Consequences of Electron Transfer in Chlorophylls, Chlorins, and

3 Consequences of Electron Transfer in Chlorophylls, Chlorins, and Porphyrins Structural and Theoretical Considerations 1

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J. Fajer , K. M. Barkigia , Κ. M. Smithy, and D. A. Goff 1

Department of Applied Science, Brookhaven National Laboratory, Upton, NY 11973 Department of Chemistry, University of California, Davis, CA 95616

Downloaded by EAST CAROLINA UNIV on December 16, 2016 | http://pubs.acs.org Publication Date: October 15, 1986 | doi: 10.1021/bk-1986-0321.ch003

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Crystallographic results are presented for a chlorin, a bacteriopheophorbide d, a nickel isobacteriochlorin derived from chlorophyll a, and for the cation radical of a magnesium porphyrin. These data form part of an increasing body of evidence that demonstrates the significant conformational flexibility of the porphyrin skeleton in the solid and in solution. Since a wide spectrum of photosynthetic and enzymatic reactions proceed via π radicals of porphyrin derivatives, the consequences of the macrocyles' flexibility are considered for distances and orientations between donors and acceptors, interactions with ligands, nearby charges or polar groups, electronic profiles and the gaps between the a and a highest occupied orbitals. 1u

2u

The primary events t h a t f o l l o w p h o t o e x c i t a t i o n of b a c t e r i a l r e a c t i o n c e n t e r s (R.C.) i n v o l v e the p i c o s e c o n d t r a n s f e r o f an e l e c t r o n from a b a c t e r i o c h l o r o p h y l l (BChl) s p e c i a l p a i r to one or more a c c e p t o r s , a l s o comprised o f b a c t e r i o c h l o r o p h y l l d e r i v a t i v e s (BChl and a dem e t a l l a t e d B C h l , b a c t e r i o p h e o p h y t i n , BPheo.)(1,2,3) R e c e n t x-ray d a t a f o r Rhodopseudomonas V i r i d i s R.C.'s (which c o n t a i n BChl b) show the donor and a c c e p t o r chromophores to be h e l d i n c l o s e p r o x i m i t y (~4A edge-to-edge) by s u r r o u n d i n g p r o t e i n r e s i d u e s . ( 4 ) Some o f these a r e i n p o s i t i o n to l i g a t e the c e n t r a l magnesium o f the BChls ( h i s t i d i n e s ) o r to p e r i p h e r a l l y hydrogen bond the 9-keto group o f one o f the BPheos ( g l u t a m i c a c i d ) ( j O . We examine here p o s s i b l e s t r u c t u r a l e f f e c t s t h a t may r e s u l t from o r accompany the g e n e r a t i o n of the primary p h o t o p r o d u c t s , and spec u l a t e about the consequences o f concomitant changes i n d i s t a n c e s , c o n f o r m a t i o n s , r e l a t i v e o r i e n t a t i o n s and charges on the e l e c t r o n i c p r o f i l e s o f and i n t e r a c t i o n s between the B C h l s , BPheos and t h e i r radicals. Because the primary events i n green p l a n t p h o t o s y n t h e s i s a l s o i n v o l v e a s e r i e s o f c h l o r o p h y l l donors and a c c e p t o r s (6) simi l a r trends s h o u l d t h e r e f o r e p r e v a i l f o r c h l o r o p h y l l r a d i c a l s as w e l l . F u r t h e r m o r e , r a d i c a l s o f p o r p h y r i n s and h y d r o p o r p h y r i n s ( s a t u r a t e d p o r p h y r i n s such as c h l o r i n s and i s o b a c t e r i o c h l o r i n s ) have been 9

0097-6156/ 86/ 0321 -0051 $06.00/ 0 © 1986 A m e r i c a n C h e m i c a l Society

Gouterman et al.; Porphyrins ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


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PORPHYRINS:

