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The stereoselectivity enhancement by metal ions in chiral micellar-promoted deacylation of long chain amino acid esters
ofMo&cuLr Cata&usip.l2(1981) 393 - 396 0 Ekevier Sequoia S-4_. Lausaone - Printed in The NstherIands
Journal
Letter to the Editor
The
promoted
by Met&
Enhankent
Stered&~ DeacyIation
OH-KUBOf
KATSUTOSHI
of Lofig
Chain
aud NORIKO
Deparhnent of SynCketic CFtemis~. Kumamoto 860 [Japrr)
Amino
Ions
irt Chid
MicelIar-
Acid Esters
MATSUMOTO
Faculty of Enginee~ring. Rumamoto
Ilniuersity.
(Received March 3.1981)
AIthough
one signScant
enzyme
model reaction has recentIy centered
around the stereoselective micellarcatalyzed hy&oIysB of amino acid esters with optically active functional&d sur factants [I - 61 cx with camicelkr systems.of uor D)-hi&dine derivatives and surfactants [7 - 121, there are few documents dealing with the stereoselective &erase activity of metalloIQ&&. In this paper, we wish fa report the rde of metal ions in the stereoselective deacyIation of NdecanoyIamino acid p-nitrophenyl esters (DecAla. DecVal, DecLeu, and DecPhe) by CorniceLIes of Ndodecanoyl-Lhistid.ine (DodHis) and (R)-(+)-Nu-methylbenzyl-~J+&di.methylhexadecyL ammonium bromide (MBzHAB) with and without divaIent metal ions (Zn”’ , Co2+, Ni2+, Ch2+, Ca”+ I and MP): enzyme
CH,
(CHf,)BCONEICHfR)C02CsH4N02-p Dec&
(R = CH3)
DecVal
(R = (CH,),CH)
DecLeu
jR = (CH3t2CHCH2)
DecPhe
(R = CBH&H,)
CllHS
CONH6HCH2 I C02H DodE-Iis
72
N
C,6HS&(CH,)26H{CHs)C,HgBrMBzHAB
The deacylation of 5.0 X lam5 M esters by comiceIles of 9.0 X LOW5 M DodHis (cmc s 5 X 10B5 M) and 2.5 X 10-a M MB&AI3 (cmc s 2 X 10S4 M) was txrried out wit.5 (0.4 - 5) x 10S4 M meti ions in 0.08 M Tris buffer including Cl.08 M KC2 at 25 “C (pH 7.61) in 10% (V/V) CH3CN/H20. The rati consbnts (k* or ksurfacrwith or without DodElis respectively) were abtahed from the dosely peudo-flrSt_otier deacylakion f&lowed by the spectrophotometrii= determinatkn of p-&rophenolate formation (400 nm), and the
*A&or
to whom co-pondence
shouId be addressed_
394
secondorder catalytic rate constant (k,,) was evaluated from k,, = (k~ k ~Jr~ct)/[DodHislJ as an average value in experiments repeated three or four iimes under identical conditions. In the present experiments, the stereaselective ability of the chiral MI3zHA.S surfactant per SE was negligibly small (k kr~ctlk~~racr = l-00 - 1.06), even though the comicelles of DadHis and MEkzHAI3 show higher selectivity (reflected in the enantiomer rate ratio of k2a,,!k$) than those of DcdHis and hexadecyltrimethy!amonium bromide 1123 The effects of metal ions on the deacylation rate and stereoselectivity were first examined in the DecPhe deacylation by the presnt comicellar system (DodHis f MBzHAB) with divalent zinc ion, and the experimental results a_reshown in Fig. 1. The deacylation rate of DecPhe gradually decreased with increasing the Znz* concentration, probably through the shielding of the imidazolyl3N-position -in DodHis by Zn’+, and the extent of rate decrease was different between the L- and D-enantiomers. Con-
Fig. l_ Concentration effects of Zn2+ on DecPhe deacylation. IDecPhe] = 5.@ x 10e5 M, [MBzHAB] = 2.5 x 10m3 M, uld in Tris buffer at 25 72 (pH 7.61) in 10% (v/v) CH$X/H20.
