RABINOWITCH--PRIMARY PROCESSES IN PHOTOSYNTHESIS

tide reactions in Chromatium. Biochim. Biophys.Acta. (In press.)

11. OLSON, J. M. and AMESZ, J. Action spectra forfluorescence excitation of pyridine nucleotide inphotosynthesizing bacteria and algae. Biochim.Biophys. Acta. (In press.)

12. SAN PIETRO, A. and LANG, H. M. Accumulation ofreduced pyridine nucleotide by illuminated grana.Science 124: 118. 1956.

13. SAN PIETRO, A. HENDRICKS, S. B., GIOVANELLI, J.and STOLZENBACH, F. E. Action spectrum for tri-phosphopyridine nucleotide reduction by illuminatedchloroplasts. Science 128: 845. 1958.

14. SHIBATA, K. Spectrophotometry of intact biologicalmaterials. Jour. Biochem. (Japan). 45: 599-623. 1958.

15. THEORELL, H. Kinetics and equilibria in the liveralcohol dehydrogenase system. Advances inEnzymol. 20: 31-49. 1958.

16. TREBST, A. V., TSUJIMOTO, H. Y. and ARNON, D. I.Separation of light and dark phases in the photo-synthesis of isolated chloroplasts. Nature 1g2:351-355. 1958.

17. WARBURG, O., SCHRODER, W., KRIPPAHL, G. andKLOTZSCH, H. Photosynthese. Angew. Chemie.69: 627-634. 1957. Translation: PhotosynthesisScience 128: 69-73. 1958.

18. WASSINK, E. C., KATZ, E. and DORRESTEIN, R. Onphotosynthesis and fluorescence of bacteriochloro-phyll in thiorhodaceae. Enzymologia 10: 285-354.1942.

19. WINER, A. D. and SCHWERT, G. W. Fluorebtencespectra of ternary complexes of dehydrogenaseswith reduced diphospyridine nucleotide and reducedsubstrates. Biochim. Biophys. Acta 29: 424-425.1958.

PRIMARY PHOTOCHEMICAL AND PHOTOPHYSICAL PROCESSESIN PHOTOSYNTHESIS 'EUGENE RABINOWITCH

DEPARTMENT OF BOTANY, UNIVERSITY OF ILLINOIS, URBANA, ILLINOIS

I. ExCITONS AND THEIR MIGRATIONIn photochemical reactions in condensed systems,

intermediary steps can occur between light absorptionand the primary photochemical reaction: the photo-physical processes of energy or charge transfer.Either the excited electron, or the hole it leaves in theabsorbing molecule, may move, to be found at the endof the excitation period in a site far removed fromthe original locus of excitation. If the electron andthe hole travel separately, the final effect is separationof charges-which can be also described as internaloxidation-reduction.

The electron-hole pair is often referred to as an

exciton. 'When the excited electron and the hole are

associated in a single molecule, the (perhaps super-

fluous) term "intramolecular exciton" is used. Figure1 illustrates the different modes of transfer of intra-molecular and intermolecular excitons (1).

The binding energy of the exciton may be so

small that it can dissociate under the influence ofthermal motion, or under the pull of an applied elec-tric field (photoconductivity). If impurities are pres-

ent in the system, or adsorbed on the surface, theymay trap the electron, or the hole, or both.

Recently, it has been pointed out that the concen-

tration of pigment molecules in the photosyntheticorganelles is so high as to justify approaching photo-synthesis from the point of view of phenomena in thesolid state, making the above considerations relevant.

II. STRUCTURE OF CHLOROPLASTS

All chloroplasts contain lamellae-flat or involuted,thin extended layers, about 20 mu thick. They are

'Received March 25, 1959.

found also in chloroplast-free cells of the blue-greenalgae. The lamellae either fill out the whole body ofthe chloroplast (lamellar chloroplasts), or form cylin-drical grana (granular chloroplasts). Figures 2 and3 show both types of chloroplasts in the same plant.Figure 3 shows that in granular chloroplasts, the lam-ellar structure extends into the intergranular stroma;the grana are merely approximately cylindrical vol-umes in which the lamellae are denser and more num-erous.

