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Vi Cation Radicals of Ferrous and Free Base Isobacteriochlorins: Models Proc. Natl. Acad. Sci. USA Vol. 78, No. 5, pp. 2652-2656, May 1981 Chemistry vi cation radicals of ferrous and free base isobacteriochlorins: Models for siroheme and sirohydrochlorin (nitrite and sulfite reductases/enzymatic intermediates/macrocycle oxidations/electron spin resonance/molecular orbital calculations) C. K. CHANGa, L. K. HANSONb, P. F. RICHARDSONb, R. YOUNGa, AND J. FAJERbC bDepartment of Energy and Environment, Brookhaven National Laboratory, Upton, New York 11973; and aDepartment of Chemistry, Michigan State University, East Lansing, Michigan 48824 Communicated by Gerhart Friedlander, January 9, 1981 ABSTRACT Theoretical and experimental optical, redox, and C02H paramagnetic results are presented for models of siroheme, the COH iron isobacteriochlorin prosthetic group of nitrite and sulfite re- HO20 X H X~~~~~~HH ductases, and of sirohydrochlorin, the metal-free siroheme that H3C' N N _CO2H N. is an intermediate in the biosynthetic pathway to vitamin B12. The facile oxidation of many isobacteriochlorins, which distinguishes them from porphyrins and chlorins, suggests that the siroheme HOC O2H macrocycle itself may undergo oxidation in the multi-electron en- CO2H CO2H zymatic cycles that reduce nitrite to ammonia and sulfite to hy- a b drogen sulfide. Extended Huckel MO calculations (i) help ration- alize the redox properties of isobacteriochlorins compared with FIG. 1. Structural formulas ofsiroheme (a) and 2,4-Me2Et8iBC (b). those of porphyrins and chlorins; (ii) indicate that Fe(IH) pyridine (See Fig. 4 for structures of the other isomers.) carbonyl [(py) (CO)] complexes ofisobacteriochlorins, unlike those of porphyrins and chlorins, should undergo oxidation from the macrocycle rather than the metal to yield i1 cation radicals; (iii) sented preliminary results suggesting that oxidation of some suggest that, in hexacoordinated Fe(II) isobacteriochlorin com- Fe(II)iBC complexes leads not to Fe(III) species but rather to plexes, the site of oxidation-i.e., the metal or the macrocycle- abstraction of an electron from the ir system to give Fe(II) ir will depend on the ligand field induced by the axial ligands; and cation radicals (24). (iv) predict similar unpaired spin density profiles for metal-free We describe here charge-iterative extended Huckel MO cal- and (py) (CO)Fe(H) isobacteriochlorin radicals. Experimental data culations that (i) provide a rationale for the redox properties of for three isomeric free-base and (py) (CO)Fe(II) complexes of di- iBCs compared with those ofporphyrins, chlorins, and bacter- methyloctaethylisobacteriochlorins support the theoretical calcu- iochlorins (BCs); (ii) indicate that ferrous pyridine-carbonyl lations and establish the existence of Fe(II) isobacteriochlorin 17 cations in vitro. [(py) (CO)Fe(II)] complexes ofiBCs, unlike those ofporphyrins or chlorins, should yield 7r cation radicals on oxidation; and (iii) Isobacteriochlorins (iBCs), porphyrins in which two adjacent predict that the unpaired spin density profiles ofmetal-free and pyrrole rings are reduced, have recently elicited (1-9) consid- (py) (CO)Fe(II)-iBC radicals should be similar. Experimental erable interest because the prosthetic groups of sulfite and ni- redox, optical, and ESR data presented herein for three iso- trite reductases contain siroheme (10), an iron-iBC that has meric models ofsiroheme and sirohydrochlorin support the the- eight carboxylic acid side chains (Fig. la). The enzymes catalyze oretical calculations and unambiguously establish the existence (10) the six-electron reductions ofsulfite to hydrogen sulfide and of Fe(II)-iBC ir cations. nitrite to ammonia. In green plants, the latter reaction, N02 + 8H+ + 6e -- NH4+ + 2H20, is light driven with photo- METHODS synthetically reduced pyridine dinucleotide serving as electron The three isomeric iron dimethyloctaethyl iBCs [Fe(II)- donor to the active enzyme (11): Me2Et8iBCs]-2,8-dimethyl-2',8'-dihydro-3,3',7,7', 12,13, 17, 18-octaethylporphyrin [1,4-Fe(II)-Me2Et8iBC]; 3,8-dimeth- Chlorophyll -+ NADPH -+ flavoprotein yl-3',8'-dihydro-2,2',7,7',12,13,17,18-octaethylporphyrin [2, -- ferredoxin -+ nitrite reductase. 4-Fe(II)-Me2Et8iBC] (see Fig. lb); and 3,7-dimethyl-3', 7'-dihydro-2,2',8,8', 12,13,17,18-octaethylporphyrin [2,3- An additional biological role has been attributed (12, 13) to Fe(II)-Me2Et8iBC]-and their free bases were synthesized as iBCs with the realization that sirohydrochlorins, demetallated described by Chang (5) and by Chang and Fajer (3). The cation sirohemes, are intermediates in the biosynthetic pathway ofthe radicals ofthe free bases were obtained by one-electron transfer corrin, vitamin B12. to the radical of zinc tetraphenylporphyrin (14) Zn- A salient feature of the iBC skeleton is its ease of oxidation Ph4PORtClO4O. The (py) (CO)Fe(II) radicals were generated and difficulty of reduction compared with those of porphyrins by oxidation with iodine (3). The techniques used for and chlorins (1-3, 