7 Chelates of Ascorbic A c i d Formation and Catalytic
Properties
ARTHUR E. MARTELL
Downloaded by UNIV OF GUELPH LIBRARY on June 5, 2012 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0200.ch007
Department of Chemistry, Texas A&M University, College Station, TX 77843
Ascorbic acid, H L, is a relatively weak bidentate ligand, which coordinates metal ions, M , to form chelates, MHL at low and intermediate p H values, and unprotonated chelates, ML at high pH. Metal ions capable of undergoing redox reactions catalyze the autoxidation of ascorbic acid through the formation of intermediate metal 2
n+
+n-1
+n-2
-ascorbate-dioxygen complexes. Catalysis of autoxidation by metal chelates seems to occur through the formation of ternary ascorbate complexes of the metal chelates. Ascorbic acid is assigned a significant catalytic role in Udenfriend's system through the formation of an initial ascorbateFe(III)-dioxygen complex in which electron transfer to dioxygen results in oxygen activation and oxygen atom insertion.
Ascorbic acid, 1, is a dibasic acid with a bifunctional ene-diol group built into a heterocyclic lactone ring (1). Although the dissociation constants of the ene-diol hydroxyls are increased somewhat over normal values by the electron-withdrawing oxygen atoms on the adjacent 1- and 4-positions, the acidity of ascorbic acid is due mainly to resonance stabilization of the monoanion (2), which distributes the negative charge between the oxygens at the 1- and 3-positions, as indicated by 2a and 2a'. Such stabilization is not possible when the 3-hydroxyl is not ionized. Formula 2b therefore represents a higher energy form that does not contribute appreciably to the structure of the monoanion. The undissociated hydroxyl group of the monoanion may be hydrogen bonded to either of the adjacent negatively charged oxygens at the 1- and 3-posi tions, as indicated by 2a and 2a'. The high acidity of the 3-hydroxyl group can be readily understood by analogy with carbonic acid mono esters, with which it has a vinylogous relationship. The nature of the monoanion has been well-characterized by x-ray crystallographic studies 0065-2393/82/0200-0153$06.00/0 © 1982 American Chemical Society In Ascorbic Acid: Chemistry, Metabolism, and Uses; Seib, P., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
154
ASCORBIC ACID
of its salts (2,4) a n d its m e t a l complexes
(3,5) as w e l l as b y I R (6)
a n d N M R ( 7 ) studies o f t h e l i g a n d a n d its m e t a l complexes. T h e s e c o n d p K c o r r e s p o n d i n g t o t h e c o n v e r s i o n o f 2a t o 3 i s r e l a t i v e l y h i g h ( ~ 11.3) because o f t h e n e g a t i v e c h a r g e o n 2a, a n d h y d r o g e n b o n d i n g t o t h e n e g a t i v e oxygens at t h e 1- a n d 3-positions. B o t h effects t e n d to increase t h e s t a b i l i t y o f t h e m o n o a n i o n r e l a t i v e t o t h e c o m pletely dissociated form. T h e b i n e g a t i v e e n e - d i o l a n i o n o f a s c o r b i c a c i d , L " , is a b i d e n t a t e 2
l i g a n d a n d is c a p a b l e o f r e a c t i n g w i t h m e t a l ions M * o f c o o r d i n a t i o n n
n u m b e r 4 o r 6 t o f o r m a series o f complexes M L * " " , M L " " ; o r M L * " " , Downloaded by UNIV OF GUELPH LIBRARY on June 5, 2012 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0200.ch007
2
2
+
4
2
M L * ~ , a n d M L * " ; respectively. C o m p a r i s o n w i t h analogous ligands n
2
4
n
3
6
h a v i n g s i m i l a r p K ' s i n d i c a t e s t h a t t h e stabilities o f t h e 1:1
ascorbate
chelates o f d i v a l e n t t r a n s i t i o n metals s h o u l d b e i n t h e r a n g e o f 10 -10 . 5
10
T h e s t a b i l i t y constant d a t a a v a i l a b l e f o r a s c o r b i c a c i d , l i s t e d i n T a b l e I , i n d i c a t e that o n l y r e l a t i v e l y v e r y w e a k chelates h a v e b e e n r e p o r t e d . F o r t h e " n o r m a l , " f u l l y d e p r o t o n a t e d chelates, t h e stabilities o f o n l y t h e 1:1 chelates o f C a ( I I ) , F e ( I I ) , C d ( I I ) , a n d A g ( I ) a r e i n d i c a t e d . T h e f o r m a t i o n constants l i s t e d a r e i n t h e r a n g e o f 10
M
t o 10 - , orders o f 3 6
m a g n i t u d e b e l o w w h a t w o u l d b e expected f o r complexes
of the type
i n d i c a t e d b y f o r m u l a 5. Table I.