[DodHis] = 9.C! x 10m5 M. [%I*~ J = (0 - 4.5) x Ibe M
sequently, the emantiomer rate ratio (k&, /kgt) reflecting the stereoselectivity increased with increasing Zn** concentration up to [Zn”‘] /[DodHis] = 3. Thus, the metal ion enhanced the stereoselectivity by changing the reaction circumstances (including asymmetric environment) through its coordination to the nucleophile and/or the ester substrate. A relatively high metal-ion concentration with respect to that of the nucleophile was required for metalion compiexation on the micellar surface because of the electrostitic repulsion between the metal cation and the cationic micelles. The contribution of divalent metal ions to selectivity enhancement was also recognized in the DecPhe deacylation with different metal ions (Co*+ Ni*+ , Cu*+ , Ca*+ , and Mn2*). and all the metal ions tested except Ca** &cl Mn*+ elevated the
395
Sterwsekctivity change caused by div&nt His + MB2EA.B comicell&
Zn
co Ni CU
ca Mn none
198 278 258 243 263 268 335
mew
56 77 82 76 98 103 113
ior~~in DecPhe dezcyktion
with Dad-
3.54 3.61 3.10 8.27 2.68 2.60 2.96
a’%.s buffer (0.08 M) including 0.08 M KCl at 25 ‘C! (pH 7.61) in 10% (v/v) Ci-Z3CN/H20r [DodXis] = 9.0 K 1OH5 M, [DeePhe] = 5.0 x 10e5 Mi. [MBzHAB] = 2.5 x IO-” M. and [M-l = 2.7 x lO-4 M.
selectivity through complexation with DodHis and/or DecPhe (Table I); the selectivity decrease with Ca2+ and Mn*+ might be attributable to insufficient coordination of both the metal ions to DecPhe and/or DodHis [13]. Since the pK, values of metal aquo complexes, such as Zn(OH,)~*, CO(OI-&)~~, etc., or imidazoIe(Im)coordinated metal aquo complexes, such as Im-Zn (OH&T1 , Im-Co(OK,)$?,, etc., are higher or around 9.0 [l3, 141, the nucleophilic attack of ‘metal hydroxide’ or ‘imidazole-coordinated metal hydroxide* to the carbonyl carbon atom in the ester might be unnecessary under present pH conditions (pK 7.61). In this relation, the rate change by the nccleophihc attack of hydroxide anions to the me’&l-ion coordinated ester substrate is also negligible in the present reaction, because the deacylation rate of DecPhe was not appreciably influenced by metal ions in the absence of DodHis. It is deduced, therefore, that the coordination of ester substrate to the imidazole (in DodHis) bound metal ion on the micellar SUTface, or the incorporation of m&&ion coordinated ester into the comicelles resuh-s in a different substratenucleophile apprcximation which changes stereoselectivity. The extent of selectivity enhancement through metal-ion complexation was, however, directly affected by structural differences in the ester substrates_ In the deacylation of N-acetylphenylalanine p-nitrophenyl ester (AcPhe) with and without Zn2+ under identical conditions, the extent of the selectivity increase was negligibly small (k,&J& = 1.79 in the D = 1 _81 in the presence of Zn2+). This imphes absence of Zn*+ and k&$,/kCat that such ester substrates as AcPhe possessing very short hydrophobic N-acyl chains are ‘nzclly incorporated into the comicehes and are not sufficiently exposed to the asymmetric environment on the micehar surface, even in the presence of metal ion. Therefore, the incorporation of ester substrates in& choral cornice&s is important for promoting metal-ion complexation with substrate and nucleophile so as to change the substitenucleophtie approximation. TabIe 2 also indicates the difference in selectivity enhancement of
396 TABLE
2
Stereaseleetivity
Ester
enhancement
acid sters eeused by divalent zinc ions”
io the absence 0E ZP*+
in ttre pence
k,jikP
k’JM-l
LDecAla DecVal DecLeu DecPhe
of amino
s--1 D
89 19
221
335
kk#D,
L
73 16
70
113
“The reaction conditions
1.06 1.21
3.16
2.96
are the -we
Sl
of Zn’+ f&/k%
D 40 14
22 8
121
37
198
56
1.82 1.75
1.72 I.45
3.27
1.03
3-54
1.17
as in Table 1.
