4489
Biological Implications from the Stability of Ternary Complexes in Solution. Mixed-Ligand Complexes with Manganese( 11) and Other 3d Ions Helmut Sigel,* Beda E. Fischer, and Bernhard Prijs Contribution from the Institute of Inorganic Chemistry, University of Basel, Spitalstrasse 51, CH-4056Basel, Switzerland. Receiued November 10, 1976
Abstract: The change in the stability of ternary complexes containing Mn2+ is quantified by A log K u n = log K M n A ~ n-Alog~ K M n ~ , corresponding ,~, to the equilibrium MnA M n B + MnAB Mn (eq 7 ) . For mixed-ligand complexes containing 2.2‘bipyridyl (bpy) and malonate, pyrocatecholate, hydrogen triphosphate, ATP4-, or ITP4-, A log K v n = 0.02 to 0.42, i.e.. 0 donors coordinate at least as well to Mn(bpy)2+ than to Mn(aq)2+, while for bpy-Mn2+-ethylenediamine (en) 1 log K w , < - 1 .O; hence, Mn(bpy)2+ favors 0 over N donors. Both the discrimination and the enhanced stabilities depend on the participation of a heteroaromatic N base like bpy; for en-Mn2+-malonate or -ATP4- A log Kwn is negative, indicating that eq 7 is on its left side. These mixed-ligand Mn2+ systems resemble those with Co2+,Ni2+, Cu2+, or Zn2+: those with Fe2+ or Fe3+ are also thought to be similar. For S donors and for the nitrogen of a deprotonated amide group, an “oxygen like” behavior in mixed-ligand complexes is deduced. In addition, it is suggested that S ligands may also have *-accepting qualities like those of heteroaromatic N bases; imidazole is shown to belong to the latter class. These low-molecular-weight mixed-ligand systems are compared with compiled data on the coordination spheres of naturally occurring complexes; in these the imidazole/O donor (or “ 0 donor like”) combination dominates. This corresponds with the enhanced stabilities and discrimination observed in model systems. Thus selectivity may be influenced by the metal ion and not only by the protein part in, e.g., enzymes, as has been generally suggested. The participation of Co 2 k and Ni2+ in biological systems, the evolution of coordination spheres, and intramolecular ligand-ligand interactions in mixed-ligand complexes are also discussed.
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The specific and distinct macroscopic structures occurring in nature are the result of a molecular order and the information stored in molecules; thus the reactions occurring on the molecular level must be highly specific. This specificity and selectivity2 is partly due to the properties of enzymes, the biological catalysts; many of them contain either “fixed” metal ions or a t least need metal ions to be catalytically a ~ t i v eIn .~ other words, many enzyme reactions are metal ion dependent, so that dialysis or addition of chelating agents causes loss of activity which is regained if further metal ions are added. Examples include the magnesium-, calcium-, and manganese-activated phosphoryl transferases, copper-activated polyphenol oxidase, and zinc-activated alkaline p h o ~ p h a t a s e . ~ The selectivity of such enzymes is often attributed to their protein part,’ but one wonders what influence the metal ion itself has, especially as it has been shown that the reactions proceed often via higher order complexes and in many cases occur within the coordination sphere of the metal The problem may be summarized in the question:1° What are the driving forces that lead to the right, Le., the catalytically active, enzyme-metal ion-substrate(s) complexes? However, the familiarity with metalloenzymes and metal ion activated enzymes tends to divert attention from the dynamic equilibria involving the many other metal-complexing species that are present in biological fluids and living tissue^.^^^ As in the mammalian body the total ligand concentration greatly exceeds the metal content: in the living tissues and fluids the various complexing species compete for the different metal ions present. Under these conditions mixed-ligand complex formation is to be and ternary complexes’0-’2-’4 have been implicated in the storage and transport of metal ions itself and of active substances through membranes.’ Taking everything together, o n e is not surprised anymore about Wood’sI6 conclusion: “If you think that biochemistry is the organic chemistry of living systems, then you are misled; biochemistry is the coordination chemistry of living systems”. As mixed-ligand complexes play such a central role it is worthwhile to assemble information on their formationl7-I9 and stability”-ls and on the mutual influence of two ligands bound to the same metal ion. Among the ternary complexes
containing 3d ions, those with Cu2+ are by far the best studied,l3-l4but for Co2+, Ni*+, and Zn2+ systems some data are also a ~ a i l a b l e . l ~ . ’ ~ .Itl ~is. ~becoming ~-~~ evident that for certain ligand combinations a discriminating behavior as well as an increased stability of the complexes is observed. As the data available for ternary Mn2+ c o m p l e x e ~were ’ ~ too ~ ~ ~ ~ ~ ~ ~ ~ limited to permit any generalization, we have now systematically studied their stability. These results together with earlier ones for the other metal ions allow conclusions about the “self-organizing” qualities that are inherent even in mixedligand systems of low molecular weight and which are evidently important in biological systems.