Stabilities of Metal Chelates of L-Ascorbic A c i d ( H L ) 2
Metal Ion
Equilibrium
Log Formation Constant (p. = 0.10M;t = 25 C)
Quotient
HL-]/[H*][L ] H L]/[H1[HL"] CaHI/]/[Ca 1[HI/] CaL]/[Ca *][L -] SrHL*]/[Sr *] [HL"] MnHI/]/[Mn *][HL-] F e H I / J / t F e * ] [HL~] FeL]/[Fe ][L "] NiHL ]/[Ni ][HL-] CuHL ]/[Cu ][HL"] ZnHI/]/[Zn *][HL-] CdHI/]/[Cd ][HL-] PbHI/]/[Pb ][HI/] AlHL ]/[AP][HL"] A1(HL) *][AP][HL-] ;AgL-]/[Ag*][L -] U0 HI/]/[U0 ][HL-] U0 (HL) ]/[U0 ][HL-]
o
11.34 4.03 0.2
2
H Ca * +
2
2
a
2
2
Sr * Mn Fe * 2
2
0. 3 1. r 0.21'
2 t
2
2
2
2 +
N P Cu Zn Cd * Pb * A P
+
2 t
:
2 t
2
2 +
+
2 t
2
2
:
2
2 t
2 t
:
2 t
2
2
2
2
2
2
2
2 +
2
2 t
2
i.r 1.6 1.0° 0.42' 1.8 1.9 3.6 3.66 2.35 3.32
• 2 5 ° C , / B = 0.16 M. »25°C,/t = 3 . 0 M . •25 C u~0. O
1.4'
a
2
( /
Source: Reference 1.
In Ascorbic Acid: Chemistry, Metabolism, and Uses; Seib, P., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
2.0'
Chelates of Ascorbic Acid
Downloaded by UNIV OF GUELPH LIBRARY on June 5, 2012 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0200.ch007
MARTELL
5b
In Ascorbic Acid: Chemistry, Metabolism, and Uses; Seib, P., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
156
ASCORBIC
ACID
M o s t of the m e t a l chelates for w h i c h s t a b i l i t y constants h a v e b e e n r e p o r t e d are t h e 1:1 m o n o p r o t o n a t e d chelates c o r r e s p o n d i n g to f o r m u l a 4.
T h e s e are q u i t e w e a k , w i t h l o g K values r a n g i n g f r o m
0.2-2.35,
b e c a u s e of the l o w n e g a t i v e c h a r g e o n the l i g a n d a n i o n a n d t h e fact t h a t one of t h e t w o d o n o r oxygens is p r o t o n a t e d . O f t h e p o s s i b l e l i g a n d donor
group
arrangements indicated b y
formulas 4 a and 4b, 4a
is
g e n e r a l l y a c c e p t e d a n d agrees w i t h the x - r a y d a t a t h u s far o b t a i n e d f o r this t y p e of chelate c o m p o u n d .
T h e b o n d i n g arrangement i n 4b, h o w
ever, is s t i l l a reasonable p o s s i b i l i t y because of
the d e r e a l i z a t i o n of
Downloaded by UNIV OF GUELPH LIBRARY on June 5, 2012 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0200.ch007
n e g a t i v e c h a r g e b e t w e e n t h e oxygens at the 1- a n d 3-positions. A l t h o u g h the m e t a l chelates of t h e c o m p l e t e l y d e p r o t o n a t e d l i g a n d are g e n e r a l l y w r i t t e n as i n d i c a t e d b y 5 a, there is also a p o s s i b i l i t y of t h e f o r m a t i o n of c o o r d i n a t e b o n d i n g m o d e s of the t y p e i l l u s t r a t e d b y 5 b, a g a i n because o f t h e d e r e a l i z a t i o n of
negative charge
between
the
oxygens b o u n d to t h e 1- a n d 3 - c a r b o n atoms. T h e l a c k of d a t a i n t h e l i t e r a t u r e o n the " n o r m a l " m e t a l chelates i n w h i c h t h e l i g a n d is f u l l y d e p r o t o n a t e d is p r o b a b l y d u e to the f a c t that f o r m o s t m e t a l ions s u c h chelates are f o r m e d o n l y i n a l k a l i n e s o l u t i o n . I n t e r e s t i n g r e d o x reactions of
a s c o r b i c a c i d , its salts, a n d its m e t a l chelates take p l a c e i n a c i d
s o l u t i o n a n d m a y b e c o n v e n i e n t l y s t u d i e d at m o d e r a t e l y l o w to l o w p H . I n a l k a l i n e s o l u t i o n the rate of a u t o x i d a t i o n of a s c o r b i c a c i d a n d t h e effect of trace i m p u r i t i e s that c a t a l y z e s u c h o x i d a t i o n reactions increase m a n y f o l d , a n d t h e p r e c a u t i o n s necessary to c a r r y out
studies i n a l k a l i n e
s o l u t i o n are s o m e w h a t i n c o n v e n i e n t . T h e r e seems to be no f u n d a m e n t a l reason, h o w e v e r ,
w h y chelate f o r m a t i o n b y a s c o r b i c a c i d w i t h
non-
o x i d i z i n g m e t a l ions c o u l d not be s t u d i e d at m o d e r a t e l y to h i g h p H u n d e r s u c h c o n d i t i o n s that the m e t a l ions d o not h y d r o l y z e extensively or p r e c i p i t a t e .
