Journal of Organometallic Chemistry, 453 (1993) 269-212 269
JOM 23472
Photolysis of methylcobalamin. Nature of the reactive excited state
Horst Kunkely and Arnd Vogler
Institut fiir Anorganische Chemie, Unil,ersitiit Regensburg Unil~ersitiitstrape .?I, W-8400 Regensburg (Germany) (Received November 13, 1992)
Abstract
Photolysis of methylcobalamin (Co”‘corrin(CH,)L + Hz0 + Oz + Co”‘(H,O)L + H,CO + OH-) shows a pronounced wave- length dependence. It is suggested that the reactive excited state is of the ligand-to-ligand charge transfer (LLCT) type and involves the promotion of an electron from the Co-C u-bond to a r*(corrin) orbital. This LLCT transition mixes with the 7;z-*(corrin) transitions. Owing to this LLCT contribution, the rrr*-absorption bands are also photoactive hut with reduced efficiency.
1. Introduction
Light sensitivity is one of the outstanding features of vitamin B,, and its derivatives such as the cobalamins [l-6]. Although the photochemistry of these com- pounds has been studied extensively, the reactive ex- cited states have in most cases not been identified.
Cyanocobalamin seems to be an exception. Evidence was obtained that the photoaquation of cyanocobal- amin is initiated by excited ligand field states [7,8].
Generally, the identification of reactive excited states of cobalamins is hampered by the fact that their ab- sorption spectra are dominated by the intense rr*
intraligand bands of the corrin ligand. Any other ab- sorptions are obscured by the corrin bands [1,51.
The photolysis of alkylcobalamins involves the ho- molysis of the Co”’ -carbon u-bond in the primary photochemical step [9]. By analogy with simple Co”’
complexes [lo], especially those with a Co-C bond [11,12], including cobaloximes [3,5], it may be assumed that the reactive excited states are of the ligand-to- metal charge transfer (LMCT) type. However, for alkylcobalamins, clear spectroscopic evidence for such an assignment has not yet been obtained. The present investigation was undertaken to explore the nature of the reactive excited state of methylcobalamin.
Correspondence to: Prof. Dr. A. Vogler.
0022-328X/93/$6.00
2. Experimental details
2. I. Materials
Methylcobalamin was purchased from Aldrich and used as received. Its absorption spectrum (A”,;,, = 267 nm, e = 16,300; A,,, = 282 nm, E = 15,400; A,,, = 290 nm, E = 14,000; A,.,, = 317 nm, E = 11,000; A,,,, = 342 nm, E = 11,700; A”,;,, = 376 nm, E = 9600; A,, = 432 nm, E = 3100; A,, = 495 nm, E = 6450; A,,, = 523 nm, E = 760; A,, = 556 nm, E = 5100) agreed well with that reported previously [13]. The water used in the photo- chemical experiments was triply distilled.
2.2 Photolyses
The light source was an Osram HBO 100 W/2 lamp. The mercury lines at 254, 280, 313, 333, 366, 436, 546, and 577 nm were selected by use of Schott PIL/IL interference filters. Solutions of methylcobalamin were photolyzed in l-cm spectrophotometer cells at room temperature. For quantum yield determinations the concentrations of methylcobalamin were such as to give essentially complete light absorption. The total amount of photolysis was limited to less than 5% to avoid light absorption by the photoproduct. Absorbed light intensities were determined by a Polytec pyroelec- tric radiometer (which was calibrated) equipped with an RkP-345 detector.
Progress of the photolysis was monitored by UV- visible spectral measurements with a Shimadzu UV- 2100 spectrophotometer. The photoproduct aquocobal-
0 1993 - Elsevier Sequoia S.A. All rights reserved
amin was identified by its absorption speclrutn (A,,,*,, cr 350 ntn, t = 3.hOO).
3. Results and discussion
Co”‘(corrin)( CH :)L ---+ C’o”(corritt)l_. 4 .(‘M I in the absence of oxygen an cfficicnt regcneralion of mcthylcobalat7iin takes place. while in the prcscnco of oxygen an irrc\,crsiblc product f(wma~ior-r occ’ur\
[l&o]:
Co”‘( corrin)( C‘H :)I_. t 0, + El ,O ~---+
(‘o”‘(corrin)(l~l,O)I.- IH,C‘O i- Oil As indicated by the spectroscopic changes that ac- company the photolyais (Fig. 1 ). the photc,con\crsion clt methylcobalamiii to ;tcluocoh~ilat7iiti is ;t wry clean rc- action that can hc driven to completion. Intcrchtinply.
