molecules Letter
Concentration Effect on Quenching of Chlorophyll a Fluorescence by All-Trans-β-Carotene in Photosynthesis Chen Chen 1 , Nan Gong 1 , Zuowei Li 1 , Chenglin Sun 2, * and Zhiwei Men 1, * 1
2
*
Coherent Light and Atomic and Molecular Spectroscopy Laboratory, College of Physics, Jilin University, Changchun 130012, China;
[email protected] (C.C.);
[email protected] (N.G.);
[email protected] (Z.L.) Key Laboratory of Physics and Technology for Advanced Batteries, College of Physics, Jilin University, Changchun 130012, China Correspondence:
[email protected] (C.S.);
[email protected] (Z.M.); Tel.: +86-431-8516-7503 (C.S.); +86-431-8516-6787 (Z.M.)
Received: 2 August 2017; Accepted: 20 September 2017; Published: 21 September 2017
Abstract: Absorption, fluorescence spectra of chlorophyll a (Chl-a) and all-trans-β-carotene (β-Car) mixing solution are investigated in different polarity and polarizability solvents. The carotenoids regulate the energy flow in photosynthesis by interaction with chlorophyll, leading to an observable reduction of Chl-a fluorescence. The fluorescence red shifts with the increasing solvent polarizability. The energy transfer in the Chl-a and β-Car system is proposed. The electron transfer should be dominant in quenching Chl-a fluorescence rather than the energy transfer in this system. Polar solvent with large polarizability shows high quenching efficiency. When dissolved in carbon tetrachloride, Chl-a presents red shift of absorption and blue shift of fluorescence spectra with increasing β-Car concentration, which implies a Chl-a conformational change. Keywords: Chl-a; fluorescence; β-Car; quenching
1. Introduction Photosynthesis is the biological basis of most life on Earth, converting sunlight to biomass, which is of great importance for the growth and development of plants. Chlorophyll and carotenoids are both building blocks of the photosynthetic apparatus [1–6]. Chl-a can be found in higher plants, green algae, and some bacteria, and is considered the major light-harvesting pigment in these organisms [7–10]. Carotenoids are alkene molecules that contain π-electron conjugated bonds, and have many functions in organic life, such as energy transfer, photoprotection and interception of singlet oxygen [11–13]. Carotenoids interacting with chlorophyll in the singlet state which will lead to a quenching of chlorophyll fluorescence. This indicates their key role in regulating the energy flow of chlorophyll excited states. Studies have shown that β-Car can quench Chl-a fluorescence in organic solvents [14]. Several studies suggest that different solutions surrounding the photosynthetic system will lead to aggregation or other structural changes inducing the photophysical changes. This means that the solution should be added with caution for photosynthetic systems. The molecular mechanism of dissipation process still remains under debate, although several models have been proposed [3,5]. This work investigates the fluorescence of Chl-a characteristics quenched by β-Car. The concentration is around 10−5 M, the quenching efficiency improves with the increasing concentration. Solvent polarizability effects on quenching are monitored by the position of the absorption of β-Car and the fluorescence position of Chl-a. The physical interaction mechanism of β-Car and Chl-a is illustrated.
Molecules 2017, 22, 1585; doi:10.3390/molecules22101585
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2. Results and and Discussion 2. Results Discussion Individual Chl-a solution bandslocated located 640~670 Individual Chl-a solutionshows showstwo two strong strong absorption absorption bands at at 640~670 nm nm and and 410~430 The absorption peaks peaks of three obvious vibronic peaks,peaks, and belong the to 410~430 nm.nm. The absorption of β-Car β-Carshow show three obvious vibronic and to belong transition S 0 → S 2 . The three peaks are the phonon assist band and are labelled, from left to right, the transition S0 S2 . The three peaks are the phonon assist band and are labelled, from left to0-right, 2, 0-1 and 0-0. In theChl-a Chl-a and mixing solution, the 0-2 band overlaps with the Chl0-2, 0-1 and 0-0. In the andβ-Car β-Car mixing solution, theabsorption 0-2 absorption band overlaps with the a 410 nm band (Figure 1). Chl-a 410 nm band (Figure 1).
Figure 1. Absorption spectra of of Chl-a solution,inset: inset:thethe corresponding Figure 1. Absorption spectra Chl-aand andβ-Car β-Car mixing mixing solution, corresponding Chl-aChl-a fluorescence emission. The excited wavelength of the emission is 532 nm. fluorescence emission. The excited wavelength of the emission is 532 nm.