Oxidation by Metal Ions and Metal Chelates A s c o r b i c a c i d is a s t r o n g t w o - e l e c t r o n r e d u c i n g agent t h a t is r e a d i l y o x i d i z e d i n one-electron steps b y m e t a l ions a n d m e t a l complexes i n t h e i r h i g h e r v a l e n c e states. A n i n n e r sphere m e c h a n i s m for t h e s t o i c h i o m e t r i c o x i d a t i o n of a s c o r b i c a c i d b y f e r r i c i o n i n a c i d s o l u t i o n is i l l u s t r a t e d b y Scheme 1 ( 8 ) .
T h e first step i n the r e a c t i o n is the f o r m a t i o n of a m o n o
protonated F e ( I I I )
c o m p l e x s i m i l a r to t h e m o n o p r o t o n a t e d
ascorbate
complexes l i s t e d i n T a b l e I. T h e i n t e r m e d i a t e m o n o p r o t o n a t e d c o m p l e x is s h o r t - l i v e d a n d r a p i d l y u n d e r g o e s
Fe(III)
a n i n t r a m o l e c u l a r one-
e l e c t r o n transfer to g i v e a d e p r o t o n a t e d F e ( I I ) c o m p l e x of t h e ascorbate r a d i c a l a n i o n , i n d i c a t e d b y 7. T h i s c o m p l e x dissociates to the free r a d i c a l a n i o n , w h i c h m a y t h e n c o m b i n e w i t h a s e c o n d f e r r i c i o n to f o r m t h e c o m p l e x 9. C o m p l e x 9 i n t u r n undergoes a s e c o n d i n t r a m o l e c u l a r e l e c t r o n
In Ascorbic Acid: Chemistry, Metabolism, and Uses; Seib, P., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
Downloaded by UNIV OF GUELPH LIBRARY on June 5, 2012 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0200.ch007
7.
MARTELL
157
Chelates of Ascorbic Acid
6
Scheme 1.
7
Direct oxidation of ascorbic acid by ferric ion.
transfer to g i v e t h e final p r o d u c t , d e h y d r o a s c o r b i c a c i d , f o r m u l a 10. T h e m o n o p r o t o n a t e d c o m p l e x 6 has b e e n i d e n t i f i e d as t h e s t a r t i n g m a t e r i a l for b o t h C u ( I I ) - a n d F e ( I I I ) - c a t a l y z e d o x i d a t i o n of a s c o r b i c a c i d o n t h e basis of t h e p H d e p e n d e n c e of t h e r e a c t i o n rate f o r o x i d a t i o n w i t h b o t h m e t a l ions, a n d b y r a p i d e q u i l i b r i u m m e a s u r e m e n t s of
chelate
f o r m a t i o n w i t h C u ( I I ) i o n . T h e p o s t u l a t i o n i n S c h e m e 1 t h a t ascorbate r a d i c a l a n i o n 8, a n d its F e ( I I ) c h e l a t e 7, as w e l l as its F e ( I I I ) chelate 9, are c o m p l e t e l y d e p r o t o n a t e d , is b a s e d o n C h a p t e r 4 i n this v o l u m e . T h e o x i d a t i o n of a s c o r b i c a c i d b y C u ( I I ) i o n is s o m e w h a t less r a p i d t h a n t h e rate of o x i d a t i o n b y F e ( I I I ) , b u t is c o n s i d e r e d to p r o c e e d b y t h e same t y p e of m e c h a n i s m . A s m a y b e seen f r o m t h a t d a t a i n T a b l e I , t h e m o n o p r o t o n a t e d m e t a l chelates of a s c o r b i c a c i d are g e n e r a l l y q u i t e w e a k a n d t e n d to b e extensively d i s s o c i a t e d i n s o l u t i o n . M o r e o v e r , as electrons are w i t h d r a w n f r o m t h e l i g a n d to g i v e first t h e r a d i c a l a n i o n a n d
finally
t h e n e u t r a l d e h y d r o a s c o r b i c a c i d , t h e affinities of these o x i d i z e d f o r m s f o r m e t a l ions are f u r t h e r d e c r e a s e d w i t h e a c h o x i d a t i o n step. T h e r e f o r e ,
In Ascorbic Acid: Chemistry, Metabolism, and Uses; Seib, P., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
158
ASCORBIC ACID
w i t h the possible
exception
of
6, the m e t a l complexes
illustrated i n
S c h e m e 1 m a y represent r a t h e r m i n o r species i n the r e a c t i o n m i x t u r e . T h e m e c h a n i s m of o x i d a t i o n of ascorbic a c i d b y v a r i o u s m e t a l c h e lates s u c h as those of F e ( I I I ) a n d C u ( I I ) is s i m i l a r to the m e c h a n i s m of o x i d a t i o n b y the m e t a l i o n , except that the rates are v e r y m u c h l o w e r (9).
