Journal of the American Chemical Society
1920
1 100:6 /
March 15, 1978
Photosynth., 4th in press. Support for the assumption that this technique generates I- comes from picosecond flash experiments5-' which indicate that, in R. viridis reaction centers so treated, photooxidation of P is effectively prevented because all electron acceptors have been reduced: P'I-Xand P'I-X-
P+I-x-
. )
20 pa
PI-x-
This value is clearly distinct from that found" f w P+, g = 2.0026. The small variation in the anion values, 2.0033-2.0036, may reflect the tightness of the ion pairs formed by the anion radicals and their gegenions. For a discussion of mechanisms which influence gvalues of porphyrins, see J. Fajer and M. S. Davis in "The Porphyrins", D. Dolphin, Ed., Academic Press, New York, N.Y.. in press. T. L. Trosper, D. L. Benson, and J. P. Thornber, Biochim. Biopbys. Acta, 460, 318 (1977). V. V. Klimov, V. A. Shuvalov, I. N. Krakhmaleva, A. V. Klevanik. and A. A. Krasnovsky. Biokhimiya, 42, 519 (1977). R. C. Prince, D. M. Tiede, J. P. Thornbet, and P. L. Dutton, Biocbim. Biophys. Acta, 462, 467 (1977). M. C. Thurnauer, J. J. Katz, and J. R. Norris. Roc. Natl. Acad. Sci. U.S.A., 72, 3270 (1975). J. R. Norris, H. Scheer. and J. J. Katz. Ann. N.Y. Acad. Sci., 244, 260 (1975). G. Feher, A. J. Hoff, R. A. Isaacson, and L. C. Ackerson, Ann. N.Y. Acad. Sci., 244, 239 (1975). G. Feher, R. A. Isaacson, and M. Y. Okamura, Biopbys. J., 17, 149a 119771. (24) [a) Diiision of Chemical Sciences; (b) Medical Research Center; (c) Department of Biology. (25) Guggenheim Memorial Fellow, on sabbatical leave from the University of California at Los Angeles.
=-2000 2 1800
4
1600 1400 1200
E-l \
t:
400 200
240 280 320 360 400 440 480 520 560 WAVE LENGTH, nm
Figure 1. Absorption spectrum of Mn02+ ( 0 )at p H 6.0 and 23.5 "C i n an air-saturated 0.5 mM sodium formate solution containing a small amount o f phosphate buffer. Absorption spectrum of M n ( O H ) 2 + (0)at p H 6.3 and 23.5 O C i n an N2O-saturated solution containing sodium phosphate and perchloric acid.
zone and mixer) after irradiation and does not enter into competition reactions with the primary radicals. Also, since in the final solution-mixture the total amount of formate is only 0.5 mM, effects due to Mn2+-HCOO- complex formaJ. Fajer,*248M. S . Davis,Z4a D. C. B r ~ n e ~tion ~ ~are minimized. All chemicals and the water were of A. F ~ r r n a n ?J.~ P. ~ T l ~ o r n b e r ~ highest ~ ~ ~ ~ p~ ~ r i t ymanganese(I1) ;~ sulfate (99.999% purity) was Brookhaven National Laboratory an Apache Chemical Inc. product. The upper limit for scatUpton, New York 1 1973 tered light was of the order of 3% at 210 nm. Received October 21, 1977 The absorption spectrum of MnO2+ (Figure 1) was obtained by monitoring the absorbance at various wavelengths under conditions of constant energy input. The spectrum has been corrected for fluctuations in the beam current which were of the order of 5%. The corresponding molar extinction coeffiProducts of Reaction of Superoxide and cients are normalized values based upon experiments in which Hydroxyl Radicals with Mn2+ Cation the absorbance of Mn02+ at 270 nm gave a linear plot as a function of beam current (which is proportional to the energy Sir: input) and was compared with a similar plot for 0 2 - at 245 nm. We would like to report the absorption spectra of Mn02+ Hence the molar extinction tE$y+ = 3000 f 150 M-' cm-I and Mn(OH)2+ which are the products of the interaction of is based upon the molar absorbance of superoxide r a d i ~ a l , ~ the Mn2+ cation with superoxide and hydroxyl radicals: t&;pn = 