DecAla, DecVal, DecLeu, and DecPhe by the divalent zinc ion. The extent of the selectivity increase followed the crder DecAla > DecVal > DecPhe = DecLeu. The stereoselectivity (and/or deacylation rate) of DecAla or DecVal was qujte low compared with DecLEu or DecPhe due to insufficient sub&rat-e-nucleophile approximation through the weak hydrophobic interaction between the CHs (in DecAla) or (CH3)&H (in DeVal) group and the imidazolyl group (in DodHis). However, me+&-ion complexation with DodHis and DecAla (or DecVal) helps to increase the approximation by decreasing the deacylation rate of the Lenantiomer rar;her than that of the Denantiomer thus increasing stireoselectivity. Acknowledgement The present work was supported by a Grant-in-Aid fcr Scientific Research from the Ministry of Edac&ion of Japan (Grant No. 5553251. References 1 J. M. 2 J. M. 3 R. A. 4 R. A. FI R. A. 6 R. A. 7 8
9 10 11 12 13 14
Brown and C. A. Bunton, J. Chem. Sot. Chem. COIRIRUR.. (1974) 969. E~~QFII,C. A\.Blrnton end S. D&z, J, Chem. Sot. Chem. Commurt., (1974) 971. Moss, T. J. Lukas and R. C. Nahas, Tetrtzf.zdron Lett., (1977) 3851. Moq R. C. Nahas and T. J. Lukas, Tetmhedron Len.. (1978) 507. Mw, Y.S. Lee and T. J. Luka.s. J. Am. Chem. Sot.. ZOZ (1979) 2499. Moss, Y.S. Lee and K. W. Alwis,J. Am. Chem. SC-.. 102 (1980) 6648. Y. Ihara. J. Chew Sot_ Chem. Common.. (1978) 984. K. Yamada, H. Shosenji and H. Shara, C%emisLry Leti.. (1979) 491. K. Yamada, H. Shosenji, H. Exra and Y. Ohtsubo, Tetiuhedrorr Letl. (1979) 2529. K. Ohkubo, K. Sugzhara. K. Yoshinaga and R. Ueoka, J. Chem. Sot. Chem. Commrc~., (1980) 63i. K. Ohkubo, K. Kawazoe, M. Tokuda ani R. Ueoka. J. &faE. Catal.. 9 (1980) 219. K. Ohkubo, K. Sugahara, H. Ohta. K:Tokuda and R. Ueoka, B&l. Chem. Sot. J.R. 51(1981) 576. M. A. Wells and T. C. Bruice, J. Am. CheR. Sot.. 99 (1977) 1083. D. A. Buckingham, in A. W. Addison, W. R. CuUen, D. Dolphin, B. R. James (eds.), Biological Aspecta of hzoqunic Chemist, wiey, New York. 1877, Chap. 5.
Journal
Letter to the Editor
The
promoted
by Met&
Enhankent
Stered&~ DeacyIation
OH-KUBOf
KATSUTOSHI
of Lofig
Chain
aud NORIKO
Deparhnent of SynCketic CFtemis~. Kumamoto 860 [Japrr)
Amino
Ions
irt Chid
MicelIar-
Acid Esters
MATSUMOTO
Faculty of Enginee~ring. Rumamoto
Ilniuersity.
(Received March 3.1981)
AIthough
one signScant
enzyme
model reaction has recentIy centered
around the stereoselective micellarcatalyzed hy&oIysB of amino acid esters with optically active functional&d sur factants [I - 61 cx with camicelkr systems.of uor D)-hi&dine derivatives and surfactants [7 - 121, there are few documents dealing with the stereoselective &erase activity of metalloIQ&&. In this paper, we wish fa report the rde of metal ions in the stereoselective deacyIation of NdecanoyIamino acid p-nitrophenyl esters (DecAla. DecVal, DecLeu, and DecPhe) by CorniceLIes of Ndodecanoyl-Lhistid.ine (DodHis) and (R)-(+)-Nu-methylbenzyl-~J+&di.methylhexadecyL ammonium bromide (MBzHAB) with and without divaIent metal ions (Zn”’ , Co2+, Ni2+, Ch2+, Ca”+ I and MP): enzyme
CH,
(CHf,)BCONEICHfR)C02CsH4N02-p Dec&
(R = CH3)
DecVal
(R = (CH,),CH)
DecLeu