T h e s e reactions are also first o r d e r i n the ascorbate
and
first
o r d e r i n m e t a l chelate.
monoanion
T h e rates decrease r a p i d l y as
the
stabilities of the m e t a l chelates increase b u t do not correlate w i t h the rates
that w o u l d
be
predicted
through a mechanism
i n v o l v i n g the
Downloaded by UNIV OF GUELPH LIBRARY on June 5, 2012 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0200.ch007
e q u i l i b r i u m d i s s o c i a t i o n of the m e t a l chelate to the free ( a q u o ) m e t a l i o n . T h e r e f o r e , the reactions are b e l i e v e d to o c c u r t h r o u g h the f o r m a t i o n of a m i x e d l i g a n d chelate i n v o l v i n g ascorbate a n i o n as a secondary l i g a n d , a n d the r a t e - d e t e r m i n i n g e l e c t r o n transfer w o u l d be d e p e n d e n t not o n l y o n the s t a b i l i t y (i.e., the o x i d a t i o n p o t e n t i a l ) of t h e m e t a l chelate itself b u t also on steric factors r e l a t e d to the o r i e n t a t i o n a n d d i m e n s i o n s of the l i g a n d d o n o r groups. T h e s t o i c h i o m e t r i c r e d o x reactions of ascorbic a c i d w i t h o x i d i z i n g m e t a l ions a n d m e t a l chelates, of t h e t y p e i l l u s t r a t e d i n S c h e m e 1, are also i n v o l v e d i n the m e c h a n i s m s of o x i d a t i o n of ascorbic a c i d b y v a r i o u s oxidants since t h e y f u n c t i o n as v e r y efficient catalysts for s u c h reactions. F u r t h e r details c o n c e r n i n g electron transfer processes i n t h e m e t a l che lates of ascorbic a c i d w i l l be p r e s e n t e d i n the f o l l o w i n g discussion of the role of s i m p l e m e t a l ascorbate chelates a n d of m i x e d l i g a n d ascorbate chelates i n the o x i d a t i o n of a s c o r b i c a c i d b y m o l e c u l a r oxygen.
Catalysis of the Autoxidation of Ascorbic Acid by Metal Ions and Metal Chelates T h e systems d e s c r i b e d a b o v e b y w h i c h m e t a l ions a n d m e t a l chelates a c c o m p l i s h t w o - e l e c t r o n o x i d a t i o n of ascorbic a c i d , m a y b e e m p l o y e d i n c a t a l y t i c systems i n w h i c h the m e t a l i o n or chelate is o n l y a m i n o r c o n stituent.
A n y o x i d i z i n g agent c a p a b l e of r e o x i d i z i n g the m e t a l i o n or
chelate f r o m its l o w e r v a l e n t state to its h i g h e r v a l e n t state m a y employed.
be
W h i l e i n the f o l l o w i n g t r e a t m e n t the o x i d a n t is m o l e c u l a r
o x y g e n , it s h o u l d b e possible to set u p analogous r e a c t i o n systems w i t h other oxidants s u c h as h y d r o g e n p e r o x i d e , halogens, n i t r i t e i o n , a n d m a n y others. Metal-Ion-Catalyzed Autoxidation.
F i g u r e 1 illustrates the v a r i a
t i o n of the first-order rate constants for the a u t o x i d a t i o n of a s c o r b i c a c i d b y m o l e c u l a r o x y g e n w i t h the c o n c e n t r a t i o n of t h e C u ( I I ) i o n , w h i c h is present i n c a t a l y t i c (i.e. l o w ) concentrations ( 8 ) . indicates second-order order i n C u ( I I ) ] .
T h e linear relationship
b e h a v i o r [first o r d e r i n a s c o r b i c a c i d a n d
first
T h e c a t a l y t i c effect of C u ( I I ) is also seen to decrease
In Ascorbic Acid: Chemistry, Metabolism, and Uses; Seib, P., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
Downloaded by UNIV OF GUELPH LIBRARY on June 5, 2012 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0200.ch007
7.