The high optical density of the lamellae in prepara-tions fixed with osmic acid (as in figs 2 and 3) isdue to higher concentration of precipitated osmium;and this in turn seems to be due to the presence oflipoids. Consequently, the lamellae must contain thegreatest concentration of lipoids, while the stroma ismore predominantly proteinaceous. In granularchloroplasts, absorption and fluorescence microscopyconfirm that the (lipophilic) pigments are located inthe grana.

Chlorophyll, whose molecule consists of a flat,slightly polar, colored conjugated ring system (chlor-in) and a long, colorless tail (phytol), should accumu-late on interfaces between the hydrophilic and thehydrophobic layers. Thomas and co-workers (2)compared the number of chlorophyll molecules in thechloroplasts of 8 species with the total surface areasof their lamellae. The results varied between 1.8 mulJand 3.8 mM2 per chlorophyll molecule, while the amountof chlorophyll per chloroplast varied by a factor of5 x 103.

Artificial chlorophyll surface layers (3, 4) existin two forms: crystalline and amorphous. In the firstform, the area requirement is about 0.45 mU2 (sug-gesting a 2-molecular layer), and the red absorption

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b.® 111(2) H()

lol

+hvT V11

0E000 ()~ 0-

C.0~~ET)EoIE* +~~~hL1 ) E I e I E I °) I <S Le I (D I 0I s I (X

FIG. 1. Transfer of intramolecular and intermolecular excitons. Q, hole; o, electron; and *, excited electron.Top row, creation (1) and transfer (2), (3) of intramolecular exciton-Forster-Heller-Marcus mechanism. Centerrow, same-Wannier mechanism. Bottom row, conversion of intra- into intermolecu,lar exciton (1); its transfer(2), (3); and dissociation (4).

band lies at 735 mju; in the second form, the area re-

quirement is 1.06 mjL2, and the red absorption band liesat 678 m,u.

In both types of artificial surface layers, the chloro-phyll ring system must be stacked obliquely, since theactual area of -this system-which would be requiredfor the molecules to lie flat on the interface-is about2.4 m/2. This is close to the average found by Thomaset al for chloroplasts. The position of the absorptionband in vivo-at 675 to 680 mjA (fig 4)-shows theabsence of crystalline, and supports the possible pres-ence of amorphous monolayers.

The electron micrographs suggest the existenceof flat chambers enclosed between pairs of lamellaein grana (fig 3). One can suggest that one product ofthe photochemical reaction (e.g., the reduction inter-mediate) is trapped in the chamber, while the other-the 02 precursor-escapes on the outside. Lamellarchloroplasts, containing no such chambers, also are

capable of photosynthesis; but some evidence suggeststhat their lamellae tend to associate in pairs; so thata similar separation mechanism could also operate inthem.

The arrangement of chlorophyll in monomolecularlayers was confirmed by Goedheer's (5) study of the(relatively weak) dichroism and the (pronounced)-

birefringence dispersion of large chloroplasts. Ac-cording to Goedheer, his observations suggest thatchloroplasts contain very thin (0.2 to 0.6 mu thick)pigment layers, which are obviously monomolecular.The axes of the chlorophyll molecules seem to be onlyvery imperfectly oriented, thus accounting for theweakness of the dichroism. Goedheer suggested thatthis may be due to the monolayer covering not a flat,but a grained surface. Spherical macromolecules canbe, in fact, seen on the surface of lamellae (6). Theyhave a diameter of 7 to 10 m,u, and thus a surface of150 to 300 m/A2 (fig 5). This should be enough to ac-commodate 150 to 300 adsorbed chlorophyll molecules,if the sphere could be covered uniformly-which issomewhat difficult if the macromolecules are incorpo-rated in lamellae.