E X C I T E D STATES A N D D Y N A M I C S

detected or proposed as transients in several enzymatic reactions (7,8); an examination of the structure of porphyrin π radicals i s thus also relevant to b i o c a t a l y t i c reactions. We present f i r s t crystallographic results for porphyrins, chlor ine and isobacteriochlorins that i l l u s t r a t e the wide range of structural variations that can be assumed by the porphyrin skeleton. A n i c k e l isobacteriochlorin which combines structural features related to chlorophylls (photosynthesis), isobacteriochlorins ( n i t r i t e and s u l f i t e reductases), as well as the nickel-containing F430 corphin (methanogenic bacteria) was recently synthesized (9) by the Raney nickel reduction of nickel (II) methylmesopyropheophorbide £ (Figure 1). An x-ray analysis of a single c r y s t a l of the compound reveals a considerable buckling of the macrocycle, which i s i l l u s trated i n Figure 2. These s t r i k i n g distortions of the skeleton are features often observed not only in nickel hydroporphyrins (10,11) but in metal-free derivatives as well (12). Examples of these are provided by the synthetic c h l o r i n , 5, 20-dimethyl, 2, 3- dihydro, 2 ,3*,7,8,12,13,17,18 octaethylporphyrin (dimethyl octaethylchlorin, Me2H2ÛEC), and by the bacteriopheophytin â (methyl 4,5-diethylbacteriopheophorbide ji, BPheo d) derived from the antenna chlorophylls of green photosynthetic bacteria (13). Figure 3 i l l u s t r a t e s the "propeller" shape of the dimethyl c h l o r i n . Note the large displacements of the 3 carbons of the saturated ring and the up (+0.47A) and down (-0.57A) t i l t s of the methyl groups that flank that r i n g . The f l e x i b i l i t y of the chlorophyll skeleton i s further demonstrated in Figure 4 which displays the deviations from planarity observed for BPheo d. The molecules c r y s t a l l i z e as c o f a c i a l dimers ( F i g . 5) formed by hydrogen bonding between the oxygens of the 9-keto groups of rings V and the protons of the hydroxy groups on rings I that distinguish these chlorophylls from those found in green plants and algae (13). Homologues of this compound can be c r y s t a l l i z e d in d i f f e r e n t aggregation modes and d i f f e r e n t conformations(14). The examples cited above represent part of an increasing body of structural information on chlorophylls, chlorins, bacteriochlorins and isobacteriochlorins (10-14 and references therein) that points to the remarkable f l e x i b i l i t y of these molecules. This a b i l i t y of the macrocycle to adjust i s not limited to hydroporphyrins but i s also observed i n porphyrins: 5,10,15,20-tetra-n-propylporphinato lead (II) assumes a "roof" shape by folding along an axis defined by two opposite methine carbons with the two planes of the "roof" inclined at 22° to one another (15). In contrast, t r i c l i n i c 5,10,15, 20tetraphenylporphinato cobalt (II) i s d i s t i n c t l y saddle shaped with the β carbons of adjacent pyrrole rings lying +0.66 and -0.66A above and below the plane of the four nitrogens (16). The f l e x i b i l i t y of these macrocycles becomes even more s i g n i f i cant when the e f f e c t s of oxidation and reduction following electron transfer are taken into account. Recent structural studies of single crystals of meso t e t r a a r y l porphyrin cation radicals reveal that their Zn(II), Cu(II), Fe(III)Cl and Mg(II) complexes are a l l saddle shaped (17-20). Since the unoxidized species are either planar or s l i g h t l y domed, oxidation to the radicals results in a major conformational change. An example of this e f f e c t i s shown in F i g . 6 which presents the displacements of the atoms that comprise the skeleton of the cation radical of Mg 1



3.

FAJER ET AL.

Electron Transfer in Chlorophylls,

Chlorins, and Porphyrins

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2

Figure 3. Deviations (AxlO ) from the plane of the four nitrogens of the dimethyl chlorin 5, 20-dimethyl, 2, 3 trans dihydro, 2', 3 , 7, 8,12,13,17,18 octaethyl porphyrin. (Space group C2/c, Z=8.) f

-II

-15

2

Figure 4. Deviations (AxlO ) from the plane of the four nitrogens of methyl 4, 5-diethylbacteriopheophorbide d . (Space group PI, Z=2).