MARTELL
159
Chelates of Ascorbic Acid
in O «— X
2
4
6
8
10
12
[Cu(ll)] X 105 Figure 1. Rate constants for the Cu(H)-ion-catalyzed autoxidation of ascorbic acid as a function of Cu(H) concentration at —log [H ] values of: A , 1.50; B, 2.00; C , 2.25; D, 2.50; E, 2.85; and F , 3.45; t = 25°C; fi = 0.10M (KN0 ). +
3
r a p i d l y as h y d r o g e n i o n c o n c e n t r a t i o n is i n c r e a s e d . T h i s v a r i a t i o n of the second-order rate constant w i t h p H m a y b e e l i m i n a t e d i f the c o n c e n t r a t i o n of the substrate is r e p l a c e d b y t h a t of the m o n o a n i o n , i n d i c a t i n g t h a t t h e latter, or the c o r r e s p o n d i n g m o n o p r o t o n a t e d C u ( I I ) chelate, is t h e r e a c t i v e species i n t h e o x i d a t i o n r e a c t i o n .
In Ascorbic Acid: Chemistry, Metabolism, and Uses; Seib, P., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
160
ASCORBIC
ACID
A s i m i l a r c a t a l y t i c effect ( 8 ) of F e ( I I I ) o n t h e o x i d a t i o n of ascorbic a c i d is i l l u s t r a t e d i n F i g u r e 2. I n this case the o b s e r v e d rates are c o n s i d e r a b l y h i g h e r t h a n those i l l u s t r a t e d i n F i g u r e 1 for C u ( I I ) catalysis. T h e pseudo
first-order
r a t e constants f o r t h e o x i d a t i o n of ascorbic a c i d
i l l u s t r a t e d i n F i g u r e 2 are seen to v a r y i n a l i n e a r f a s h i o n w i t h
the
c o n c e n t r a t i o n of F e ( I I I ) , w h i c h is present i n c a t a l y t i c a m o u n t s .
The
rates i n d i c a t e d i n F i g u r e 2 are also seen to increase w i t h h y d r o g e n i o n c o n c e n t r a t i o n , a n d here a g a i n the p H v a r i a t i o n i n the rate i n d i c a t e s t h a t the m o n o a n i o n , or its m o n o p r o t o n a t e d
i r o n chelate, is f o r m e d i n a p r e -
Downloaded by UNIV OF GUELPH LIBRARY on June 5, 2012 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0200.ch007
e q u i l i b r i u m step p r i o r to t h e r a t e - d e t e r m i n i n g electron transfer r e a c t i o n . F r o m the d a t a i l l u s t r a t e d i n F i g u r e 2, second-order the F e ( I I I ) - c a t a l y z e d
o x i d a t i o n of the a s c o r b i c
rate constants
acid monoanion
for may
be c a l c u l a t e d . T h e second-order rate constants for t h e a u t o x i d a t i o n of ascorbic a c i d d e t e r m i n e d f r o m d a t a of the t y p e i l l u s t r a t e d i n F i g u r e s 1 a n d 2 f o u n d to be p r o p o r t i o n a l to the d i o x y g e n c o n c e n t r a t i o n .
At low
were
oxygen
concentrations this d e p e n d e n c e o n o x y g e n c o n c e n t r a t i o n w a s f o u n d
to
l e v e l off i n d i c a t i n g a change i n r e a c t i o n m e c h a n i s m . A l s o the rates i n the presence of o x y g e n w e r e f o u n d to be m u c h m o r e r a p i d t h a n the d i r e c t s t o i c h i o m e t r i c rates of o x i d a t i o n b y t h e C u ( I I ) a n d F e ( I I I ) ions i n t h e absence of m o l e c u l a r o x y g e n .
F i g u r e 3 illustrates the d e p e n d e n c e o n
o x y g e n c o n c e n t r a t i o n of the specific rate constants
(i.e., rate constants
b a s e d o n c o n c e n t r a t i o n of the m o n o p r o t o n a t e d a n i o n ) f o r the a u t o x i d a t i o n of ascorbic a c i d i n the presence of c a t a l y t i c amounts of F e ( I I I ) . r e l a t i o n s h i p s w e r e o b t a i n e d for C u ( I I ) i n d i c a t e t h i r d - o r d e r b e h a v i o r for C u ( I I )
catalysis.
T h e data
and Fe(III)
Similar therefore
catalysis of
the
a u t o x i d a t i o n of a s c o r b i c a c i d — f i r s t o r d e r i n substrate, first o r d e r i n m e t a l i o n , a n d over a l i m i t e d r a n g e of c o n c e n t r a t i o n , first o r d e r i n d i o x y g e n concentration.