The hypothetical picture of protein macromoleculescarrying 200 or 300 molecules is similar to the empiri-cal picture of phycobilins as protein macromolecules(mol. weight -_ 280.000) carrying 50 to 100 chromo-phores (S. and M. Brody (7)). The main differencemay be in the additional association of the chlorophyllswith lipoids (probably through the hydrophobicphytol tail), which prevents the extraction of chloro-phyll macromolecules with water.

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RABINOWITCH-PRIMARY PROCESSES IN PHOTOSYNTHESIS

III. STATES OF CHLOROPHYLL IN THE CHLOROPLASTSVarious evidence indicates that chlorophyll in the

chloroplasts is present in at least two forms (see e.g.,Krasnovsky et al (8) and Smith et al (9)). It hasbeen suggested that one is monomeric and anotheraggregated. Apparently discordant measurements oflife-time and of the quantum yield of fluorescence (inChlorella) by Brody (10) and Latimer (11) can bereconciled by the assumption that about one fourthof the chlorophyll is in the fluorescent state (with ayield of approximately 10 %), while three fourths isnon-fluorescent (giving an average yield of 3 %).Other observations (Brody (12)) make it appearlikely that one non-fluorescent form of chlorophyll invivo has an absorption band near 705 mu, and afluorescence band at 720 m,u (which appears only atlow temperatures).

The monomeric and polymeric, fluorescent andnon-fluorescent forms of chlorophyll in vivo probablymean different states in a monolayer-possibly dis-tinguished by the density of the monolayer, or the kindof molecules with which it is associated. Franck(13) concluded, from kinetic considerations, that twotypes of chlorophyll molecules in vivo are character-ized by contact with water, or with non-polar mole-cules, respectively. The former are fluorescent; nofree electrons can occur in this hydrated part of themonolayer. The lipoid-protected part could be non-fluorescent, and could support the existence of freeelectrons. According to Platt (14), the non-fluores-cent nr excited state must be lower than the fluores-cent =r state in lipoid-bound, and higher than the latterin the hydrated form, thus accounting for the latter'scapacity for fluorescence (fig 6).

Franck suggested that for the primary photochem-ical process in photosynthesis, 2 excited moleculesare needed, one of which must be in the jrr excitedstate, while the other can be in the nit state.

IV. ENERGY AND CHARGE TRANSFER INCHLOROPLASTS

The problem of energy transfer in photosynthesisarose in 1936, when Gaffron and Wohl (15). inter-preted the results of Emerson and Arnold's flashinglight experiments (showing that the maximum yieldof 02 per brief flash (10' sec) is about 1 moleculeof 02 per 2000 molecules of chlorophyll), by the as-sumption that about 2000 chlorophyll molecules act asa photosynthetic unit, an energy catch-basin associatedwith a single reduction site, in which the photochemicalprocess takes place. The number 2000 must be re-duced (perhaps, by a factor of 8) because the libera-tion of 1 molecule of 02 requires several (perhaps 8)elementary photochemical processes. The photosyn-thetic unit is therefore most likely to consist of about250 chlorophyll molecules. This makes it possibleto consider identifying the unit with the pigment-bearing macromolecules in the lamellae, which accord-ing to section II, may carry 100 to 300 chlorophyllmolecules.

Gaffron and Wohl said that cooperation betweenthe many light-absorbing chlorophyll molecules anda single reaction center in a unit can be attributed tothe migration, either of energy-rich particles generatedat each chlorophyll molecule, or of energy quanta.The second picture has fascinated workers in photo-synthesis in the last 20 years, without a definiteanswer being found as to its relevance.

Goedheer (5) observed a very low fluorescencepolarization of phycobilin, suggesting that the excita-tion energy moves among the differently orientedchromophores attached to the macromolecule. Inanalogy, one could expect energy exchange also be-tween chlorophyll molecules attached to a commonmacromolecule. It may perhaps take in severaladjacent macromolecules, thus accounting for the rela-

.J ..4.,~~~~~

¢X - ^(- W.j.. , ) x.ix__z

-li) Ii.''1r.T.-1lI;i-]l s Zal ;7 '. nI'1''.Tt'eiT Ii, i v T th lilt dL biiUldr, tilc 2 uji tilc Iiglit arclamellar. (After Vatter.)FIG. 3 (right). Chloroplast structure (Zea maYs). Cross section of a granular chloroplast, showing intergranu-

lar lamellae. (After Vatter.)