Experimental Section Materials and Apparatus. Pyrocatechol and malonic acid (both purissimum) were obtained from Fluka AG, Buchs, Switzerland. Sodium t r i p ~ l y p h o s p h a t eethylenediamine ,~~ dihydrochloride,27the other materials, and the apparatus were as described before.’ Determination of Equilibrium Constants. These were measured by automatic potentiometric pH titrations (25 “C; I = 0.1, NaC104),28 titrating 25 mL of aqueous solutions of the reactants under N2 with 0. I N NaOH. Some data from earlier work were also used: the acidity constants of 2,2‘-bipyridyl and the stability constants of its binary V n 2 + c o m p l e ~ e s ,K ~M ~ . n~ A ~ un for~ 2ethylenediamine3’ and pyro~ a t e c h o l a t e , ~as ~ -well ) ~ as some acidity constant^.^^-^^ Those which significantly influence the calculation of the stability constants were from our own earlier ~ o r kand ~ were ~ . confirmed ~ ~ in this study. The concentrations used for the redetermination of the acidity of H2(er1)~+were [H2(en)2+] = 1.5-2.4 X M. The stability constants of the binary Mn2+ complexes were measured under the following conditions: [H2(en)2+] = 2.4 X M and M and [Mn(C104)2] = 2.4or 3.6 X lo-* M; [H2(Mal)] = 1.2 X [Mn2+] = 1.2 or 2.4 X M; [H>(Pyr)] = 1.5 X M and M. The conditions for the ternarysystems [Mn2+] = 1.2 or 3.6 X were [ H ~ ( e n ) ~ = + ]2 X M, [Mal2-] = 1-1.4 X IO-2 M, and [Mn2+] = 1-2 X M ; [H2(Mal)] = 1.2 or 2.4 X M, [bpy] M , a n d [Mn2+] = 4.8 X = 2.4-4.8 X M; [H*(Pyr)] = 1.5-3 X M , and [Mn2+] = 6 X M to I X M, [bpy] = 6 X M ; [H2(en)2+] = 2.4 X M to 1.6 X M , [bpy] = 8 X M . The calculations for the M, and [Mn2+] = 1.2-2.4 X binary and ternary systems were carried out as described before.28 Under our experimental conditions the formation of MAB2 or MA2B is negligible; this was confirmed by Daniele et and can also be seen from the distribution curves of the complex species in dependence
Sigel, Fischer, Prijs
/ Biological Implications f r o m Mixed-Ligand Complexes
4490 Table I. Negative Logarithms of the Acidity Constants of Ligands and Logarithms of the Stability Constants of Their Binary Mn2+ Complexes (25 OC; I = 0.1, NaC104) Ligand
PK~H,A
bPY en Mal2Pyr2HTP4ATP4ITP4-
-0.20 7.15 f 0.01 2.62d 9.32d 2.68 4.06h,' 2.1'
PK~HA
Log
4.476 10.10 f 0.02 5.30d 13.00' 5.50 f 0.01 6.421 6.45 f 0.01
2.62b 2.74 f 0.06 2.72 f 0.03 7.97 f 0.02 4.31 f 0.02 4.70 f 0.02 4.66 f 0.02
Log K M n A M n A 2
KMnMnA
2.00b 2.10c 5.7f
Reference 30. Reference 29. Reference 31 (25 "C; I = 1.4). Reference 28. e Reference 35. f Reference 32. 37. Reference 38a (37 "C; I - 0.3). jReference 38b. kReference 34.
g
Reference 36.