T h i s b e h a v i o r , together w i t h the fact t h a t the
r e a c t i o n is m u c h m o r e r a p i d i n t h e presence
observed
of d i o x y g e n t h a n i n its
absence, p r o v i d e s e v i d e n c e for the f o r m a t i o n of a n i n t e r m e d i a t e ascorbate-copper-dioxygen
complex
transfer takes
The
place.
i n w h i c h t h e r a t e - d e t e r m i n i n g electron
experimental observation
that the
metal-
c a t a l y z e d o x i d a t i o n b y m o l e c u l a r o x y g e n is m u c h m o r e r a p i d t h a n the s t o i c h i o m e t r i c o x i d a t i o n of ascorbic a c i d b y the m e t a l i o n or chelate i n the absence of m o l e c u l a r o x y g e n , w a s n o t e d some t i m e ago b y D e k k e r and D i c k i n s o n (10)
b u t this o b s e r v a t i o n w a s i n t e r p r e t e d i n terms of the
r e a c t i v i t y of the ascorbate r a d i c a l a n i o n , a n d its i n v o l v e m e n t i n a free radical chain reaction. A r e a c t i o n m e c h a n i s m f o r the m e t a l - i o n - c a t a l y z e d a u t o x i d a t i o n of a s c o r b i c a c i d , i n v o l v i n g the f o r m a t i o n of a n i n t e r m e d i a t e t e r n a r y ascorb a t e - m e t a l i o n - d i o x y g e n c o m p l e x , is i l l u s t r a t e d i n S c h e m e 2.
Although
the b o n d i n g b e t w e e n the m e t a l i o n a n d the d i o x y g e n i n t h e i n t e r m e d i a t e
In Ascorbic Acid: Chemistry, Metabolism, and Uses; Seib, P., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
Chelates of Ascorbic Acid
161
Downloaded by UNIV OF GUELPH LIBRARY on June 5, 2012 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0200.ch007
MARTELL
I
I
I
I
I
I
0
2
4
6
8
10
I— 12
[Fe (III)] x 105 Figure 2. Rate constants for the Fe(III)'ion~catalyzed autoxidation of ascorbic acid as a function of Fe(III) concentration at —log [H*] values of: A , 1.50; B, 2.00; C , 2.42; D, 2.94; E, 3.44; t = 2S°C; p. = 0 . I 0 M (KNO ). s
In Ascorbic Acid: Chemistry, Metabolism, and Uses; Seib, P., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
162
ASCORBIC
ACID
Downloaded by UNIV OF GUELPH LIBRARY on June 5, 2012 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0200.ch007
3.0
0
[ 0 ] X 1(H 2
Figure 3. Variation of second-order rate constants for the Fe(HI) catalyzed autoxidation of ascorbic acid as a function of oxygen concentration at -log [H ] values of: A, 3.85; B, 3.45; C, 3.00; t = 25°C; /x = 0.10M (KNO ). +
s
In Ascorbic Acid: Chemistry, Metabolism, and Uses; Seib, P., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
MARTELL
Chelates of Ascorbic Acid
Downloaded by UNIV OF GUELPH LIBRARY on June 5, 2012 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0200.ch007
7.
In Ascorbic Acid: Chemistry, Metabolism, and Uses; Seib, P., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
163
164
ASCORBIC
ACID
d i o x y g e n c o m p l e x w o u l d seem to b e e x t r e m e l y w e a k , i t m a y b e s t a b i l i z e d b y resonance
of the t y p e i n d i c a t e d b y
suggested b y H a m i l t o n ( I I ) .
11a a n d l i b
(Scheme 2)
as
T h e r a t e - d e t e r m i n i n g e l e c t r o n transfer step
i l l u s t r a t e d i n S c h e m e 2 is i n d i c a t e d as o c c u r r i n g t h r o u g h a n i o n i c shift of t w o electrons i n a c c o r d a n c e w i t h the suggestion of H a m i l t o n ( I I )
to
give directly a C u ( I I ) complex containing w e a k l y coordinated dehydro a s c o r b i c a c i d a n d a m o r e s t r o n g l y c o o r d i n a t e d h y d r o p e r o x i d e d o n o r , as i n d i c a t e d b y f o r m u l a 12.
T h i s c o m p l e x r a p i d l y dissociates to t h e free
m e t a l i o n , h y d r o g e n p e r o x i d e , a n d the o x i d a t i o n p r o d u c t .
