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Photolysis of methylcobalamin. Nature of the reactive excited state
Journal of Organometallic
Chemistry,
453 (1993) 269-212
269
JOM 23472
Photolysis
of methylcobalamin.
Horst Kunkely November
of the reactive
excited state
and Arnd Vogler
Institut fiir Anorganische (Received
Nature
Chemie,
Unil,ersitiit Regensburg
Unil~ersitiitstrape
.?I, W-8400 Regensburg
(Germany)
13, 1992)
Abstract
Photolysis of methylcobalamin (Co”‘corrin(CH,)L + Hz0 + Oz + Co”‘(H,O)L + H,CO + OH-) shows a pronounced wavelength 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
2. Experimental
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 compounds has been studied extensively, the reactive excited states have in most cases not been identified. Cyanocobalamin seems to be an exception. Evidence was obtained that the photoaquation of cyanocobalamin 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 absorption spectra are dominated by the intense rr* intraligand bands of the corrin ligand. Any other absorptions are obscured by the corrin bands [1,51. The photolysis of alkylcobalamins involves the hou-bond in the primary molysis of the Co”’ -carbon 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-tometal 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.
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 photochemical experiments was triply distilled.
Correspondence
0022-328X/93/$6.00
to: Prof. Dr. A. Vogler.
details
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 pyroelectric radiometer (which was calibrated) equipped with an RkP-345 detector. Progress of the photolysis was monitored by UVvisible spectral measurements with a Shimadzu UV2100 spectrophotometer. The photoproduct aquocobal0 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 accompany 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 avelength. the quantum yield drops. ‘l‘his ricct-case i% not tnonotonotts.
Between
33.1 and
4%
ntn.
;t [tlateau
reached
A
0.8
0.4
0.0
I
300
I
400
500
nm
600
is
H. Kunkely, A. I/ogler / Photolysis of methylcobalamin
regular position. It appears with reduced intensity. The other band appears at shorter wavelength. Well-documented 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 and located at a2U symmetry in Ddh metalloporphyrins 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 = tetraphenylporphyrin 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 expected to occur at comparable energies and can mix efficiently. A similar hyper spectrum was also observed for [Ir “‘(OEP)(C,H ,,)(CO)] with OEP = octaethylporphyrin [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,, uorbital of the axial ligands is quite stable and occurs at much lower energies than the a2,, rr porphyrin orbital, mixing will be rather small. LLCT/rr r * (porphyrin) 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 complicated by the lower symmetry of the corrin [1,51. Fortunately, 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 cyanocobalamin show a regular (or “typical”) [l] ~TT~T*corrin
271
spectrum (Soret band h,,, = 361 nm> while alkylcobal[I] or, in the terminology of amins are “atypical” porphyrins, are hyper-type [16]. On the basis of the quantum-yield profile of methylcobalamin and in analogy 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 corrin transitions with decontribution to the rrr* Calculations on methylcobalamins creasing energies. [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 complications that can be avoided by using porphyrin complexes as suitable models. Acknowledgment Support of this research by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie is gratefully acknowledged. References 1 J.M. Pratt, Inorganic Chermstry of Vitamin EIz, Academic Press, London, 1972. 2 D.G. Brown, Prog Inorg. Chem., 18 (1973) 177. 3 G.N. Schrauzer, Angew. Chem.. ht. Ed. Eng., 15 (1976) 417. 4 H.P.C. Hogenkamp, in D. Dolphin (ed.), B,,, Wiley, New York, 1982. p. 295. 5 C. Giannotti, in D. Dolphin (ed.), B,,, Wiley, 1982, p. 393. 6 P.J. Toscano and L.G. Marzilli, Prog. Inorg. Chem., 31 (1984) 105. 7 A. Vogler, R. Hirschmann, H. Otto and H. Kunkely, Ber. Bunsenges. Phys. Chem., NO (1976) 420. 8 (a) A.J. Thomson, J. Am. Chem. Sot., 91 (1969) 2780; (b) M. Gardiner and A.J. Thomson, J. Chem. Sot., Dalton Trans., (1974) 820. 9 (a) J.F. Endicott and G.J. Ferraudi, J. Am. Chem. Sot., 99 (1977) 243: (b) J.F. Endicott and T.L. Netzel, J. Am. Chem. Sot., 101 (I 979) 4000. in A.W. Adamson and P.D. Fleischauer feds.), 10 J.F. Endicott, Concepts of Inorganic Photochemistry, Wiley, New York, 197.5, p. 81. 11 G.L. Geoffroy and MS. Wrighton, Organometallic Photochemis@,, Academic Press, New York, 1979. 12 H.G. Ah, Anger+. Chem., Int. Ed. Engl., 23 (1984) 766. 13 R.T. Taylor, L. Smucker, M.L. Hanna and J. Gill, Arch. Biochem. Biophys.. 156 (1973) 521. 14 A. Voyler and H. Kunkely, Comments Inorg Chem., 9 (1990) 201. 15 M. Kaupp, H. Stall, H. Preuss, W. Kaim, T. Stahl, G. van Koten, E. Wissing, W.J.J. Smeets and A.L. Spek, J. Am. Chem. Sot., 113 (1991) 5606.