Reference
Table 11. Logarithms of Stability Constants of Ternary Mn2+ Complexes Together with the Values of A Log KM,, (25 OC; I = 0.1, NaC104) Ligande
en bPY bPY bPY bPY bPY bPY en
Mal2Mal2Pyr2en H(TP)4ITP4ATP4ATP4-
5.3 f 0.2 5.36 f 0.07 11.01 f 0.03 44.3 7.23 f 0.03 7.40 f 0.03 7.35 f 0.04 6.6 & 0.2
2.58 2.64 3.04 41.6 2.92 2.74 2.65d 1.9a 1.80 f 0.086
I
2.56 2.74 8.39 41.7 4.61 4.78 4.73
-0.2 0.02 0.42
c- 1 .o
3.8'
-0.9'
0.30 0.12 0.03
Calculated according to procedure I as described in ref 26. Calculated with procedure I1 of ref 26. Average from the results of procedures I and I I . dThis value agrees well with the one determined by Hague and e Reference 34. on pH given in Figure 4 of the work of Scharff and G e n i ~ within ~;~~ the first pH unit in which MAB is significantly formed, [MAB2] and [ MA2BJ are negligible. The stability constant of Mn(en)(ATP)2- was determined from solutions in which [Mn2+] = [ATP4-] = 1.9 or 2.4 X lo-* M and [ H ~ ( ~ I I )=~ +I .8] X M. The data were evaluated by procedures I and I I as described earlier.26 The constants of the systems containing A T P - , I T P - , or H(TP)4were determined as described,' although a twofold excess of bpy was sometimes used, and the reaction solutions had only a volume of 25 tnL; i.e.. the concentrations were twice those given in ref. 1. The equilibrium constants were calculated from a t least six (in average ten) independent titration curves. The range of error given in the tables I S three times the standard error of the mean.
log K
M
B= log ~ P~
~'
~ log~ K ~ M
~ ( 5~) One way to quantify the stability of ternary complexes of the kind studied here is according to eq 612,'4,42 A log K M = log K M A M A B- log K = log K
M
M
~
~
B- log ~ K'r\.l~ ~ ~
(6) i.e., by comparing the difference in stability, A log K M ,e.g., for the reaction between Mn(bpy)2+ or Mn(aq)2+ and pyrocatecholate. In addition, A log K M is identical with the constant for equilibrium 7.
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MA MB +MAB M (7) Results and Discussion Since more coordination positions are available for bonding Mixed-Ligand Systems Containing Mn2+.As an increased by the first ligand than for the second one (eq 6), A log K M is stability of the ternary complexes containing a heteroaromatic expected to be n e g a t i ~ e and , ~ ~ the statistical value for the N base and an 0 donor ligand has been o b ~ e r v e d ' ~ - ' ~ J - ~ ~coordination , ~ ~ - ~ ' of two different bidentate ligands to an octahedral for Co2+,Ni2+, Cu2+,and Zn2+,we used the same combina(=oh) coordination sphere is A log K,h = -0.4.14 tions of ligands for the study of the ternary Mn2+ complexes. The value of A log K M " for the en-Mn2+-Mal system (cf. The acidity constants of the ligands and the stability constants Table 11) is the same as this statistical value within experiof their binary Mn2+complexes are given in Table I. The mental error, while for bpy-Mn2+-Mal A log K M is~ about stability constants of the corresponding mixed-ligand systems zero, suggesting a stabilizing effect of the heteroaromatic N are defined by eq 1-3 and are given in Table 11. The overall base. This is confirmed by the A log K M " values of bpystability constant P M ~which ~ ~was, computed, is related to Mn2+-ATP (+0.03) and en-Mn2+-ATP (-0.9). Comparison the constants K M A ~ * and 6 K M Bby ~eq 4~and~ 5. with the bpy-Mn2+-H(TP) system,34 in which stacking cannot occur, indicates that the increased stability of Mn(bpy) M A B=MAB (ATP)2- is not due to an intramolecular stacking'.44 between (1) P M = [MABl/[Ml[Al ~ ~ ~ [BI bipyridyl and the purine moiety of ATP. LeongZ2obtained the MA+B+MAB same result for bpy-Mn2+-3-oxoglutarate: A log KM,, = log (2) KMn(bPY)~n(bpy) ( olog ~ ~K) M n ~ n ( ~= ~1.18 A ) - 0.90 = K M A = [MABl/ ~ ~ ~[MA1 [Bl +0.28.23 It should be noted that positive values for A log K M M B -I-A + M A B mean that equilibrium 7 is shifted toward the right; hence, li(3) = [MABl/[MBl[Al gands with 0 donors coordinate preferably with the binary log KMAMAB = log P ' ~ , t , 6 - log K M ~ * (4) Mn(bpy)2+ complex compared to the Mn(aq)2+ ion.