W h i l e the
Downloaded by UNIV OF GUELPH LIBRARY on June 5, 2012 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0200.ch007
r e d o x r e a c t i o n i n v o l v i n g transfer of t w o electrons is i l l u s t r a t e d i n the m e c h a n i s m i n S c h e m e 2 as o c c u r r i n g i n a single step, i t w o u l d also b e q u i t e reasonable to i l l u s t r a t e the r e d o x r e a c t i o n as o c c u r r i n g i n t w o successive one-electron transfers w i t h the f o r m a t i o n of a n i n t e r m e d i a t e c o m p l e x i n w h i c h C u ( I I ) is b o u n d to a d e p r o t o n a t e d ascorbate r a d i c a l anion and a superperoxide anion. W i t h kinetic data presently available i t is i m p o s s i b l e to d i s t i n g u i s h b e t w e e n these a l t e r n a t i v e r e a c t i o n m e c h anisms. T h e w o r k d e s c r i b e d h e r e o n the C u ( I I ) - a n d a u t o x i d a t i o n of ascorbic
a c i d has b e e n
extended
Fe(III)-catalyzed
to c a t a l y t i c systems
i n v o l v i n g v a n a d y l ( 1 2 ) a n d u r a n y l ( 1 3 ) ions. O n the basis of the results d e s c r i b e d a b o v e i t w o u l d seem that there are p o t e n t i a l l y m a n y other m e t a l ions t h a t are c a p a b l e
of
undergoing redox
reactions w i t h
the
ascorbate i o n , a n d that m a y f u n c t i o n as catalysts i n the a u t o x i d a t i o n of a s c o r b i c a c i d . A n a l o g o u s m e c h a n i s m s m a y also a p p l y to systems i n v o l v i n g m e t a l - i o n catalysis of ascorbate o x i d a t i o n i n w h i c h the p r i m a r y o x i d a n t is a reagent other t h a n m o l e c u l a r o x y g e n . Metal-Chelate-Catalyzed Autoxidation of Ascorbic A c i d .
Kinetic
d a t a for r e a c t i o n systems i n w h i c h m e t a l chelates r a t h e r t h a n m e t a l ions serve as catalysts for the a u t o x i d a t i o n of ascorbic a c i d are i l l u s t r a t e d i n F i g u r e s 4, 5, a n d 6 ( 9 ) . oxygen
T h e rate constants are i n d e p e n d e n t of m o l e c u l a r
concentration a n d are m u c h lower
t h a n those
observed
for
a u t o x i d a t i o n of ascorbic a c i d i n the presence of free ( a q u o ) m e t a l ions. T h e m e t a l - c h e l a t e - c a t a l y z e d reactions are therefore e x p e c t e d to p r o c e e d t h r o u g h single e l e c t r o n transfer steps, w i t h the
first
e l e c t r o n transfer
f o l l o w e d b y m e t a l i o n d i s s o c i a t i o n a n d r e c o m b i n a t i o n of t h e d e p r o t o n a t e d ascorbate r a d i c a l a n i o n w i t h the h i g h e r v a l e n c e f o r m of the m e t a l chelate. T h u s the r e a c t i o n m e c h a n i s m is s i m i l a r to the s t o i c h i o m e t r i c r e a c t i o n s c h e m e i l l u s t r a t e d i n S c h e m e 1 w i t h a m e t a l chelate r e p l a c i n g the free metal ion.
I n a c a t a l y t i c system i n the presence
of excess m o l e c u l a r
o x y g e n a n d o n l y a r e l a t i v e l y s m a l l a m o u n t of m e t a l chelate t h e g e n e r a t i o n of the l o w e r v a l e n c e f o r m of t h e m e t a l chelate b y o x i d a t i o n of
ascorbic
a c i d is c o u n t e r - b a l a n c e d b y r a p i d r e o x i d a t i o n of the m e t a l chelate to t h e higher valence form by molecular oxygen, resulting a cyclic catalytic
In Ascorbic Acid: Chemistry, Metabolism, and Uses; Seib, P., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
Downloaded by UNIV OF GUELPH LIBRARY on June 5, 2012 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0200.ch007
7.
MARTELL
Chelates of Ascorbic Acid
1
2
3
165
4
5
6
MOLARITY OF CATALYST X 10* Figure 4. Variation of rate constants for the autoxidation of ascorbic acid as a function of concentration of Cu(II) chelates at 25°C and —log [H ] of 3.45: EDTA = ethylenediaminetetraacetic acid; HEDTA = hydroxyethylethylenediaminetetraacetic acid; NTA = nitrilotriacetic acid; HIMDA = hydroxyethyliminodiacetic acid; IMDA = iminodiacetic acid. +
process of t h e t y p e i l l u s t r a t e d i n S c h e m e 3. T h e p s e u d o
first-order
rate
constants p l o t t e d i n F i g u r e s 4 a n d 5 s h o w l i n e a r d e p e n d e n c e of the r a t e constants o n c o p p e r c h e l a t e a n d o n i r o n c h e l a t e c o n c e n t r a t i o n s , r e s p e c t i v e l y , t h u s i n d i c a t i n g t h e f o r m a t i o n of a m i x e d l i g a n d c o m p l e x
with
ascorbate as t h e r e a c t i v e i n t e r m e d i a t e i n w h i c h t h e s l o w r a t e - d e t e r m i n i n g e l e c t r o n transfer occurs.
