Owing to the double bonds in the molecule, all carotenoids exhibit cis-trans isomerization stereomutation. A cis double bond implies a configuration with the highest-priority group on the same side, whereas in the trans configuration they are on opposite sides.
The absorption spectrum of a cis isomer presents a subsidiary peak in the near-ultraviolet, the cis peak; generally, it is located nm from the longest wavelength maximum. For example, cis peak will appear at nm if the longest wavelength maximum is nm. In photosynthetic systems, carotenoid has essential functions.
First, carotenoid is an accessory pigment in the collection of light energy in the spectral region which chl does not absorb and in transferring energy to a chl pigment [ 11 , 12 ]. Second, carotenoid functions in a process called photoprotection by quenching triplet state of chl before it reacts with oxygen to form singlet oxygen species ROS [ 13 , 14 ].
Third, carotenoid regulates energy transfer in the light-harvesting antenna through a process called xanthophyll cycle, to avoid over-excitation of the photosynthetic system by safely dissipating excess energy [ 15 , 16 ].
Chlorophylls and carotenoids are embedded in PS II and PSI, large pigment-protein clusters, the structures of which are perfectly adopted to ensure that almost every absorbed photon can be utilized to drive photochemistry.
Both PSII and PSI consist of two moieties, that is, core complex or the reaction center that is responsible for charge separation and light-harvesting antenna complexes that surround the core complex and have functions to increase the capture of light energy and energy transfer to the reaction center in the core complex.
The Soret band of chl a in the complexes was detected at nm while in the MeOH it was found at nm Figure 2a black line. It is shown here that Chl a acts as the main contributor to the excitation band at nm and it shows that excitation at nm Soret band produces stronger emission intensity, while the excitation at and nm, correspond to Chl b Soret band and carotenoid, respectively, produces weaker emission intensity.
Measurements were conducted at ambient temperature. The isolation of chloroplast was carried out as follows: 20 g of suji leaves Pleomele angustifolia were washed with running water and cut. The current high-resolution structural models of antenna complexes have been obtained only for LHCII 2. LHCII shows trimeric structure. One monomeric subunit contains eight chlorophyll Chl a pigments, six Chl b , two luteins Lut , neoxanthin and one additional xanthophyll [ 17 , 19 ].
The 14 chlorophylls are non-covalently attached in the protein cavity. Four carotenoid binding sites per monomer have also been characterized, but in this case the type of carotenoid bound can vary. Typically, two lutein molecules are in groves on both sides of helices a and b and have been likened to a cross-brace.
A third carotenoid, 9- cis neoxanthin, is located in the Chl b -rich region near helix c. The fourth carotenoid is located at monomer-monomer interfaces in the trimer. It has been suggested that this site accommodates carotenoids that can participate in the xanthophyll cycle. It depends on the external stress level of the plant; the fourth carotenoid is either violaxanthin no or low stress or zeaxanthin high stress [ 20 ]. In this structure, the carotenoids are in van der Waals contact with the chlorophylls [ 9 ].
This is essential as carotenoids in LHCII act as accessory light-harvesting pigments and photoprotectors. The accessory light-harvesting function represents singlet-singlet energy transfer from the carotenoid to the chlorophylls. Since the singlet excited state lifetime of the carotenoid is quite short, approximately fs, the carotenoid must be in close distance to a chlorophyll molecule if the energy transfer is to be efficient.
Photoprotection function represents the quenching of triplet excited state of chlorophylls and so preventing the formation of singlet oxygen. This triplet-triplet exchange reaction also requires the carotenoid to be in close contact with the chlorophylls. Regarding CP29, it binds 3 carotenoids and 13 chlorophyll molecules [ 18 ]. Each monomer is colored magenta, yellow and pale green. The three-transmembrane helices a, b and c present in a monomer are labeled and are easily visible. Chl a molecules are in red, Chl b colored green and carotenoids colored orange.
The two branches are related by the pseudo-twofold symmetry axis. The two main proteins that comprise a monomer are PsaA yellow and PsaB magenta. The electron transport chains are in the center of each monomer. The current high-resolution crystal structure of PS II and PSI core complexes is limited to that from cyanobacteria and from pea, respectively [ 21 , 22 ]. The core of PSII is a multi-subunit complex.