P O R P H Y R I N S : E X C I T E D STATES A N D D Y N A M I C S


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F i g u r e 1. S t r u c t u r e of the i s o b a c t e r i o c h l o r i n Raney n i c k e l r e d u c t i o n o f n i c k e l ( I I ) methyl mesopyropheophorbide a.

obtained

by the

a

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b

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F i g u r e 2. a) D e v i a t i o n s ( A x l O ) from the l e a s t squares p l a n e d e f i n e d by the f o u r n i t r o g e n s of the n i c k e l i s o b a c t e r i o c h l o r i n . b) An edge-on-view of the molecule t h a t i l l u s t r a t e s the p u c k e r i n g o f the m a c r o c y c l e . S u b s t i t u e n t s have been removed f o r clarity. (Space group: 14^, Z=16).


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FAJER E T A L .

Electron Transfer in Chlorophylls,

Chlorins, and Porphyrins

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0^

Figure 5. Dimer of the bacteriopheophorbide d. The arrows point to the hydrogen bonds between the protons of the 2 - ( l hydroxyethyl) groups and the oxygens of the 9-keto groups.

-4K-57)

-42(-54)

7(-22)

-I0(-I9) -3(-8)

26(5)

27(29)

24(3)

32(34)

2H8) -IO(-22) 45(-57)

-43 (-52) 2

Figure 6. Deviations (AxlO ) from the least squares plane of the macrocycle and, in parenthesis, from the plane of the four nitrogens of the cation radical of magnesium tetraphenyl porphyrin (Space group P2^/c, Z=4) (Reproduced from Ref. 20. Copyright 1983 American Chemical Society.)


PORPHYRINS: E X C I T E D STATES A N D DYNAMICS


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tetraphenylporphin (perchlorato 5,10,15,20-tetraphenylporphlnato magnesium ( I I ) ) (20). The question then arises as to whether these deformations of the porphyrin radicals are simply due to packing forces or whether they are found in solution as well. Electron spin resonance (ESR) and double resonance (ENDOR) results for the cation r a d i c a l of BPheo £ (21) strongly suggest that the saturated rings of the chromophore can twist, in accord with the x-ray data for methy1bacteriopheophorbides a and ά (13,21). ESR data for cation radicals of Co(II) n i t r o s y l chlorine (8) also provide evidence of conformational changes, again, i n good agreement with c r y s t a l l o graphic s t e r i c effects observed i n similar compounds (12 and the c h l o r i n results presented above). If s i g n i f i c a n t variations in conformations can be induced by oxidations, crystals forces, and s t e r i c effects i n solution, simi l a r effects may presumably also be caused by protein environments. What might be the consequences of conformational changes i f they accompany or follow electron transfer reactions i n vivo? In gener a l , they might a l t e r interactions with ligands and other protein residues i n the immediate v i c i n i t y of the porphyrin, i . e . interac tions with charged, polar or aromatic groups. In the cases of porphyrin and chlorin cation radicals, i n which the highest occu pied orbitale (HOMO) are nearly degenerate ( a ^ and a2 f o r symmetry)(2), the gap between the two HOMOs could be s i g n i f i c a n t l y altered i f the radical puckers and the metal-to-ligand distance increases (for an a2 porphyrin cation, i t e r a t i v e extended Huckel calculations (23) suggest that the gap can vary from 0.2 to 0.5 eV). In heme r a d i c a l s , such as the compound I transients found i n catalases and peroxidases, the magnetic coupling between the para magnetic metal (S=l) and the porphyrin radical (S=l/2) may also depend on the structure of the intermediate. An intriguing i n v i t r o sample of such an e f f e c t i s observed i n high spin Fe(III) tetraarylporphyrin π cations: a planar radical exhibits a magnetic s u s c e p t i b i l i t y of s i x unpaired electrons whereas i n a saddle-shaped complex, S=4, i . e . the metal (S«5/2) and the r a d i c a l are ferro and antiferromagnetically coupled, respectively (18,19). Perhaps the most important e f f e c t of conformational variations in electron transfer reactions would be to a l t e r the distances and the r e l a t i v e orientations of donors and acceptors. In photosynthetic RC's, where the primary donors and acceptors l i e within 4-5A of each other (4), small structural displacements (^0.5A) may s i g n i f i c a n t l y a f f e c t rates of back reactions, i f they occur rapidly (24). (Conformational movements on a picosecond time scale are not inconsistent with resonance Raman data on photo-dissociated heme-CO complexes (25^)). On a longer time scale, protein rearrangements triggered by and propagating from the chromophores may also help subsequent reactions such as the transport of protons that i s i n i t i a t e d by the primary photochemical event i n the R.C. (26). The close proximity of the photosynthetic donors and acceptors may also account f o r some of the o p t i c a l changes observed on short time scales ( 1 0 ~ to Ι Ο " sees) following the primary elec tron transfer (1-3). In b a c t e r i a l R.C.'s, this event generates a BChl cation (the special pair) and a BPheo anion (27). According to the x-ray structure of the R. v i r i d i s R.C. (4), these two u