jR = (CH3t2CHCH2)
DecPhe
(R = CBH&H,)
CllHS
CONH6HCH2 I C02H DodE-Iis
72
N
C,6HS&(CH,)26H{CHs)C,HgBrMBzHAB
The deacylation of 5.0 X lam5 M esters by comiceIles of 9.0 X LOW5 M DodHis (cmc s 5 X 10B5 M) and 2.5 X 10-a M MB&AI3 (cmc s 2 X 10S4 M) was txrried out wit.5 (0.4 - 5) x 10S4 M meti ions in 0.08 M Tris buffer including Cl.08 M KC2 at 25 “C (pH 7.61) in 10% (V/V) CH3CN/H20. The rati consbnts (k* or ksurfacrwith or without DodElis respectively) were abtahed from the dosely peudo-flrSt_otier deacylakion f&lowed by the spectrophotometrii= determinatkn of p-&rophenolate formation (400 nm), and the
*A&or
to whom co-pondence
shouId be addressed_
394
secondorder catalytic rate constant (k,,) was evaluated from k,, = (k~ k ~Jr~ct)/[DodHislJ as an average value in experiments repeated three or four iimes under identical conditions. In the present experiments, the stereaselective ability of the chiral MI3zHA.S surfactant per SE was negligibly small (k kr~ctlk~~racr = l-00 - 1.06), even though the comicelles of DadHis and MEkzHAI3 show higher selectivity (reflected in the enantiomer rate ratio of k2a,,!k$) than those of DcdHis and hexadecyltrimethy!amonium bromide 1123 The effects of metal ions on the deacylation rate and stereoselectivity were first examined in the DecPhe deacylation by the presnt comicellar system (DodHis f MBzHAB) with divalent zinc ion, and the experimental results a_reshown in Fig. 1. The deacylation rate of DecPhe gradually decreased with increasing the Znz* concentration, probably through the shielding of the imidazolyl3N-position -in DodHis by Zn’+, and the extent of rate decrease was different between the L- and D-enantiomers. Con-
Fig. l_ Concentration effects of Zn2+ on DecPhe deacylation. IDecPhe] = 5.@ x 10e5 M, [MBzHAB] = 2.5 x 10m3 M, uld in Tris buffer at 25 72 (pH 7.61) in 10% (v/v) CH$X/H20.
[DodHis] = 9.C! x 10m5 M. [%I*~ J = (0 - 4.5) x Ibe M
sequently, the emantiomer rate ratio (k&, /kgt) reflecting the stereoselectivity increased with increasing Zn** concentration up to [Zn”‘] /[DodHis] = 3. Thus, the metal ion enhanced the stereoselectivity by changing the reaction circumstances (including asymmetric environment) through its coordination to the nucleophile and/or the ester substrate. A relatively high metal-ion concentration with respect to that of the nucleophile was required for metalion compiexation on the micellar surface because of the electrostitic repulsion between the metal cation and the cationic micelles. The contribution of divalent metal ions to selectivity enhancement was also recognized in the DecPhe deacylation with different metal ions (Co*+ Ni*+ , Cu*+ , Ca*+ , and Mn2*). and all the metal ions tested except Ca** &cl Mn*+ elevated the
395
Sterwsekctivity change caused by div&nt His + MB2EA.B comicell&
Zn
co Ni CU
ca Mn none
198 278 258 243 263 268 335
mew
56 77 82 76 98 103 113
ior~~in DecPhe dezcyktion
with Dad-
3.54 3.61 3.10 8.27 2.68 2.60 2.96
a’%.s buffer (0.08 M) including 0.08 M KCl at 25 ‘C! (pH 7.61) in 10% (v/v) Ci-Z3CN/H20r [DodXis] = 9.0 K 1OH5 M, [DeePhe] = 5.0 x 10e5 Mi. [MBzHAB] = 2.5 x IO-” M. and [M-l = 2.7 x lO-4 M.