2?15

At.: X.

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PLANT PHYSIOLOGY

I-

za-i

400 450 500 550 600 650 700 750WAVELENGTH IN MU

FIG. 4. Absorption curves of 2 types of chlorophyllsurface layers on water (arrow indicates the position ofthe bands in solution). (After Jacobs et al.)

tively large number (at least 250) molecules in theunit; however, in this case, only one out of severalmacromolecules could carry an enzymatic reactioncenter.

Plant cells need a high concentration of pigmentto provide sufficient light absorption; but they do nothave enough space to provide a separate enzymaticconveyor belt for each chlorophyll molecule. A highratio (chlorophyll): (enzyme) is possible becauseeven in direct sunlight, a chlorophyll molecule absorbsquanta only about once every 0.1 second, while enzymescan handle their substrates in a small fraction of thistime. Excitation energy migration in the photosyn-thetic unit provides an elegant solution to the problemof how to utilize the light-absorbing capacity of sev-eral hundred pigment molecules to feed a single en-zymatic conveyor belt.

It has been asked (Franck and Teller (16))whether enough time is available between light ab-sorption and energy dissipation to permit effectiveenergy transfer in the unit. If the transfer is byrandom walk, considerably more than 250 transferswill be needed to assure effective delivery of thequantum to the reaction center in a 250-molecule unit.(On the other hand, it may be sufficient to conductthe quantum into the neighborhood of the center,rather than to a single specific pigment molecule).The natural life-time of chlorophyll in the Chl* excitedstate is 15 m/u sec. (Brody and Rabinowitch (10)).With 3 % fluorescence yield (Latimer, Bannister andRabinowitch (11) ), the actual life-time is 0.5 mnu sec;with 10 % yield, 1.5 my sec. A thousand energytransfers during this period means a visiting time of0.5 to 1.5 X 102 mu sec. The width of the absorp-tion band of chlorophyll in vivo suggests undisturbedcoupling with intramolecular vibrations, and this re-quires electronic excitation to last > 10' m,u sec.Comparison of the 2 figures (0.5 to 1.5 X 102 m,u sec

available and v 10-4 mju sec needed) shows justabout enough time for the postulated number of visits.

The range of effective migration could be extendedif one could assume that fluorescence is limited bytransfer of 'Chl* into the metastable 3Chl-state (whichis plausible), and that energy migration can continueby the WA'igner mechanism 'Chl, + 3Chll --* 3Chl1+ 'Chlll. Resonance transfer in the triplet statewas demonstrated by Terenin and co-workers (17)in frozen hydrocarbon solutions. However, it re-quires an overlapping of the electron clouds of the ex-change partners to make their total spin a significantinvariant, and what we know about the chlorophyllmonolayers in vivo may not justify this assumption.

The mechanism of energy migration considered sofar corresponds to figure 1 (a), (b)-the migrationof an intramolecular exciton. It may be askedwhether the formation and transfer of intermolecularexcitons (fig 1 (c) ), with separation of charges, alsois possible. This was postulated by Arnold (18) toaccount for light-induced electron trapping in driedchloroplast layers, and by Commoner et al (19), andCalvin et al (20), to account for light production ofunpaired spins, revealed by paramagnetic resonance.However, the postulate that light absorption in thechloroplast leads to electron transfer into a conductanceband, or at least, to the formation of a loosely boundintermolecular exciton, meets with difficulty. Theabsorption band of chlorophyll in vivo is very similarto that in vitro; this suggests that the excitation leadsto the same intramolecular excited state. Simultane-ous hole-electron transfer can then occur as an "after-thought" because of resonance between chlorophyllmolecules. A similar delayed transfer of the electronwithout the hole-i.e., conversion of an intramolecularinto an intermolecular exciton, indicated in figure1 (c)-could occur only if the energy level of thesecond is below that of the first, which seems unlike-ly. (See note added in proof). It is, however, pos-sible that charge separation occurs when the migratingexciton encounters an impurity, or a chlorophvll mole-

FIG. 5. Granular surface of a lamella (after Stein-mann).