u

u

12

13



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FAJER E T A L .

Electron Transfer in Chlorophylls, Chlorins, and Porphyrins

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radicals are positioned on opposite sides of a BChl b molecule within van der Waals contacts. Theoretical INDO calculations suggest that point charges placed within 3-4A of BChls would induce s i g n i f i c a n t o p t i c a l s h i f t s (14,28). Interestingly, positive charges placed near rings I and IV or negative charges situated near rings II and III are predicted to cause red s h i f t s whereas reversing the charges should r e s u l t i n blue s h i f t s (29). The primary charge separation i n a R.C. should therefore induce a s i g n i f i c a n t e l e c t r o chromic e f f e c t on the o p t i c a l spectrum of the BChl that "bridges" the primary donor and the BPheo. Shifts of more than a thousand wave numbers are predicted for oppositely placed positive and negative point charges (29). Since the charges i n the cation BChl special pair and anion BPheo are delocalized (30), the net e f f e c t w i l l be smaller (at a given distance, the s h i f t i s proportional to the charge). Experimentally, spectral changes i n the bridging BChls have indeed been observed on subpicosecond or picosecond time scales and they have been variously attributed to the formation of a BChl anion, to a charge transfer state or to electrochromic e f f e c t s , as suggested here (1-3). Again, these effects would be susceptible to additional modulation i f changes i n distances or orientations between the chromophores follow electron transfer. In summary, experimental results c l e a r l y indicate that porphyrin derivatives are f l e x i b l e and can undergo rearrangement following electron transfer. If similar conformational changes occur i n vivo as the porphyrins f u l f i l l their b i o l o g i c a l electron transport r o l e s , the changes may help regulate r e l a t i v e orientations and distances between donors and acceptors as well as their inter actions with the protein environment with resulting effects on o r b i t a l occupancies, and on spectral and magnetic properties. A cknowledgments We thank the authors of references 3, 5, 16, 23 and 29 f o r communicating their results p r i o r to publication. This work was supported by the D i v i s i o n of Chemical Sciences, U.S. Department of Energy (Contract DE-AC02-76CH00016) at Brookhaven National Laboratory and by the National Science Foundation (Grant No. CHE81-20891) at the University of C a l i f o r n i a .

Literature Cited 1. "Antennas and Reaction Centers of Photosynthetic Bacteria. Structure, Interactions and Dynamics"; Michel-Beyerle, Μ. Ε., Ed.; Springer-Verlag, Berlin, 1985. 2. Kirmaier, C.; Holten, D.; Parson, W. W. Biochim. Biophys. Acta, 1985, 819, 49-61. 3. Martin, J. L.; Breton, J.; Hoff, A. J.; Migus, Α.; Antonetti, A. Proc. Nat'l. Acad. Sci. U.S.A., 1986, 83, 957-961. 4. Deisenhofer, J.; Epp, O.; Miki, K.; Huber, R.; Michel, H. J. Mol. Biol., 1984, 180, 385-398. 5. Deisenhofer, J.; Epp, O.; Miki, K.; Huber, R.; Michel, H. Nature, 1985, 318, 618-624. 6. Fajer, J.; Fujita, I.; Davis, M. S.; Forman, Α.; Hanson, L. K.; Smith, Κ. M. Adv. Chem. Ser., 1982, 201, 489-513. 7. Hanson, L. K.; Chang, C. K.; Davis, M. S.; Fajer, J. J. Am. Chem. Soc., 1981, 103, 663-670.