selectivity through complexation with DodHis and/or DecPhe (Table I); the selectivity decrease with Ca2+ and Mn*+ might be attributable to insufficient coordination of both the metal ions to DecPhe and/or DodHis [13]. Since the pK, values of metal aquo complexes, such as Zn(OH,)~*, CO(OI-&)~~, etc., or imidazoIe(Im)coordinated metal aquo complexes, such as Im-Zn (OH&T1 , Im-Co(OK,)$?,, etc., are higher or around 9.0 [l3, 141, the nucleophilic attack of ‘metal hydroxide’ or ‘imidazole-coordinated metal hydroxide* to the carbonyl carbon atom in the ester might be unnecessary under present pH conditions (pK 7.61). In this relation, the rate change by the nccleophihc attack of hydroxide anions to the me’&l-ion coordinated ester substrate is also negligible in the present reaction, because the deacylation rate of DecPhe was not appreciably influenced by metal ions in the absence of DodHis. It is deduced, therefore, that the coordination of ester substrate to the imidazole (in DodHis) bound metal ion on the micellar SUTface, or the incorporation of m&&ion coordinated ester into the comicelles resuh-s in a different substratenucleophile apprcximation which changes stereoselectivity. The extent of selectivity enhancement through metal-ion complexation was, however, directly affected by structural differences in the ester substrates_ In the deacylation of N-acetylphenylalanine p-nitrophenyl ester (AcPhe) with and without Zn2+ under identical conditions, the extent of the selectivity increase was negligibly small (k,&J& = 1.79 in the D = 1 _81 in the presence of Zn2+). This imphes absence of Zn*+ and k&$,/kCat that such ester substrates as AcPhe possessing very short hydrophobic N-acyl chains are ‘nzclly incorporated into the comicehes and are not sufficiently exposed to the asymmetric environment on the micehar surface, even in the presence of metal ion. Therefore, the incorporation of ester substrates in& choral cornice&s is important for promoting metal-ion complexation with substrate and nucleophile so as to change the substitenucleophtie approximation. TabIe 2 also indicates the difference in selectivity enhancement of
396 TABLE
2
Stereaseleetivity
Ester
enhancement
acid sters eeused by divalent zinc ions”
io the absence 0E ZP*+
in ttre pence
k,jikP
k’JM-l
LDecAla DecVal DecLeu DecPhe
of amino
s--1 D
89 19
221
335
kk#D,
L
73 16
70
113
“The reaction conditions
1.06 1.21
3.16
2.96
are the -we
Sl
of Zn’+ f&/k%
D 40 14
22 8
121
37
198
56
1.82 1.75
1.72 I.45
3.27
1.03
3-54
1.17
as in Table 1.
DecAla, DecVal, DecLeu, and DecPhe by the divalent zinc ion. The extent of the selectivity increase followed the crder DecAla > DecVal > DecPhe = DecLeu. The stereoselectivity (and/or deacylation rate) of DecAla or DecVal was qujte low compared with DecLEu or DecPhe due to insufficient sub&rat-e-nucleophile approximation through the weak hydrophobic interaction between the CHs (in DecAla) or (CH3)&H (in DeVal) group and the imidazolyl group (in DodHis). However, me+&-ion complexation with DodHis and DecAla (or DecVal) helps to increase the approximation by decreasing the deacylation rate of the Lenantiomer rar;her than that of the Denantiomer thus increasing stireoselectivity. Acknowledgement The present work was supported by a Grant-in-Aid fcr Scientific Research from the Ministry of Edac&ion of Japan (Grant No. 5553251. References 1 J. M. 2 J. M. 3 R. A. 4 R. A. FI R. A. 6 R. A. 7 8
9 10 11 12 13 14
Brown and C. A. Bunton, J. Chem. Sot. Chem. COIRIRUR.. (1974) 969. E~~QFII,C. A\.Blrnton end S. D&z, J, Chem. Sot. Chem. Commurt., (1974) 971. Moss, T. J. Lukas and R. C. Nahas, Tetrtzf.zdron Lett., (1977) 3851. Moq R. C. Nahas and T. J. Lukas, Tetmhedron Len.. (1978) 507. Mw, Y.S. Lee and T. J. Luka.s. J. Am. Chem. Sot.. ZOZ (1979) 2499. Moss, Y.S. Lee and K. W. Alwis,J. Am. Chem. SC-.. 102 (1980) 6648. Y. Ihara. J. Chew Sot_ Chem. Common.. (1978) 984. K. Yamada, H. Shosenji and H. Shara, C%emisLry Leti.. (1979) 491. K. Yamada, H. Shosenji, H. Exra and Y. Ohtsubo, Tetiuhedrorr Letl. (1979) 2529. K. Ohkubo, K. Sugzhara. K. Yoshinaga and R. Ueoka, J. Chem. Sot. Chem. Commrc~., (1980) 63i. K. Ohkubo, K. Kawazoe, M. Tokuda ani R. Ueoka. J. &faE. Catal.. 9 (1980) 219. K. Ohkubo, K. Sugahara, H. Ohta. K:Tokuda and R. Ueoka, B&l. Chem. Sot. J.R. 51(1981) 576. M. A. Wells and T. C. Bruice, J. Am. CheR. Sot.. 99 (1977) 1083. D. A. Buckingham, in A. W. Addison, W. R. CuUen, D. Dolphin, B. R. James (eds.), Biological Aspecta of hzoqunic Chemist, wiey, New York. 1877, Chap. 5.