The mixing solution shows overlap of the absorption spectra of Chl-a and β-Car. When the βThe mixing solution shows overlap of the absorption spectra of Chl-a and β-Car. When the β-Car Car concentration is increased, the 0-0 peak shows a blue shift, which leads to a much greater spectral concentration is increased, the 0-0 peak shows a blue shift, which leads to a much greater spectral overlap with the absorption of Chl-a. The superposition between Chl-a and β-Car 0-2 peaks enhances overlap with the absorption of Chl-a. The superposition between Chl-a and β-Car 0-2 peaks enhances the absorption at 410 nm. Simultaneously, the fluorescence decreases with the increasing concentration. the absorption nm. Simultaneously, the fluorescence with the increasing Figureat2410 presents the fluorescence intensity variationdecreases after adding β-Car. The acetoneconcentration. solution Figure 2 presents the fluorescence intensity variation after adding β-Car. acetone solution shows the highest quenching efficiency, due to its large dielectric constant, which isThe somewhat larger showsthan the20. highest efficiency, due to its largesolvent dielectric constant, which is somewhat larger Large quenching quantum efficiency is observed in polar because the solute tends to aggregate. Thus, the photophysical properties will vary in in polar solvents with different polar than 20. Large quantum efficiency is observed solvent because polarities. the soluteMoreover, tends to aggregate. are analogousproperties to adding an electric among the molecules, which generatesMoreover, the intrinsicpolar Thus,solvents the photophysical will varyfield in solvents with different polarities. dipole moment. With intrinsic dipole moments, intermolecular electrostatic interaction will solvents are analogous to adding an electric field among the molecules, which generates theoccur intrinsic between two molecules. Upon the interaction of an excited singlet state of β-Car with Chl-a, the dipole moment. With intrinsic dipole moments, intermolecular electrostatic interaction will occur energy can be partitioned between energy transfer and an electron transfer process [14]. Since β-Car between two molecules. Upon the interaction of an excited singlet state of β-Car with Chl-a, the has a lower ionization potential, the electron transfer should predominate in the center of reaction. energy can befast partitioned transfer and electron transfer [14]. exciplex Since β-Car Through interactionbetween between energy β-Car ground state andan Chl-a excited state, a process non-emissive has aintermediate lower ionization potential, the electron transfer should predominate in the center of reaction. is produced. β-Car+ forms and a rapid ground state reaction appears with neighbor Through fast interaction between β-CarChl−·Chl+ ground state and Chl-a state,species a non-emissive chlorophyll molecules. As a result, is produced, andexcited this dimer is involvedexciplex in + forms and a rapid ground state reaction appears with neighbor intermediate is produced. β-Carthe photosystem II [15]. Among different solvents, we have about 50% quenching or more at a carotenemolecules. concentration × 10−5 Chl M. −· Assuming quenching, corresponds to involved a Stern in chlorophyll Asofa 2result, Chl+ is dynamic produced, and thisthis dimer species is 4 M−1. For a singlet decay time of the chlorophyll of 10−8 s, this would Volmer constant of 5 × 10 photosystem II [15]. Among the different solvents, we have about 50% quenching or more at a 12 M−1 s−1. Actually, 10−8 s is really an correspond to a fluorescence quenching rate constant of 5 × 10 −5 M. Assuming carotene concentration of 2 × 10 dynamic quenching, this corresponds to a Stern −9 s making the fluorescence quenching rate upper limit for the singlet decay time, which is in fact 10 4 − 1 Volmer constant of135 × 10 M . For a singlet decay time of the chlorophyll of 10−8 s, this would constant 5 × 10 M−1 s−1. This is two to three orders of magnitude higher than the rate constant for a correspond to a fluorescence quenching rate constant of 5 × 1012 M−1 s−1 . Actually, 10−8 s is really an diffusion-controlled process, which is the maximum−9rate constant possible for fluorescence upperquenching limit for the decay time, as which is transfer in fact 10 s making quenching by asinglet short-range process Dexter electron transfer.the Forfluorescence the same reason, we can rate 13 − 1 − 1 constant × 10 the M dynamic s . This is two to orders of magnitude higher than the constant for a also5exclude quenching bythree Förster transfer, as this would correspond torate an unrealistic diffusion-controlled process, which is the maximum rate constant possible for fluorescence quenching R0 value of 8 to 80 nm. Additionally, static Förster transfer to randomly distributed carotenes can be by a short-range process as Dexter transfer electron transfer. For the same reason, we can also exclude the dynamic quenching by Förster transfer, as this would correspond to an unrealistic R0 value of 8 to 80 nm. Additionally, static Förster transfer to randomly distributed carotenes can be excluded.