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RABINOWN'ITCH-PRIMARY PROCESSES IN PHOTOSYNTHESIS

cule in a special state, which can serve as a "trap"for either the electron or the hole.

Since the quantum yields of the observed creationof trapped electrons (Arnold) and of free electronspinls (Commoner, Calvin) appear to be low, these mayfollow exceptional absorption acts, e.g., in chlorophyllmolecules immediately adjacent to traps. Further-more, effects observed with dried chloroplasts maywell be due to aggregates formed in drying. S. Brody(21) observed that illumination of crystalline chloro-phyll does lead to abundant production of free spins.

Franck suggested that free electrons may be viablein the water-free "protected" parts of the chlorophylllayers, but Franck's mechanism of photosynthesis (13)does not require their occurrence.

Calvin and co-workers (20) believe that photo-chemical separation of charges, followed by electronand hole migration across the lamella can separatethe oxidation products from the reduction productsin photosynthesis. It was often pointed out by thisauthor that this separation is the real crux of photo-synthesis; in non-living photochemical processes,high energy intermediates often are produced, buttend to disappear rapidly by recombination. However,the pigment layers in chloroplasts probably are mono-

molecular; therefore, all that is needed to separatethe oxidation product from the reduction product, isfor them to be produced on different sides of this laver.They could then be drained into alternate interstices,without the need for more electron conductance thanis made possible by the aromatic structure of chlorin.

Electron and hole migration in the plane of thelamella is a different matter. One could conceive ofit as helping to initiate reactions in 2 different enzy-matic sites-the reduction of an appropriate inter-mediate (such as TPN), and the generation of free02. However, this mechanism would be less efficientin preventing recombination; and it, too, is made un-

likely by the above-mentioned spectroscopic evidence.To sum up, we suggest, as a working hypothesis,

that migration of an intramolecular exciton is themost important type of energy migration in the chloro-plast, bringing excitation energy close to the photo-enzymatic center, located in each, or in each few,macromolecules in the lamellae.

We still need to specify the location of the differ-ent types of chlorophyll, as well as of the accessory

IChlxSc -.

Chl-A

FIG. 6. Term scheme of chlorophyll in the fluorescentstate, B; and in the non-fluorescent state, A. Dashed hori-zontal lines represent nit levels, solid lines, n levels.

pigments, particularly in view of Emerson's observa-tions (22) suggesting their distinct photochemicalfunctions. In the case of the phycobilins, it is likelythat their macromolecules also lie in the lamellae.The carotenoids (other than the photosyntheticallyhighly efficient fucoxanthol of brown algae), must bedistributed in a different way; if Platt's picture (23)of them as "electron pipelines" between excited chloro-phyll molecules and electron acceptors (or donors) iscorrect, some carotenoids must be associated with theenzymatic reaction centers.

V. PHOTOCHEMIICAL FUNCTION OF CHLOROPHYLLThere are other, chemical data which require fit-

ting into the picture. These are findings by Kras-novsky and co-workers (24), Rabinowitch and Weiss(25) and Bannister (26) of the capacity of chlorophyllfor reversible photochemical reduction and oxidation,suggesting that chlorophyll sensitizes photosynthesisby undergoing one, or both, of these reactions. Fromthe studies of chlorophyll-sensitized reductions ofvarious oxidants by ascorbic acid in pyridine, it ap-pears (Evstigneev and Gavrilova (27), Bannister(26) ), that chlorophyll is first reduced to an unstableproduct (a free radical ?), which can reduce such un-willing oxidants as riboflavin; if not immediately re-oxidized, it is then transformed into the more stable,probably valence-saturated, reduced form, pink in thecase of chlorophyll a (eosinophyll). Examination ofdifference spectra of illuminated Chlorella cells byColeman and Rabinowitch (28) suggested-but didnot prove-that this last product may accumulate incells-up to 1/300 of total chlorophyll when photosyn-thesis becomes light-saturated. The similarity of thisfigure and the size of the unit suggests that chlorophyllmolecules associated with the reaction centers maybe the ones to become reduced (perhaps, only to un-stable radicals as long as no light saturation occurs,and then to saturated eosinophyll molecules).