Most of the chromophores involve light harvesting as well as electron transfer reaction and are bound to four main subunits, that is, D1, D2, CP43 and CP When the core of PSII and PSI reaction center structures is compared, the arrangement of the pigments and other electron transfer co-factors is also very similar Figure 4c and d.
Here, first we look at the PSII core reaction center. These two helices clasp each other like two cupped hands holding on to each other. The redox cofactors are arranged into two arms that lie on either side of the point where the two groups of helices interact.
This arrangement of the helices and the cofactors introduces a pseudo two-fold symmetry axes that runs through the center of reaction center normal to the plane of the membrane. Then each arm contains, in order, one monomeric chlorophyll molecule, one pheophytin a chlorophyll derivative and one plastoquinone molecule. Here, only the D1 arm is active in electron transport.
Upon excitation P becomes oxidized and one electron is injected out and passes down the active branch to the quinone Q A. P is re-reduced by electron transfer from a special tyrosine residue called Z Tyr z. A second turnover of P delivers a second electron to the plastoquinone and the secondary quinone Q B is now reduced to Q B H 2. The hole on Tyr z is filled by electron transfer from the manganese cluster, the oxygen evolving complex. Every four turnovers of P stores four positive charges in the manganese cluster that are then used to oxidize water and evolve oxygen.
While in CP43 and CP47, there are a total of 49 Chl a molecules that are bound and that function as internal antenna and allow excitation energy transfer from the peripheral antenna system to the reaction center.
The 3. These are byproducts of cellular processes within the body. A review notes that when there is an accumulation of ROS in the cells, they can cause cellular damage and stress. ROS has links to aging , cancer , and diabetes. For example, a study on rats found that herbal melanin may be able to prevent the formation of stomach ulcers. This suggests that melanin could play a role in the protection of the gut.
Additionally, previous research also showed that melanin may contribute to the reduction of inflammation in the body, preventing injuries to the liver. It may also play a role in the immune system. The amount of melanin in the skin will vary from person to person.
Melanocytes house melanin in cells called melanosomes. The amount of melanin in the skin is a result of the quantity and distribution of melanocytes. A article notes that skin pigmentation differences are due to the number of melanocytes present in the skin, as well as the ratio of eumelanin to pheomelanin.
Typically, those with light skin have melanocytes that have clusters of two or three melanosomes. In contrast, those with dark skin generally have individual melanosomes that can also produce melanin for keratinocytes more quickly. Other factors that can affect the level of melanin in the skin include :. It is possible for a person to produce too much melanin within the body. Experts call it hyperpigmentation.
It can occur due to certain conditions or the presence of excess melanocyte-stimulating hormone. For example, daily application of sunscreen with an SPF of 50 or higher can help minimize the effects of UV light on the skin.
A person should consult a healthcare professional before using any topical treatments, as these could have side effects, such as increased sensitivity.
Learn more about treatment for hyperpigmentation here. At times, a person can have too little melanin in the skin. This results in the skin becoming lighter. Healthcare professionals may refer to this as hypopigmentation. Two examples of hypopigmentation are vitiligo and albinism. Vitiligo is a skin condition that results in white patches of skin due to the loss of melanocytes. Albinism is a genetic condition that causes people to have very little or no melanin pigment in the eyes, skin, or hair.
Although tanning is an indication that the skin is releasing melanin, it is not a safe way to increase melanin levels. The Skin Cancer Foundation notes that tanning increases the risk of developing skin cancer. Learn more about increasing melanin levels here.
Over time, this cellular damage can potentially lead to skin cancer. Outside of tanning, there have been some reports that suggest certain vitamin or herbal supplements can increase melanin levels in the skin. Some of the more commonly recommended supplements are antioxidants, such as vitamin A and vitamin E. In addition to providing pigmentation for the cells, melanin also absorbs harmful UV rays and protects against cellular damage from UV light exposure.
Melanin levels are generally determined by genetics, but they can be influenced by outside sources, such as sun exposure, hormones, or even age.
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