths of 589.0 nm and 589.6 nm, both in the yellow band, so we see bright yellow flames in the flame color reaction of Na element.
So back to the meat we eat, why does vb appear different red? This is mainly derived from myoglobin. First, let's take a look at myoglobin, a single-stranded protein made up of 153 amino acids with a molecular weight of 16,700 daltons. Myoglobin is present in muscles to store and release oxygen and plays an important role in muscle movement. The process of myoglobin storage and release of oxygen is accompanied by changes in the coordination of iron elements in it, as shown in Figure 5.
Fig. 5 Structure of myoglobin[5]
When myoglobin is not bound to oxygen, iron is positive divalent, and each ferrous ion is coordinated with five nitrogen atoms to form a (FeN) group with a pyramidal structure. Four of the five nitrogen atoms come from the pyrrole ring and one from histidine in the polypeptide chain. After combining with oxygen, the valence of iron is controversial, and may have both bivalent and trivalent. At this time, in addition to coordinating with nitrogen, iron ions are also directly bonded with oxygen molecules to form (FeNO) groups, which are octahedral structures. Below we analyze in detail the orbital arrangement of electrons in it:
When oxygen is not bound, the electron arrangement of Fe ions in myoglobin is 1s2s2p3s3p3d. In general, the 3D electrons of these electrons have the most obvious influence on the physicochemical properties of Fe ions. The 3d orbit can be subdivided into five orbits: d, d, d, d, d. In isolated Fe ions, these 5 orbitals have the same energy, but the shape or orientation of the electron cloud is not the same, see Figure 6.
Figure 6 Electron cloud in a 3d orbit[6]
When fe ions are coordinated with five nitrogen atoms, the interaction of the 3d orbital electron cloud with the nitrogen atoms causes the energy of the five orbitals to differ, from low to high, d/d, d, d (the energy of d and d is equal) [7], as shown in the left figure of Figure 7. The horizontal lines in the figure represent the electron orbits, and their height represents the amount of orbital energy. Each arrow represents an electron, and their orientation represents the direction of the electron's spin. The electron arrangement of anaerobic Fe ions in the left figure of Figure 7 only shows the case of high spin state, and the actual anaerobic myoglobin has a variety of electron arrangements of Fe ions.
Fig. 7 Electron arrangement in myoglobin iron ion coordination group[8]
Another case is when oxygen molecules enter myoglobin. First, for oxygen molecules, electrons from the 2p orbitals of different oxygen atoms form σ/σ* bonds and π/π* bonds, and the electrons are filled in the order of σ bonds, π bonds, π* bonds, and σ* bonds. The result of the fill is that the σ and π keys are filled, σ there are no electrons occupied on the * key, while the π * key is only half filled. When myoglobin binds to oxygen, the iron ion is bonded to the oxygen atom, and the two electrons on the π* bond in the O-O bond will bond with the 3d electron of the iron ion, which is: σ/σ* bond with the d orbital, and π/π* bond with the d orbital. After bonding, the iron ion 3d orbital and the Fe-O bond orbital form a common Fe-O energy level system, and the energy level of the electron orbital in the system is shown in the middle of Figure 7. The electron arrangement in the figure only shows the low spin situation, and the electrons in the actual Fe-O system also have a variety of arrangements.
Now we discuss the electron transitions associated with myoglobin color. The experiment found that in both the oxygen-free Fe ion group and the Fe-O system, electrons can transition between orbits shown in Figure 7, and the transition process can absorb visible light. However, the energy level arrangement of the two is different, resulting in different absorption spectra. Figure 8 shows the absorption spectrum of oxygenated, anaerobic, high-iron, and nitric oxide myoglobin. Oxygenated myoglobin has a significant absorption of light with a wavelength of less than 600 nm, resulting in its reflected light being red in appearance (red band: 622-770 nm). In contrast, anaerobic myoglobin has weakened light absorption at wavelengths less than 550 nm, and light absorption in the wavelength range of 600-700 nm is enhanced relative to oxygenated myoglobin, resulting in a purple-red appearance. The muscles in live animals appear bright red due to the sufficient amount of oxygen carried by myoglobin. The fresh meat delivered to the supermarket is separated from the oxygen supply of the mother, and the myoglobin releases oxygen to make it temporarily purple-red. After a long time in the air, the oxygen-free myoglobin on the surface of the meat will combine with the oxygen in the air again to form an oxygen-containing myoglobin and appear bright red, but the inside of the meat is still in a state of less oxygen, showing a purple-red color. When over-oxidized, for example, to make bacon, myoglobin is converted into methemyoglobin, which takes on a tan color. Some merchants add a certain amount of nitrite when curing meat, in order to keep the meat red. After the nitrite is reacted by its own redox, part of it will be converted into nitric oxide, and the nitric oxide will bind to myoglobin to form nitric oxide myoglobin, and its absorption spectrum is similar to that of oxygenated myoglobin, so the meat appears red. However, nitrite has certain harm to the human body, and the state has strict restrictions on its addition.
Fig. 8 Absorption spectrum of myoglobin[9]
Having said that for so long, I went back to the bells and whistles of the salmon next door again. When the proportion of myoglobin in meat changes, it will show different red colors from the macroscopic level. Before making sashimi, some of the fish are fitness "da fish", jumping up and down, such as dark red moonfish and tuna with a color like beef; and some fish are slowly leisurely, lying flat on the bottom of the sea, such as the yellow-banded trevally of the puff puff. Oh yes, and the beautiful orange salmon, which can prey on small crustaceans to obtain astaxanthin, and store it in its own muscle red cells, which in turn stains the reddish color of the flesh into orange. Different living environments, habits and other factors lead to different myoglobin content in animals, which in turn show different flesh colors. This is very similar to the pigment toning in Chinese paintings, when white is mixed with different proportions of red, you can call up the transition from milky white, light pink, dark pink, rose red to positive red.
Fig. 9 Fast-swimming moonfish[10]
Fig. 10 Yellow-banded trevally sashimi[11] (left) and salmon sashimi[12] (right)
Fig. 11 Chinese painting pigment color grading[13]
Finally, friends, exercise a lot, and our flesh will change color.
bibliography
[1] From ZCOOL, https://img.zcool.cn/community/0780db6246a8b40002db57064bdee2.jpg
[2] From Wikipedia, https://wikipedia.org/[J].
[3] Excerpt from Instrument Network, https://www.yiqi.com/retiao/detail_2596.html
[4] Excerpt from Zhihu "Why is the flame color reaction a physical change?" Doesn't metal burn when put on fire? Kevin Wayne's answer, https://www.zhihu.com/question/436486528/answer/2375592574
[5] From Wikipedia, https://wikipedia.org/
[6] From Wikipedia, https://wikipedia.org/
[7] Churg A K, Makinen M W. The electronic structure and coordination geometry of the oxyheme complex in myoglobin. The Journal of Chemical Physics, 1978, 68(4): 1913-1925.
[8] Excerpt from Libretexts-Chemistry, https://chem.libretexts.org/
[9] Millar S J, Moss B W, Stevenson M H. Some observations on the absorption spectra of various myoglobin derivatives found in meat. Meat Science, 1996, 42(3): 277-288.
[10] From The Sina Weibo of Fish in Salzburg
[11] Excerpt from The Fish in Salzburg's Sina Weibo
[12] From Wikipedia, https://wikipedia.org/
[13] Excerpt from the public account of Chinese Painting Art, ID: quicksnowfall
Original contributions from teachers and students of the Institute of Physics
Author: Zhou Junyan
Reviewer: Jin Shifeng
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