H K.;!M'M,. t . I 7<./,'r " Phozo(wis qf tin'lily!, ot,al.nius
222
lh M. ( } o u t c r m a r l . ill l), I ) o l p h i n led.). /71¢ Pou~le','~in~. \cadcmic' [Jrt.".;s. Nc~<'~ Y o r k . 197X. Vol. 111. |:'ai-I \ . p I 17 I..K. t|anson. \V,:\. l}:thm. ~ ( } . Sligal. 1.(. (itli~suht% M. (]OtJtCFI1Hill aIlC! ( . ] . ( ' l m u c l l . ] I ~ : f/U'*J, .VU~. OS (]')76) 2672. lS ( i . l l Ioc~'< ~l~lct M - M , t~.Mm~cl, ./ t , ( k , v n P'~' (I~)XI!) 3655. Iq M, N4pp,t amcl . I S \:lh.:nHi/<.:. I .tts/ (/w;Jt <",,,< . 0{7 {ItJ7N) 5O57. 2(} .l,'v, Nt~Itio. M. Stun,, uild ,1 tt. l)>OlX ]H
M
".'h~ik I
(k,'~+
~,,, . I)
24 M. S~itoh. ~ ()hbu ?", %am,mchi und M. h~ai>'un~i. /nor:~. C & m . . .,'l(199!) 2clS, 25 I . S a l e m , (), [~isct~slcin N , ] ' . \ u h . t{,[:L t:hlrgi, ,\. I)L'\:lqUCt, (~ "~Cg;ll ~llld A< \,ctiil~Hl,k \'¢)[t; ; , ! L){~:.~i "4~, 2"7' ,,\. 'vcill:~Id ;i~!ct 15 Pti!imarl. ,/ 771<>,t t;io!, h' { i?1~5)~(17, 2S 1' I M > [7i~,.,i ('hi,'H .-l~&t,. 7 tlgf'~7).:~,]{-,. 1~(). ()llCllh;lI[I/. t<~1t ()lit:lib lrt," ;iFJd ',,l.hl [:tm!:..7 A . i ( M ' n l 3¢J(. ~l~ {97{}j ~]U{)i~ {<~> { i . N Sctu:am~c? X,zeen.~
Chemistry,
453 (1993) 269-212
269
JOM 23472
Photolysis
of methylcobalamin.
Horst Kunkely November
of the reactive
excited state
and Arnd Vogler
Institut fiir Anorganische (Received
Nature
Chemie,
Unil,ersitiit Regensburg
Unil~ersitiitstrape
.?I, W-8400 Regensburg
(Germany)
13, 1992)
Abstract
Photolysis of methylcobalamin (Co”‘corrin(CH,)L + Hz0 + Oz + Co”‘(H,O)L + H,CO + OH-) shows a pronounced wavelength 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
2. Experimental
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 compounds has been studied extensively, the reactive excited states have in most cases not been identified. Cyanocobalamin seems to be an exception. Evidence was obtained that the photoaquation of cyanocobalamin 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 absorption spectra are dominated by the intense rr* intraligand bands of the corrin ligand. Any other absorptions are obscured by the corrin bands [1,51. The photolysis of alkylcobalamins involves the hou-bond in the primary molysis of the Co”’ -carbon 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-tometal 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.
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 photochemical experiments was triply distilled.
Correspondence
0022-328X/93/$6.00
to: Prof. Dr. A. Vogler.
details
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 pyroelectric radiometer (which was calibrated) equipped with an RkP-345 detector. Progress of the photolysis was monitored by UVvisible spectral measurements with a Shimadzu UV2100 spectrophotometer. The photoproduct aquocobal0 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 accompany 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 avelength. the quantum yield drops. ‘l‘his ricct-case i% not tnonotonotts.
Between
33.1 and
4%
ntn.