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−5 Theexcluded. average The distance to distance a carotene about 25is nm at 25 a concentration of 2 × 10 is much average to aiscarotene about nm at a concentration of 2 ×M. 10−5This M. This is → larger than any value reported for R , especially considering that the S S transition in carotene 0 0 1 S0 → S1 transition in is much larger than any value reported for R0, especially considering that the Molecules 2017, 22, 1585 of 6 symmetry Therefore, must conclude that the carotene chlorophyll form a 3complex, caroteneforbidden. is symmetry forbidden.we Therefore, we must conclude that theand carotene and chlorophyll form anda that insideand this that complex, very quenching of thequenching chlorophyll by electron transfer of complex, inside thisefficient very efficient of singlet the chlorophyll excluded. The average distance tocomplex, a carotene is about 25 nm at a concentration of 2 × 10−5 M.singlet This is by Dexter type energy transfer occurs [16–18]. electron of Dexter typereported energy transfer occurs [16–18]. much transfer larger than any value for R0, especially considering that the S0 → S1 transition in
carotene is symmetry forbidden. Therefore, we must conclude that the carotene and chlorophyll form a complex, and that inside this complex, very efficient quenching of the chlorophyll singlet by electron transfer of Dexter type energy transfer occurs [16–18].
Figure Stern-Volmertype typeplot plotof ofChl-a Chl-a and and β-Car β-Car mixing Figure 2. 2.Stern-Volmer mixingin indifferent differentsolvents. solvents.
Direct energy transfer from carotenoid considered because the Figure 2. Stern-Volmer type plot of was Chl-atraditionally and β-Car mixing in differentimpossible, solvents. Directofenergy transfer from carotenoid wasbelow traditionally considered impossible, energy the excited singlet state of Chl-a lies that of β-Car. It is suggested the because symmetrythe energy ofDirect theS0excited singlet lies below that of considered β-Car. isimpossible, suggested the symmetry energy transferstate from of carotenoid was traditionally because the forbidden → S1 transition leads to Chl-a a small transition dipole moment.It Therefore, the Förster type forbidden S S transition leads to a small transition dipole moment. Therefore, the Förster → 0 1 energy of the seems excitedto singlet state of Chl-aand lies the below that electron of β-Car. exchange It is suggested the symmetry energy transfer be inappropriate, Dexter mechanism would type be energy transfer to be inappropriate, and Dexter electron exchange mechanism would forbidden S0seems → S1However, transition leads to a small dipole moment. Therefore, the Förster typethebe more appropriate. recent reports oftransition a the symmetry effect on carotenoids shows that even energy transferHowever, seems tonot berecent inappropriate, and Dexter mechanism be isthe more appropriate. reports of aCsymmetry effect onexchange carotenoids shows thatThere even β-Car in solution should have the C2h, but 2the should beelectron the proper symmetry group would [4]. appropriate. However, recent reports of in a 2symmetry effect on carotenoids shows that the β-Car in solution should not have thetransition C2h , but C shouldThe be the proper symmetry group [4]. There nomore symmetry-forbidden electronic β-Car. S1 state of β-Car lies on the even ~4000 cm−1 is β-Car in solution should not have the C 2h, but C2 should be the proper symmetry group [4]. There is no lower symmetry-forbidden electronic transition in β-Car. The S state of β-Car lies on the ~4000 in energy than that of S2 [18]. If the number of carbon-carbon double bonds exceeded 10, thecm S1−1 1 no symmetry-forbidden electronic transition in β-Car. The S 1 state of β-Car lies on the ~4000 cm−1 stateinwould below that of2 Chl-a. this context, transfer could appear the Chl-a10, andthe β- S1 lower energyliethan that of S [18]. IfInthe number ofenergy carbon-carbon double bondsinexceeded lower in energy than that of S2 [18]. If the number of carbon-carbon double bonds exceeded 10, the S1 Car system, and thethat β-Car wouldInbethis able to act energy as a trap for thecould dissipation energy. state would lie below of Chl-a. context, transfer appearofinexcess the Chl-a and The β-Car state would lie below that of Chl-a. In this context, energy transfer could appear in the Chl-a and βproposed energy transfer scheme Chl-a and β-Car different solvents is shown in Figure 3. system, and the β-Car would be ableinto act as a trap forinthe dissipation of excess energy. The proposed Car system, and the β-Car would be able to act as a trap for the dissipation of excess energy. The energy transferenergy scheme in Chl-a andinβ-Car different shownisin Figure 3. proposed transfer scheme Chl-a in and β-Car insolvents different is solvents shown in Figure 3.