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Journal of the American Chemical Society
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99:13
/ June 22, 1977
449 1
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%
"1001
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8
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9
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i
10
PH
Figure 1. Effect of pH on thc concentrations o f the species i n an aqueous solution ( I = 0.1; 15 "C) o f Mn?+.Z.Z'-bipyridyl. and pyrocatecholatc (uppcr part) or ethylenediamine (lower part: [Mn(bpy)z2+] 6 I .7%; [Mn(cn)??+]6 I .3?6).given as the percentage of the total [Mn?+]. [bpy]. and [Pqr] or [en];computcduith theconstantsofTables I and II for IO--' M reactant concentrations. 4s the stability constant for Mn(bpy)(en)*+ is a n upper limit (Table 11). the same is true for its calculated concentration. The dotted line represents [Mn(bp>)(en)'+] under the above conditions but assuming 1 log K = t 0 . 4 . uhich gives then "log d M n ~ n , b p ) ) ( c n ) = 5.76" (cf. text).
PH
Figure 2. Effect of pH on the concentrations of the species in aqueous solutions ( / = 0.1: 25 "C) of binary and ternarq Mn2+systems. given as the percentage of the total [Mn2+]:computed w i t h the constants of Tables I and I I and log K " " ~ n ( ~ =~ 2.39 ~ p (cf. ) ref 37). The dotted lines represent the free ATP species and the solid ones the ATP complexes: twofold protonated ATPcomplexes do i n this pH range not occur.47Upper part: M d t and ?,2'-bipyridyI [dashed lines: [Mn2+],,, = hl. [bp)],,, = 2X M: [Mn(bpq)j?+]< 0.1 I%calculated w i t h log K'4n(bp?1+4n(~p))3 = I.I,cf.ref29] and Mn'+ andATP([Mn"],,, = [ATP],,,, = 10-3 M ) . Middle part: ATP. Mn?+.and 2.?'-bipyridyl [[ATP],,, = [Mn?+],,t = M . [bpy]lu, = 2 X IO-' M: [Mn(bpy)??+] < 2.1%: [Mn(bpq) (HATP)-] 6 2.3% calculated with log K M n l b p ? ) ~ n ( b p ! l ( ~ , , ~ = ~ ) 2.39. cr, ref 481. Lower part: ATP. Mn?+, and ethylenediamine [[ATP],,, = [Mn?+],,, = M. [en],,, = 1 X IO-' M: [Mn(en)>?+]< I.l?6: [Mn(cn)(HATP)-] negligible traces only. caiculated w i t h log KMn(c"JMn(en),H,Tp)= 1.5. cf. ref 481.