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8. Fujita, E.; Chang, C. K.; Fajer, J. J. Am. Chem. Soc., 1985, 107, 7665-7669. 9. Smith, K. M.; Goff, D. A. J. Am. Chem. Soc., 1985, 107, 4954-4964. 10. Kratky, C.; Waditschatka, R.; Angst, C.; Johansen, J. E.; Plaquevent, J. C.; Schreiber, J.; Eschenmoser, A. Helv. Chim. Acta, 1985, 68, 1312-1337. 11. Suh, M. P.; Swepston, P. N.; Ibers, J. A. J. Am. Chem. Soc., 1984, 106, 5164-5171. 12. Barkigia, Κ. M.; Fajer, J.; Chang, C. K.; Williams, G. J. B. J. Am. Chem.Soc., 1982, 104, 315-317. 13. Smith, Κ. M.; Goff, D. Α.; Fajer, J.; Barkigia, K. M. J. Am. Chem. Soc., 1982, 104, 3747-3749. 14. Fajer, J.; Barkigia, Κ. M.; Fujita, E.; Goff, D. A.; Hanson, L. K.; Head, J. D.; Horning, T.; Smith, K. M.; Zerner, M. C. In reference 1, 324-338. 15. Barkigia, Κ. M.; Fajer, J.; Adler, A. D.; Williams, G. J. B. Inorg. Chem., 1980, 19, 2057-2061. 16. Hanson, L. K.; Kabuto, C.; Silverton, J. V.; Kon, H. Private communication. 17. Spaulding, L. D.; Eller, P. G.; Bertrand, J. Α.; Felton, R. H. J. Am. Chem. Soc., 1974, 96, 982-987. 18. Scholz, W. F.; Reed, C. Α.: Lee, Y. J.; Scheldt, W. R.; Lang, J. J. Am. Chem. Soc., 1982. 104, 6791-6793. 19. Buisson, G.; Deronzier, Α.; Duee, Ε.; Gans, P.; Marchon, J. C.; Regnard, J. R. J. Am. Chem. Soc., 1982, 104, 6793-6796. 20. Barkigia, Κ. M.; Spaulding, L. D.; Fajer, J. Inorg. Chem., 1983, 22, 349-351. 21. Horning, T. L.; Fujita, E.; Fajer, J. J. Am. Chem. Soc., 1986, 108, 323-325. 22. Barkigia, Κ. M.; Fajer, J.; Smith, Κ. M.; Williams, G. J. B. J. Am. Chem. Soc., 1981, 103, 5890-5893. 23. Hanson, L. K. Private communication. 24. Marcus, R. Α.; Sutin, N. Biochim. Biophys. Acta., 1985, 811, 265-322. 25. Findsen, E. W.; Scott, T. W.; Chance, M. R.; Friedman, J. M.: Ondrias, M. R. J. Am. Chem. Chem., 1985, 107, 3355-3357. 26. Dutton, P. L.; Prince, R. C. In "The Photosynthetic Bacteria"; Clayton, R. C.; Sistrom, W. R., Eds.; Plenum Press, NY 1978; p. 525. 27. Fajer, J.; Brune, D. C.; Davis, M. S.; Forman, A.;. Spaulding, L. D. Proc. Nat'l. Acad. Sci. U.S.A., 1975, 72, 4956-4960. 28. Eccles, J.; Honig, B. Proc. Nat'l Acad. Sci. U.S.A., 1983, 80, 4959-4963. 29. Hanson, L. K.; Thompson, M.; Zerner, M. C.: Fajer, J. Unpublished results. 30. Davis, M. S.; Forman, Α.; Hanson, L. K.; Thornber, J. P.; Fajer, J. J. Phys. Chem., 1979, 83, 3325-3332. RECEIVED April 3, 1986


Consequences of Electron Transfer in Chlorophylls, Chlorins, and

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