Figure 3. Proposed energy transfer scheme in Chl-a and β-Car in different solvents. Figure 3. Proposed energytransfer transferscheme scheme in in in different solvents. Figure 3. Proposed energy in Chl-a Chl-aand andβ-Car β-Car different solvents.
Anotherfactor factorthat thatshould shouldbebetaken takeninto intoaccount accountininthe thequenching quenchingprocess processisissolvent solvent Another polarizability.Figures Figures4 4and and5 5show showthe theChl-a Chl-aand andβ-Car β-Carabsorption absorptionand andfluorescence fluorescenceversus versussolvent solvent polarizability. polarizability.Interestingly, Interestingly,both bothspectra spectrashow showa ared-shift red-shiftfollowing followingthe thesolvent solventpolarizability polarizability polarizability. increase.With Withananincrease increaseininsolvent solventpolarizability, polarizability,the thefluorescence fluorescenceposition positionofofthe theChl-a Chl-ared-shift red-shiftisis increase. Molecules 2017, 22, 1585 4 of 7 about1010nm. nm.The Theabsorption absorptionspectra spectraofofthe theChl-a Chl-aand andβ-Car β-Carsystem systemininnon-polar non-polarsolvent solventare areshown shown about Figure5.5.Neither Neitherthe theChl-a Chl-anor norβ-Car β-Car absorptionpeaks peaksshow showa aparticularly particularlylarge largeshift. shift.Considering Considering ininFigure absorption thatthe theβ-Car β-CarS1Sstate 1 state moresolvent-sensitive, solvent-sensitive,the thered-shift red-shiftversus versussolvent solventpolarizability polarizabilitywill willbebe that isismore Another factor that should be taken into account in the quenching process is solvent polarizability. relativelylarge. large.The Theβ-Car β-CarS1Sstate 1 state theroute routeofofthe theenergy energytransfer, transfer,and andred-shifts red-shiftswith withincreasing increasing relatively isisthe Figures 4 and 5 show the Chl-a and β-Car absorption and fluorescence versus solvent polarizability. solventpolarizability. polarizability.Therefore, Therefore,energy energytransfer transfershould shouldmake makea acontribution, contribution,and andisisdependent dependentonon solvent Interestingly, both spectra show a red-shift following the solvent polarizability increase. With an moleculardistance. distance.The Theenvironment environmentininantennae antennaeisisknown knowntotobebedifferent differentfrom fromthat thatofofthe thereaction reaction molecular increase in solvent polarizability, the fluorescence position of the Chl-a red-shift is about 10 nm. center,and andenergy energytransfer transferisisrelative relativetotothe theenvironment environmentofofthe thecarotenoids carotenoids[19]. [19].Absorption Absorptionand and center, The absorption spectra of the Chl-a and β-Car system in non-polar solvent are shown in Figure 5. vibrationaltechniques techniqueshave haveshown shownthat thatpolarizability polarizabilityisisananimportant importantenvironmental environmentalparameter parameterboth both vibrational Neither the Chl-a nor β-Car absorption peaks show a particularly large shift. Considering that the vivoand andininvitro. vitro.Conformational Conformationalchange changecorrelates correlatessignificantly significantlywith withthe theconjugation conjugationlength lengthofof ininvivo β-Car S1 state is more solvent-sensitive, the red-shift versus solvent polarizability will be relatively carotenoids.This Thismeans meansthat thatcarotenoids carotenoidsinindifferent differentpolarizability polarizabilityenvironments environmentsshould shouldhave have carotenoids. large. The β-Car S1 state is the route of the energy transfer, and red-shifts with increasing solvent differentroles rolesininregulating regulatingenergy energyflow. flow.We Wesuggest suggestthat thatboth bothelectron electrontransfer transferand andenergy energytransfer transfer different polarizability. Therefore, energy transfer should make a contribution, and is dependent on molecular