The discriminating qualities of Mn(bpy)'+ are also evident from the systems with pyrocatecholate or ethylenediamine: With OL A log K b f n is positive (+0.42; Table I I ) while only an upper limit ( A log K\ln 6-1.O) could be determined with N L . This is also reflected i n the formation of the complex species as a function of pH (Figure I ) . I n the bpy-Mn'+-Pyr system the ternary complex reaches -36.5% of [Mn2+],,,, and in the bpy-Mn?+-en system only -0.5%. of [Mn2+Itol(or of [ATP],,,). The concentration of the latter The low concentration of Mn(bpy)(en)'+ (Figure 1) is not ternary species is nearly the same as that of Mn(bpy)'+ under mainly due to the lower absolute size of the constants P M n ~ n , , ~corresponding conditions ([Mn2+],,, = I O p 3 M. [bpy],,,, = 2 and KMn~'~,n,l~ (see Table I I ) of this system, compared with X IOw3 M) in the binary system: i e., ATP converts Mn(bpy)'+ those of bpy-Mn'+-Pyr,j' but due to the different relative predominantly into the ternary complex. On the contrary, in magnitudes of the constants within these two systems. Indeed, the binary Mn2+-en system Mn(en)2+ reaches about 40% of if 1 log K u n for bpy-Mn'+-en were +0.4, log / 3 M n ~ n ( b p l ) ( , n ) [ Mn2+Itot, while in the corresponding ternary system would be 5.76 and the concentration of this ternary complex Mn(en)(ATP)*- is formed only to about 4.7% (cf. bottom part would be about 13.5% (dotted line in the lower part of Figure of Figure 2). As a consequence, the concentrations of the ATP I ) , i.e., about one-third of [Mn(bpy)(Pyr)]. Thus, the concomplexes in en-Mn*+-ATP (bottom part) are about the same centration of the ternary complex depends more on the position as those in Mn*+-ATP (upper part) Again the discriminating of equilibrium 7 (and on that of eq 8, see later) than on the qualities of Mn(ATP)*- are clearly seen. absolute size of the other constants listed in Tables I and I I . Comparison of the Ternary Mn2+ Systems with Those This is even more evident for the systems bpy-Mn2+-,4TP Containing Co2+, Ni2+, Cu2+, or Zn2+ and Some Tentative and en-Mn'+-ATP: The constants of the binary complexes Conclusions. In the preceding section the stability of ternary . k f ~ ~ ( e nand ) ~ +Mn(bpy)'+ are about the same,J6 but the forcomplexes was quantified by A log K M (eq 6 and 7). The stamation degrees of the ternary complexes are very different tistical value of A log K & for a regular octahedral (oh) coor(middle and bottom parts of Figure 2). Hence, the discrimidination sphere is -0.4. For the distorted octahedron (do) of nating qualities of the Mn2+ complex with a heteroaromatic Cu(aq)*+ and two different bidentate ligands, the statistical N base regarding N L or OL can also be looked at beginning value was deduced as A log Kdo/Cu N -0.9.14 However, besides with M n ( 0 L ) (cf. eq 6). Thus Mn(ATP)'- discriminates beA log K w the "disproportionation" equilibrium 8 can be used tween the two amines which have about the same coordination to quantify the stability of ternary complexes. tendency toward Mn(aq)'++; the heteroaromatic N base is now MA2 MB2 * 2 MAB strongly favored, i.e., by a factor of about 10 corresponding to (8) the difference between the A log K v values. X = [MABI2/[MA2][MB*] It is also worthwhile comparing the binary systems (upper part of Figure 2) with the corresponding mixed-ligand systems The corresponding constant X may be calculated with eq 9. (middle and bottom parts). The concentration of Mn1% = 2 1% P M M A B - (1% OMMA* -k log P'MB2) (ATP)*- drops from about 87% of [Mn2+Itotin the binary = (log KvBvBA - log K M A v A 2 ) (log K M A M A B system to about 50% if 2,2'-bipyridyl is present as well, due to the formation of Mn(bpy)(ATP)'- which reaches about 36% - log K M B ~ (9) ~ J
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Sigel, Fischer, Prijs
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/ Biological Implications f r o m Mixed-Ligand Complexes
4492 Table 111. Examples for the Discriminating Qualities of M(2.2'Bipyridyl)?+, Expressed by the Stability of the Ternary Complexes Formed with Ethylenediamine or Pyrocatecholate, i.e., by the Values due to 1 Log K v (Eq 6 and 7) and Log X (Eq 8 and 9) (25 "C: I = 0.1. NaCI04)"
Table IV. Comparison of the Stability Constants of the Ternary Nitrilotriacetate-M*+-lmidazole Complexes with the Corresponding Data of the Binary M2+-lmidazole Complexes
M"
A-M'+-bpy
A log K,,,,
Log X
Co?+ hi?+
en
6-1.0 -0.27 -0.18 - 1.29 -0.49 0.42 0.76 0.36 0.43 -0.01