the quantum yield i5 not indcpcndcnt of the irradiating wavelength (‘I‘ablc 1 ). Thix quantum yield profile has obscrxd qualitatively by Taylor P/ tri. / 1.31. hut the variations observed in the pracnt stud) arc I~LICII
larger. and should be useful for the identification and
characterization r~f the rextivc excited \tatt‘. ‘1%~
quantum yield maximum coincides with tlic absorption maximum at A == 3 i 7 nm (Fig. 1 J. Towards lcxipcr u ave- length. the quantum yield drops. ‘l‘his ricct-case i% not tnonotonotts. Between 33.1 and 4% ntn. ;t [tlateau is reached
A 0.8
0.4
0.0 I I
300 400 500 nm 600
H. Kunkely, A. I/ogler / Photolysis of methylcobalamin 271
regular position. It appears with reduced intensity. The other band appears at shorter wavelength. Well-docu- mented cases are p-type hyperporphyrins with metals such as Sn2+, Pb2+ and Sb’+ which possess an extra electron pair in their pz valence orbital [16,17]. It is of a2U symmetry in Ddh metalloporphyrins and located at relatively high energies. The azU (p,) to e, r*
(porphyrin) metal-to-ligand charge transfer (MLCT) transition mixes with the azU rr to ep r* intraligand porphyrin transition. Accordingly, a split Soret band is observed.
Six-coordinate metalloporphyrins M(porphyrin)LL’
may be also of the hyper type if the axial ligands provide appropriate filled orbitals. Cytochrome P-450 displays a characteristic hyper spectrum with a split Soret band. It consists of a longer wavelength 7~7~*
absorption at 450 nm and a shorter wavelength LLCT band which is assigned to the transition from the axial mercaptide ligand to the rTT* orbitals of the porphyrin [14,16-201.
Typical hyper spectra are apparently also displayed by organometallic db metalloporphyrins of the form M(porphyrin)(R)L with R = alkyl [21]. For example, Co”‘(TPP)R(pyridine) with TPP = tetraphenylporphy- rin and R = CH,, C,H, or C,H, show the split Soret band at 370 and 430 nm. We suggest that both bands originate from two a2” + eg transitions which are of the mixed LLCT (R to porphyrin)/intraligand rr~i’
(porphyrin) type. The axial ligands are characterized by a q-orbital of azU symmetry, which is derived from the t ,” orbitals in O,, symmetry. In the case of the alkyl complexes, this a2,, orbital should occur at rather high energies owing to the presence of the cobalt-carbon a-bond. Accordingly, both a2” + e, transitions are ex- pected to occur at comparable energies and can mix efficiently. A similar hyper spectrum was also observed for [Ir “‘(OEP)(C,H ,,)(CO)] with OEP = octaethyl- porphyrin [22]. The influence of a-donation by axial ligands on the a,, rr porphyrin orbital seems to be of general significance [23,24]. However, if the a,, u- orbital of the axial ligands is quite stable and occurs at much lower energies than the a2,, rr porphyrin orbital, LLCT/rr r * (porphyrin) mixing will be rather small.
The spectrum is then regular as it is observed for many other d6 metalloporphyrins [ 161.
Let us now return to methylcobalamin. Although the corrin ligand is related to the porphyrin ring, detailed assignments of absorption bands are compli- cated by the lower symmetry of the corrin [1,51. Fortu- nately, the basic pattern of the rrr* spectra is similar to that for the porphyrins [1,5]. Cobalamins display a Q band that consists of (Y and /3 components and a B or Soret (y) band. Some compounds such as cyanocobal- amin show a regular (or “typical”) [l] ~TT~T* corrin
spectrum (Soret band h,,, = 361 nm> while alkylcobal- amins are “atypical” [I] or, in the terminology of porphyrins, are hyper-type [16]. On the basis of the quantum-yield profile of methylcobalamin and in anal- ogy to alkylmetalloporphyrins (see above), we assign the band at A,,, = 317 nm to the LLCT transition from the cobalt-carbon a-bond to the porphyrin r*
orbitals. The absorptions at A,,, = 342 and 376 nm are then assigned to the Soret transition which, however, has considerable LLCT character, as indicated by their photochemical activity. The wavelength-dependence of the quantum yield seems to reflect a decreasing LLCT contribution to the rrr* corrin transitions with de- creasing energies. Calculations on methylcobalamins [25] seem to support our conclusions. However, the interpretation of the electronic spectra [25-301 does not lead to unambiguous assignments [1,5]. The low symmetry of the corrin ligand introduces serious com- plications that can be avoided by using porphyrin com- plexes as suitable models.
Acknowledgment
Support of this research by the Deutsche Forsch- ungsgemeinschaft and the Fonds der Chemischen In- dustrie is gratefully acknowledged.
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