contributetotoquenching quenchingChl-a Chl-afluorescence fluorescenceininvitro. vitro.Moreover, Moreover,polar polarenvironments environmentswith withgreater greater contribute distance. The environment in antennae is known to be different from that of the reaction center, and polarizabilityare areconsidered consideredtotolead leadtotohigher higherquenching quenchingefficiency. efficiency. polarizability energy transfer is relative to the environment of the carotenoids [19]. Absorption and vibrational Unlikeother other observationsininthis thiswork, work,Chl-a Chl-aininCCl CCl 4 solvent undergoesa ared-shift red-shiftofofabsorption absorption Unlike observations 4 solvent undergoes techniques have shown that polarizability is an important environmental parameter both in vivo and anda ablue-shift blue-shiftofoffluorescence fluorescence(Figures (Figures5 5and and6).6).The Thered redshift shiftofofabsorption absorptionimplies impliesconformational conformational and in vitro. Conformational change correlates significantly with the conjugation length of carotenoids. change the Chl-a. Two effects should taken into account: One the out-of-plane distortion effect change ofof the Chl-a. Two effects should bebe taken into account: One isis the out-of-plane distortion effect This means that carotenoids in different polarizability environments should have different roles in betweenChl-a Chl-aand andβ-Car β-Carmixing mixingmolecules moleculesand andCCl CCl theother otherisisthe thelaser-heating laser-heatinginduced induced between 4, 4,the regulating energy flow. We suggest that both electron transfer and energy transfer contribute to conformationalchange change[20,21]. [20,21].The Theblue blueshift shiftofoffluorescence fluorescenceimplies impliesananenhanced enhancedpopulation populationinin conformational quenching Chl-a fluorescence in vitro. Moreover, polar environments with greater polarizability are highervibrational vibrationallevel, level,which whichleads leadstotoincrease increaseofofthe theradiation radiationphoton photonenergy. energy. higher considered to lead to higher quenching efficiency. 10nm 10nm H42Cl (0.265) C2HC42Cl (0.265) 2 H6(0.295) C6HC66(0.295)
Intensity Intensity
(0.275) CClCCl (0.275) 4 4
600600
620620
640640
660660
680680
700700
720720
740740
Wavelength/ nm / nm Wavelength Figure Fluorescence spectra red-shift increasing solvent polarizability. Figure 4. Fluorescence spectra red-shift by increasing solvent polarizability. Figure 4.4. Fluorescence spectra red-shift byby increasing solvent polarizability. 2 2 1 1
C2C H24H Cl4Cl 2 2
0M0M -6 -6 0.60.6 1010 M M -5 -5 1.81.8 1010 M M -5 -5 2.02.0 1010 M M
Absorbance Absorbance
0 0 4 4 3 3 2 2
C6C H66H6
1 1 0 0 3 3 2 2
CCl CCl 4 4
1 1
0M0M -6 -6 0.60.6 1010 M M -5 -5 1.01.0 1010 M M -5 -5 2.02.0 1010 M M
0M0M -6 -6 0.60.6 1010 M M -5 -5 1.01.0 1010 M M -5 -5 1.21.2 1010 M M
0 0 350350
400400
450450
500500
550550
600600
650650
700700
750750
Wavelength/ nm / nm Wavelength Figure Absorption spectra Chl-a and β-Car different polarizability non-polar solvents. Figure 5. Absorption spectra of Chl-a and β-Car in different polarizability non-polar solvents. Figure 5.5. Absorption spectra ofof Chl-a and β-Car inin different polarizability non-polar solvents.
Unlike other observations in this work, Chl-a in CCl4 solvent undergoes a red-shift of absorption and a blue-shift of fluorescence (Figures 5 and 6). The red shift of absorption implies conformational change of the Chl-a. Two effects should be taken into account: One is the out-of-plane distortion
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effect between Chl-a and β-Car mixing molecules and CCl4 , the other is the laser-heating induced conformational change [20,21]. The blue shift of fluorescence implies an enhanced population in higher Molecules 2017, 22, 1585 5 of 6 vibrational level, which leads to increase of the radiation photon energy.