This picture of the photophysical and primaryphotochemical stages in photosynthesis is speculative,and may prove wrong in parts or in toto. It seemed,however, tempting to try to develop such a picture fromthe data accumulated by recent research in variouslaboratories.

The experimental work in part utilized in this pa-per was carried out in the Photosynthesis Laboratoryof the Botany Department at the University of Illinoisby E. E. Jacobs, S. S. Brody, M. Brody, P. Latimer,and T. T. Bannister, with the assistance of the Officeof Naval Research and continuous ready and invaluablehelp of the late Dr. Robert Emerson. The electronmicrographs were obtained by A. Vatter at the Elec-tron Microscope Laboratory of the University.

Note added in proof: Rosenberg (Faraday So-ciety Discussion on Energy Transfer, April, 1959)pointed out that, in some molecular melts, strongphotoconductivity is observed without the appearance

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PLANT PHYSIOLOGY

of a new absorption band leading to a conductancelevel; such conductance could result, for example,from coalescence of the metastable triplet levels ofadjoining molecules, so that transfer into the tripletstate, commonly postulated to precede the primaryphotochemical act, becomes identical with transferinto a conductance band. It remains to be seen, how-ever, whether such processes can occur with highefficiency in non-crystalline chlorophyll monolayers.

BIBLIOGRAPHY1. See, for example, Halbleiterprobleme, Vol. I, article

1 (M. VoLz), and Vol. IV, articles 1 (H. HAKEN),and 1 a (W. SCHOTTKY).

2. J. B. THOMAS, K. MINNAERT and P. F. ELBERTS

Acta Botanica Neerl. 5: 315. 1956.3. E. A. HANSON Rec. Trav. Botan. Neerland. 36:

183. 1939.4. E. E. JACOBs, A. S. HOLT, R. KROMIHOUT and E.

RABINOWITCH Arch. Biochem. Biophys. 72: 495.1957.

5. J. C. GOEDHEER, Thesis, University of Utrecht,Utrecht 1957.

6. A. FREY-WYSSLING anid E. STEINMANN, Viertel-jahresshr. naturforsh. Ges. Zurich 98: 20. 1953;E. STEINMANN Exp. Cell. Res. 3: 367. 1952;Experentia VIII 8: 300. 1952.

7. S. S. BRODY and M. BRODY, unpublished.8. A. A. KRASNOVSKY and G. P. BRIN Comp. rend.

(Doklady) acad. sci. USSR, 62: 163. 1948; A. A.KRASNOVSKY and L. M. KOSOBUTSKAYA, ibid., 91:343. 1953; A. A. KRASNOVSKY, L. M. KoSOBUTS-KAYA, ibid., 92: 1201. 1953.

9. J. H. C. SMITH, D. W. KUPKE, J. E. LOEFFLER, A.BENITEZ, I. AHRNE, A. T. GIESE In: Research inPhotosynthesis. P. 464 Interscience New York1957; K. Shibata Jour. Biochemistry (Japan) 44:147. 1957.

10. S. S. BRODY and E. RABINOWITCH Scienice 125:565. 1957.

11. P. LATIMER, T. T. BANNISTER and E. RABINOWITCHScience 124: 585. 1956.

12. S. S. BRODY Science 128: 838. 1958.13. J. FRANCK Article in Handbuch der Pflanzenphysio-

logie. (In press.) J. E. BRUGGER and J. FRANCKArch. Biochem. Biophys. 75: 465. 1958; J. FRANCKProc. Natl. Acad. Sci., U.S. 44: 941. 1958.

14. J. R. PLATT In Radiation Biology. Vol. III p.

71. McGraw Hill, New York 1956.15. H. GAFFRON and K. WOHL Natuirwiss. 24: 81.

1936.