2350 f 120 M-] cm-l at 23.5 OC. Control experiments carried out in presence of a wide concentration range Mn2+ 0 2 Mn02+ (1) of MnS04 (2.5-100 mM) suggest that neither Mn2+ nor Mn(OH)2+ S042- affects the molar absorbance of Mn02+. Absence of Mn2+ O H (2) absorbance above 480 nm in the system indicates the absence Although the formation of these species had been correctly of reaction 2. diagnosed in an earlier pulse radiolysis the reported Reaction 2. The spectrum of the reaction product of OH spectra were not well resolved (see Figure 2 in ref 1) and hardly with Mn2+ was determined in the same apparatus as the differed from each other. Because of the importance of these spectrum of Mn02+. A 1 mM MnS04 solution, pH 6.3, satspecies in biological reaction^,^,^ where it had been erroneously urated with N20 was irradiated, mixed with nonirradiated suggested that 0 2 - is capable of oxidizing Mn2+ to Mn3+,we solution, and monitored for absorbance at the various waveinvestigated the system using the stopped-flow radiolysis lengths: technique5 by which reactions 1 and 2 can be studied separately N20 and in the absence of interfering reactions. H20 --+OH, H202, €32 (4a) Reaction 1. The superoxide radical was generated at 23.5 OC by a 2-MeV electron beam impinging on an air-saturated Mn2+ OH Mn(OH)*+ (4b) 1 .O mM sodium formate solution pH IO.O? The molar absorbance of Mn(OH)2+ was determined from a linear plot of the absorbance at 225 nm as a function of energy (3) input and compared with an 0 2 - absorbance vs. energy input curve at 245 nm.All other points in the spectrum were norThe 0 2 - solution (6 pM) was rapidly mixed with a 5 mM malized in terms of the molar absorbance2+)"'&$'' = 1640 f MnS04 solution of such acidity (H2SO4) that the final mixture 80 M-1 cm-l at 23.5 OC. The numerical values of the extincwas at pH 6.0. Effective scavenging of 0 2 - was monitored at tion coefficients of Mn(OH)2+reported here are based upon 270 nm; it was found to be independent of Mn2+ concentration the assumption that G ( O H ) N ~ O = G ( ~ ~ - ) O ~ . H C= OO 6.05, in the range studied (0.5-50 mM). With the stopped-flow where G is the number of primary radicals formed per 100-eV technique one can use relatively low formate concentrations energy dissipated. Since the fate of the H atom is unknown in for the conversion of the primary radicals into 0 2 - , because this system, GH = 0.55, the values of tMn(OH)2+ given in Figure the Mn2+ cation is added -1 5 ms (dead time between radiation
+ +
-
+
0002-7863/78/1.500-1920$01 .OO/O
-
0 1978 American Chemical Society
1921
Communications to the Editor
1 could be low by as much as 17%. In general the spectrum of Mn(OH)2+ determined in this study is in good agreement with an earlier published spectrum of manganic pyrophosphate of p H 6.0.7 As is apparent from Figure 1, the spectrum of Mn02+ is quite different from the spectrum of Mn(OH)2+. With the exception of the small absorbance a t 420 nm, it resembles the spectrum of 0 2 - which has undergone a red shift due to complexing. Since in the present study Mn02+ was formed directly from 0 2 - and Mn2+ and in absence of other oxidizing chemicals, we conclude that the superoxide radical is not capable of oxidizing Mn2+ to Mn3+. Preliminary kinetic studies of the MnOz+ and MnOH2+ species indicate that their decay mechanisms are very complex. For similar initial concentrations the observed ratio of overall half-lives of Mn02+/02- a t pH 6.0 is 10. The decay of MnOH2+ which can be observed for up to 1 s under present experimental conditions is complicated by the formation of a precipitate.