;t [tlateau
reached
A
0.8
0.4
0.0
I
300
I
400
500
nm
600
is
H. Kunkely, A. I/ogler / Photolysis of methylcobalamin
regular position. It appears with reduced intensity. The other band appears at shorter wavelength. Well-documented 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 and located at a2U symmetry in Ddh metalloporphyrins 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 = tetraphenylporphyrin 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 expected to occur at comparable energies and can mix efficiently. A similar hyper spectrum was also observed for [Ir “‘(OEP)(C,H ,,)(CO)] with OEP = octaethylporphyrin [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,, uorbital of the axial ligands is quite stable and occurs at much lower energies than the a2,, rr porphyrin orbital, mixing will be rather small. LLCT/rr r * (porphyrin) 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 complicated by the lower symmetry of the corrin [1,51. Fortunately, 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 cyanocobalamin show a regular (or “typical”) [l] ~TT~T*corrin
271
spectrum (Soret band h,,, = 361 nm> while alkylcobal[I] or, in the terminology of amins are “atypical” porphyrins, are hyper-type [16]. On the basis of the quantum-yield profile of methylcobalamin and in analogy 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 corrin transitions with decontribution to the rrr* Calculations on methylcobalamins creasing energies. [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 complications that can be avoided by using porphyrin complexes as suitable models. Acknowledgment Support of this research by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie is gratefully acknowledged. References 1 J.M. Pratt, Inorganic Chermstry of Vitamin EIz, Academic Press, London, 1972. 2 D.G. Brown, Prog Inorg. Chem., 18 (1973) 177. 3 G.N. Schrauzer, Angew. Chem.. ht. Ed. Eng., 15 (1976) 417. 4 H.P.C. Hogenkamp, in D. Dolphin (ed.), B,,, Wiley, New York, 1982. p. 295. 5 C. Giannotti, in D. Dolphin (ed.), B,,, Wiley, 1982, p. 393. 6 P.J. Toscano and L.G. Marzilli, Prog. Inorg. Chem., 31 (1984) 105. 7 A. Vogler, R. Hirschmann, H. Otto and H. Kunkely, Ber. Bunsenges. Phys. Chem., NO (1976) 420. 8 (a) A.J. Thomson, J. Am. Chem. Sot., 91 (1969) 2780; (b) M. Gardiner and A.J. Thomson, J. Chem. Sot., Dalton Trans., (1974) 820. 9 (a) J.F. Endicott and G.J. Ferraudi, J. Am. Chem. Sot., 99 (1977) 243: (b) J.F. Endicott and T.L. Netzel, J. Am. Chem. Sot., 101 (I 979) 4000. in A.W. Adamson and P.D. Fleischauer feds.), 10 J.F. Endicott, Concepts of Inorganic Photochemistry, Wiley, New York, 197.5, p. 81. 11 G.L. Geoffroy and MS. Wrighton, Organometallic Photochemis@,, Academic Press, New York, 1979. 12 H.G. Ah, Anger+. Chem., Int. Ed. Engl., 23 (1984) 766. 13 R.T. Taylor, L. Smucker, M.L. Hanna and J. Gill, Arch. Biochem. Biophys.. 156 (1973) 521. 14 A. Voyler and H. Kunkely, Comments Inorg Chem., 9 (1990) 201. 15 M. Kaupp, H. Stall, H. Preuss, W. Kaim, T. Stahl, G. van Koten, E. Wissing, W.J.J. Smeets and A.L. Spek, J. Am. Chem. Sot., 113 (1991) 5606.
H K.;!M'M,. t . I 7<./,'r " Phozo(wis qf tin'lily!, ot,al.nius
222
lh M. ( } o u t c r m a r l . ill l), I ) o l p h i n led.). /71¢ Pou~le','~in~. \cadcmic' [Jrt.".;s. Nc~<'~ Y o r k . 197X. Vol. 111. |:'ai-I \ . p I 17 I..K. t|anson. \V,:\. l}:thm. ~ ( } . Sligal. 1.(. (itli~suht% M. (]OtJtCFI1Hill aIlC! ( . ] . ( ' l m u c l l . ] I ~ : f/U'*J, .VU~. OS (]')76) 2672. lS ( i . l l Ioc~'< ~l~lct M - M , t~.Mm~cl, ./ t , ( k , v n P'~' (I~)XI!) 3655. Iq M, N4pp,t amcl . I S \:lh.:nHi/<.:. I .tts/ (/w;Jt <",,,< . 0{7 {ItJ7N) 5O57. 2(} .l,'v, Nt~Itio. M. Stun,, uild ,1 tt. l)
M
".'h~ik I
(k,'~+
~,,, . I)
24 M. S~itoh. ~ ()hbu ?", %am,mchi und M. h~ai>'un~i. /nor:~. C & m . . .,'l(199!) 2clS, 25 I . S a l e m , (), [~isct~slcin N , ] ' . \ u h . t{,[:L t:hlrgi, ,\. I)L'\:lqUCt, (~ "~Cg;ll ~llld A< \,ctiil~Hl,k \'¢)[t; ; , ! L){~:.~i "4~, 2"7' ,,\. 'vcill:~Id ;i~!ct 15 Pti!imarl. ,/ 771<>,t t;io!, h' { i?1~5)~(17, 2S 1' I M > [7i~,.,i ('hi,'H .-l~&t,. 7 tlgf'~7).:~,]{-,. 1~(). ()llCllh;lI[I/. t<~1t ()lit:lib lrt," ;iFJd ',,l.hl [:tm!:..7 A . i ( M ' n l 3¢J(. ~l~ {97{}j ~]U{)i~ {<~> { i . N Sctu:am~c? X,zeen.~