0M -5 0.610 M -5 1.010 M -5 2.010 M
Intensity
CCl4
600
620
640
660
680
700
720
740
760
Wavelength / nm Figure6.6.Fluorescence Fluorescenceblue-shift blue-shiftwith withincreasing increasingconcentration concentrationofofβ-Car. β-Car. Figure
Materialsand andMethods Methods 3.3.Materials Chl-apowder powder was purchased Bomei (Hefei, andunder storedcold under cold condition and dark Chl-a was purchased fromfrom Bomei (Hefei, China)China) and stored and dark condition before use. The purity was checked by comparing its absorption bands. β-Car from Sigma before use. The purity was checked by comparing its absorption bands. β-Car from Sigma Aldrich Aldrich (Shanghai, China) was used without further purification. samplesthe contained the with same (Shanghai, China) was used without further purification. All samplesAll contained same Chl-a −5 M. The concentration gradients of β-Car were 0.6, 1.0 − 5 − Chl-a with the same concentration at 10 the same concentration at 10 M. The concentration gradients of β-Car were 0.6, 1.0 and 2.0 × 10 5 and M, 2.0 × 10−5 M, Spectrograde respectively. Spectrograde were used. The fluorescence Chl-a was respectively. solvents weresolvents used. The fluorescence of Chl-a was of measured at measured ~670 nm at ~670 nm perpendicular to the incident light,recorded and wasby recorded Ocean Optics Maya 2000Optical (Ocean perpendicular to the incident light, and was Ocean by Optics Maya 2000 (Ocean Optical Corporation, Dunedin, FL, USA). The excited wavelength was 532 nm. The detector was fixed Corporation, Dunedin, FL, USA). The excited wavelength was 532 nm. The detector was fixed perpendicular to the quazt cell at an appropriate distance to record the fluorescence. Absorption perpendicular to the quazt cell at an appropriate distance to record the fluorescence. Absorption spectrawere were measured a double-beam UV-Vis spectrophotometer TU-1901 Co., Ltd., spectra measured on on a double-beam UV-Vis spectrophotometer TU-1901 (Persee (Persee Co., Ltd., Beijing, Beijing, China) with a step of 0.5 nm at room temperature. China) with a step of 0.5 nm at room temperature. Conclusions 4.4.Conclusions Inconclusion, conclusion,absorption absorptionand andfluorescence fluorescenceshow showan aneffective effectivequencher quencherrole roleofofβ-Car. β-Car.The TheChl-a Chl-a In fluorescenceisisreduced reducedby byincreasing increasingthe theconcentration concentrationofofβ-Car. β-Car.Increasing Increasingthe thesolvent solventpolarizability polarizability fluorescence leadstotoaared-shift red-shiftofofthe thefluorescence fluorescenceofofChl-a. Chl-a.The Theresults resultssuggest suggestthat thatelectron electrontransfer transferdominates dominates leads theChl-a Chl-aand andβ-Car β-Carsystem. system.Different Differentsolvent solventenvironments environmentsdetermine determinethe therelative relativeimportance importanceofof ininthe thesetwo twoprocesses, processes, which related to the distance between the molecules. solvents with these which areare related to the distance between the molecules. PolarPolar solvents with high high polarizability higher quenching On excitation in CCl 4, of a red shift of Chl-a polarizability show ashow highera quenching efficiency.efficiency. On excitation in CCl4 , a red shift Chl-a absorption absorption andofafluorescence blue shift are of fluorescence observed. All tothese results lead to a strong and a blue shift observed. Allare these results lead a strong environmental effect environmental effect on quenching Chl-a fluorescence. Morewill experimental studies willissue be conducted on quenching Chl-a fluorescence. More experimental studies be conducted on this and the on this issue the mechanism quenchingin ofphotosynthesis. Chl-a fluorescence in photosynthesis. mechanism ofand quenching of Chl-a of fluorescence Acknowledgments: Acknowledgments:The Theauthors authorsgratefully gratefullyacknowledge acknowledgethe thefinancial financialsupported supportedby byNational NationalNatural NaturalScience Science Foundation of China (NSFC) (11574113, 1374123, 11374123 and 11604024); Graduate Innovation Foundation of China (NSFC) (11574113, 1374123, 11374123 and 11604024); Graduate InnovationFund FundofofJilin Jilin University (2016155). University (2016155). Author Contributions: Zhiwei Men and Chenglin Sun conceived and designed the experiments; Nan Gong Author Contributions: ZhiweiChen Men and SunGong conceived and designed experiments; Nan Gong performed the experiments; ChenChenglin and Nan analyzed the data;theZuowei Li contributed performed the experiments; Chen and Nan Gong analyzed the data; Zuowei Li contributed reagents/materials/analysis tools; ChenChen Chen wrote the paper. reagents/materials/analysis tools;declare Chen Chen wrote of theinterest. paper. Conflicts of Interest: The authors no conflict
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds all-trans-β-Carotene and Chlorophyll-a are available from the authors. © 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