8). This facile oxidation has led us to propose obtaining the ESR spectra have been described (15). (1-6) that the siroheme macrocycle itself may undergo redox reactions in the multi-electron enzymatic reductions ofthe sub- Abbreviations: py, pyridine; Ph4POR, tetraphenylporphine; iBC, iso- strates to ammonia and hydrogen sulfide, and we have pre- bacteriochlorin; Me2Et8iBC, dimethyloctaethylisobacteriochlorin; BC, bacteriochlorin; HOMO, highest occupied molecular orbital; LUMO, The publication costs ofthis article were defrayed in part by page charge lowest unoccupied molecular orbital; E112, polarographic half-wave payment. This article must therefore be hereby marked "advertise- potential. ment" in accordance with 18 U. S. C. §1734 solely to indicate this fact. CTo whom reprint requests should be addressed. 2652 Downloaded by guest on September 29, 2021 Chemistry: Chang et al. Proc. Natl. Acad. Sci. USA 78 (1981) 2653 -7fF -8 g9 eg Or w z -J am -10 00r - Pa - pyrrole N p' pyrrole N Pa, pyrrole N -.-pyrrole-N- -Ha~~~~~~~~~~~~~~~yrlNyrlp&_|/lLT______+aup Olu.(T(7r) II 2(7r)_.-.. '2u(71 -*-2--(7T)- --- - * a2u(r) "a o(r)l$ Y- -fF- flY Y.. f Y- A ,fAd_X rVTf-,m. z.9V.0orrmmergy levelloual AiaVOMuagram.LurfarwthOe I\ HOMOs and LUMOs ofporphine, chlorin', -121 Zn iBC, and BC complexes of Zn(II). Oxida- Nr n tion results in abstraction of an electron from an "a" (ir) orbital whereas reduction PORPHINE CHLORIN ISOBACTERIOCHLORIN BACTERIOCHLORIN adds an electron to the lowest ir* orbitals. The self-consistent charge-iterative extended Huckel pro- ment (1, 2, 14, 24-26). In addition, as the ligand is saturated, gram and parameters have been presented by Gouterman and the energy required for oxidation should parallel the energy of coworkers (16, 17). Iterations were continued until the esti- the HOMOsf and decrease in the order: (hard) porphine mated (input) and calculated (output) charges for each atom >chlorin >iBCBC (easy), as is observed experimentally (Ta- agreed to within 0.015 electrons. Coordinates for the aromatic ble 1). Conversely, Fig. 2 suggests that iBC will be the hardest portions of the porphine, chlorin, iBC, and BC skeletons are to reduce and that the other three complexes shouldhave nearly given in ref. 18. The bond distances ofthe reduced pyrrole rings equal reduction potentials. This conclusion is also verified ex- in chlorin, iBC, and BC were set at 1.452 A for C,3-C,3(6) and perimentally for a series oftetraphenyl and octaethyl derivatives 1.500 A for CO.Ca,(6). In the Zn complexes, the Zn atom was (Table 1 and refs. 1-3 and 8). displaced 0.33 A from the macrocycle plane (6, 19, 20) with The energy differences between the HOMOs and LUMOs Zn-N(pyrrole) = 2.08 A. In the Fe complexes, the Fe was kept shown in Fig. 2 also provide an indication of the energy of the in plane, with Fe-N(pyrrole) = 2.01 A, Fe-N(saturated pyrrole) first absorption band ofeach complex. This difference increases = 2.03 A, Fe-N(pyridine) = 2.10 A (21), and Fe-CO = 1.77 in the order BC < iBC-chlorin <porphine and is reflected in A (21). the position of the lowest energy absorption bands for each tetraphenyl species-Zn-Ph4BC, A = 755 nm (= 1.64 eV); RESULTS AND DISCUSSION Zn-Ph4iBC, A = 602 (2.06); Zn-tetraphenylchlorin, A = 610 Calculated energy levels for the highest occupied (HOMO) and (2.03), and Zn-tetraphenylporphine, A = 585 (2.12). (See Table lowest unoccupied (LUMO) MOs of porphine, chlorin, iBC, 1 for a similar comparison for the metal-free compounds.) and BC complexes of Zn(II) are shown in Fig. 2. (For clarity, To a first approximation, these energy differences can also the D4h symmetry labels ofa,. and a2. for the two highest filled be related (28) to the polarographic redox span for formation of Xr orbitals of porphines have been retained for all the macro- the cation and anion radicals (neglecting solvation terms) such cycles.) As the porphyrin skeleton is saturated, the a2. iorbitals that the energy of the first absorption band Eab, AE, where remain effectively isoenergetic whereas the a,. orbitals are pro- AE+ = E1/2 (ox) - E1/2 (red) and E1/2 is the polarographic half- gressively raised.d The degenerate eg(ir*) orbitals of the por- wave potential. This comparison is made in Table 1 and roughly phine split for chlorin, iBC, and BC without altering the LUMO parallels the trend expected from the molecular orbital scheme. energies ofthe chlorin and BC relative to those ofporphine. For The particularly easy oxidations of the iBCs raise the possi- the iBC, the LUMO is raised significantly. Similar energy level bility that sirohemes mediate electron transport to the substrate trends have been noted in other calculations (17, 22, 23). via X- cation radicals ofthe porphyrin moiety. We demonstrate Because oxidation of the macrocycles involves abstraction of now that such Fe-iBC XT cations can indeed be generated in electrons from the HOMOs, examination ofFig. 2 suggests that vitro. The absorption spectrum;of the pyridine-carbonyl com- electron removal should occur from the a2.
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