Acknowledgments. We thank Drs. R. Holroyd and H. A. Schwarz for the many stimulating discussions and helpful suggestions. This research was carried out a t Brookhaven National Laboratory under contract with the U S . Department of Energy and supported in part by its Division of Basic Energy Sciences and N I H Grant 1 R01 GM23656-01. References and Notes (1) M. Pick-Kaplan and J. Rabani, J. Phys. Chem. 80, 1840 (1976). (2) D. M. Braun, F. S. Dainton. D. C. Walker, and J. P. Keene, “Pulse Radiolysis”, Academic Press, New York, N.Y.. 1965, p 221. (3) J. T. Curnutte, M. L. Karnovsky, and B. M. Babior, J. Clin. Invest. 57, 1059 (1976). (4) J. Lumsden and D. 0.Hall, Biochem. Biophys. Res. Commun., 64, 595 (1975). (5) B. H. J. Bielski and H. R. Richter, J. Am. Chem. Soc., 99, 3019 (1977). (6) B. H. J. Bielski, Roc. Int. Conf. SingIet Oxygen Relat. Spec. Chem. BioI. (Aug 21-26, 1977). (7) A. Y. Drummond and W. A. Waters, J. Chem. Soc., 435 (1953).
Benon H. J. Bielski* Department of Chemistry, Brookhaven National Laboratory Upton, New York 11973
Phillip C. Chan Department of Biochemistry, State University of New York Downstate Medical Center, Brooklyn, New York 11 203 Received October 27, 1977
Novel Bonding Mode of Diphenylketene to a Transition Metal: a Metal Anchored Olefin System Sir: Diphenylketene has been regarded as an activated olefin in its interaction mode with a transition Nevertheless, Ph C-C=O
7t a I
b
Figure 1. A view of the molecular structure of the dimer [cp2Ti(Ph2C20)]2, showing the atomic numbering scheme. The phenyl rings have been omitted for clarity.
two potential coordination sites (a and b) are, a priori, susceptible to the metal’s attack. The a-bonding attachment of I to a Mn(1) complex2 was recently confirmed by x-ray analysis, while interaction with C=O moiety is invoked as an intermediate in the metal-induced deoxygenation or decarbonylation of ketene^.^-^ The structure here reported shows that diphenylketene is q2-C,0 bonded to the bis(q-cyclopentadienyl)titanium, Ticp2, unit. This mode of attachment gives an anchored free C=C bond, highly affected by the metal. Until now, the T interaction of a ketonic C=O group was restricted to the complexes of hexafluoroacetone with dl0 metals.7 It was reported that the reaction of cp2Ti(CO)2 (11) with diphenylketene gives an orange-yellow monomeric complex, which was formulated as a n olefin-like compound.8 Reaction between I and I1 must be, however, described by the following steps,9 2cp2Ti(C0)2 2Ph2C20 I1 I 60 ‘C [cp2Ti(Ph2C20)]2 4CO (1) d I11 [cp2Ti(Ph2C20)]2 2Ph2C2O 2cp2Ti(Ph2C20)2 (2) I11 IV CP = qS-C5H5
+
+
+
giving 111, orange-yellow,1° and IV,” black, crystalline solids, respectively. Regarding complex 111, the dramatic absence of any C=O band above 1600 cm-I (C=O is at 2093 cm-I in free Ph2C20), together with the observed C=O stretching in the range 1700-1800 cm-’ for M(C=C) coordinated diphenylketene,1%2suggests that CO is involved in the metalketene interaction. An x-ray crystallographic study was required to prove the nature of the product. This was performed on [cp2Ti(PhzC20)]2.2THF ( T H F = tetrahydrofuran). Crystal data: C48H4002Ti2.2C4H80; M = 889; triclinic; a = 9.990 ( l ) , b = 11.058 (2), c = 10.820 (2) A; a = 73.86 (2), /3 = 94.98 (2), y = 98.85 (1)’; = 1; dcalcd = 1.302 g/Cm3; space group P1. Intensity data were collected on an “on-line” single-crystal automated Siemens AED diffractometer using nickel-filtered Cu K a radiation (A = 1.54178 A, 6 < 28