16. J. FRANCK and E. TELLER Jour. Chem. Phys. 6:861. 1938.

17. A. N. TERENIN and V. L. ERMOLAEV Compt. rend.(Doklady) acad. sci. USSR 85: 547. 1952; V.

ERMOLAEV News (Izvestig) Acad. Sci. USSR,ser. fys., 20: 514. 1956; V. L. ERMOLAEV and A. N.TERENIN Jour. chimie physique 55: 698. 1958.

18. W. ARNOLD and H. K. SHERWOOD Proc. Natl. Acad.Sci., U.S. 43: 105. 1957.

19. B. COMMONER, J. TOWNSEND and G. E. PAKENature 174: 689. 1954; B. COMMONER, J. J. HEisE,and J. TOWNSEND. Proc. Natl. Acad. Sci., U.S.42: 71. 1956; B. COMMONER, J. J. HEISE, B. B.LIPPINco,rT, R. E. NORBERG, J. V. PASSONNEAUand J. TOWNSEND Science 126: 57. 1957.

20. D. F. BRADLEY and M. CALVIN Proc. Natl. Acad.Sci., U.S. 42: 710. 1956; C. TOLLIN, P. B. SoGo,and M. CALVIN, Jour. chimie physique 55: 919.1958; M. CALVIN and P. B. SOGo Science 125:499. 1957.

21. S. S. BRODY, unpublished.22. R. EMERSON, R. CHALMERS, and C. CEDERSTRAND,

Proc. Natl. Acad. Sci., U. S. 43: 133. 1956; R.EMERSON and B. CHALMERS Phycological Soc.Bull. 11: 51. 1958.

23. J. R. PLATT, Science 129: 372. 1958.24. A. A. KRASNOVSKY Compt. rend. (Doklady) acad.

sci. USSR, 60: 421. 1948; this and subsequentpapers (up to 1953) by A. A. KRASNOVSKY, V. B.EVSTIGNNEV and co-workers are available in transla-tion by E. RABINOWITCH, AEC-tr-2156, Office ofTechnical Services, Dept. of Commerce, Washing-ton 25, D. C. More recent papers*: Compt. rend.(Doklady) acad. sci. USSR, 95: 841. 1954; 96:1025, 1201 and 1219. 1954; 98: 1017. 1954; 100:131. 1954; 103: 97 and 283. 1955; 104: 440 and882. 1955; 108: 507. 1956; 112: 911. 1957: 114:1066. 1957; also Biokhimiya 18: 340. 1953; 21:126. 1956; News (Izvestia) Acad. Sci. USSRser. biol., 122: 1955; Fizyol. rastenyi 4: 124. 1957:Biofizika. 1: 120. 1956; Zhurnal fyz. khimii 30;968. 1956.

25. E. RABINOWITCH and J. WEISS, Proc. Royal Soc.(London) (A) 162: 251. 1937.

26. T. T. BANNISTER Thesis, Univ. of Illinois, Urbana1958; T. T. BANNISTER In:Proceedings of Endi-cott House Conference on Photochenmical Storageof Energy, Sept., 1957. (In press.)

27. V. B. EvSTIGNNov and V. A. GAVRILOVA Compt.rend. (Doklady) acad. sci. USSR, 92: 381. 1953;95: 841. 1954; 96: 1201. 1954; 100: 131. 1955;103: 97. 1955; 114: 1066. 1957.

28. E. RABINOWITCH In: Proceedings of EndicottHouse Conference on Photochemical Storage ofEnergy, Sept., 1957. (In press.); J. W. COLEMAN,A. S. HOLT and E. RABINOWITCH In: Research inPhotosynthesis. P. 68 Interscience, New York1957. J. W. COLEMAN and E. RABINOWITCH Jour.Phys. Chem. 63: 30. 1959; J. W. COLEMAN,Masters Thesis, Univ. of Illinois, Urbana 1957.

* Many of these available in translations by E. Milner,from the Crerar Library, Chicago.

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