T h e r e a c t i o n rates are also seen to decrease
r a p i d l y w i t h a n i n c r e a s e i n the stabilities of t h e c o p p e r a n d i r o n chelates
In Ascorbic Acid: Chemistry, Metabolism, and Uses; Seib, P., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
166
ASCORBIC ACID
Downloaded by UNIV OF GUELPH LIBRARY on June 5, 2012 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0200.ch007
HIMDA
5
10
15
20
25
30
MOLARITY OF CATALYST X 106 Figure 5. Variation of rate constants for the autoxidation of ascorbic acid as a function of concentration of Fe(HI) chelates at 25° C and —log [H*] of 2.45; DTPA = diethylenetriaminepentaacetic acid; CDTA = trans-1,2-diaminocyclohexanetetraacetic acid; other terms as in caption of Figure 4. involved.
T h i s effect m a y be i n t e r p r e t e d i n one of t w o w a y s :
(i)
that
the r e a c t i o n occurs t h r o u g h a dissociative m e c h a n i s m r e l e a s i n g a s m a l l a m o u n t of the free m e t a l i o n , w h i c h t h e n acts as a catalyst for
ascorbic
a c i d o x i d a t i o n , i n t h e m a n n e r i l l u s t r a t e d i n S c h e m e 2; or ( i i ) t h a t t h e r e d o x p o t e n t i a l of t h e c o p p e r i o n i n the m i x e d
ligand-ascorbate-carrier
l i g a n d c o m p l e x stabilizes t h e h i g h e r v a l e n t f o r m of t h e m e t a l i o n to a greater extent w h e n a m o r e h i g h l y stable m e t a l chelate is i n v o l v e d as t h e catalyst.
In Ascorbic Acid: Chemistry, Metabolism, and Uses; Seib, P., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
Downloaded by UNIV OF GUELPH LIBRARY on June 5, 2012 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0200.ch007
Figure 6. Dependence of second-order rate constants for Fe(III)-chelatecatalyzed autoxidation of ascorbic acid on hydrogen ion concentration at 25°C. Abbreviations are those given in Figures 4 and 5.
In Ascorbic Acid: Chemistry, Metabolism, and Uses; Seib, P., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
In Ascorbic Acid: Chemistry, Metabolism, and Uses; Seib, P., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
Scheme 3.
Proposed mechanism for metal-chelate-catalyzed autoxidation of ascorbic acid.
Downloaded by UNIV OF GUELPH LIBRARY on June 5, 2012 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0200.ch007
9
•
o o » S o >
oo
7.
169
Chelates of Ascorbic Acid
MARTELL
T h e p o s s i b i l i t y that the d e c r e a s e d c a t a l y t i c a c t i v i t y w i t h increase i n m e t a l chelate s t a b i l i t y represents a d i s s o c i a t i v e m e c h a n i s m i n w h i c h t h e free m e t a l i o n is a c t u a l l y the c a t a l y s t w a s e x p l o r e d b y c o m p a r i n g the o b s e r v e d rates w i t h t h e concentrations of free m e t a l i o n i n e q u i l i b r i u m w i t h the v a r i o u s chelates i n v e s t i g a t e d . S i n c e the rate constants f o r freem e t a l - i o n catalysis are k n o w n i t w a s possible to c a l c u l a t e a n d p r e d i c t t h e observed
c a t a l y t i c rate constants, since t h e e q u i l i b r i u m constants
for
d i s s o c i a t i o n of the m e t a l chelates are also k n o w n . T h e values c a l c u l a t e d i n this m a n n e r d i d n o t c o r r e l a t e w i t h the o b s e r v e d rates, i n d i c a t i n g t h a t Downloaded by UNIV OF GUELPH LIBRARY on June 5, 2012 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0200.ch007
the o b s e r v e d catalysis p r o b a b l y p r o c e e d s b y e l e c t r o n transfer f r o m t h e r e d u c t a n t to the m e t a l i o n i n the m i x e d l i g a n d chelates i l l u s t r a t e d i n S c h e m e 3.
of the
type
I n s u c h a m e c h a n i s m t h e m e t a l chelates
v i s u a l i z e d as r e m a i n i n g i n t a c t i n b o t h t h e o x i d i z e d a n d r e d u c e d t h r o u g h the
entire c a t a l y t i c c y c l e .
T h u s the d e p r o t o n a t e d
are
forms
ascorbate
r a d i c a l a n i o n a n d the c a r r i e r - l i g a n d are v i s u a l i z e d i n f o r m u l a
15
as
r e m a i n i n g s i m u l t a n e o u s l y b o u n d to the r e d u c e d m e t a l i o n a n d r e m a i n c o m b i n e d w i t h t h e m e t a l i o n w h e n it is r e o x i d i z e d to the h i g h e r v a l e n c e state [i.e., f r o m C u ( I )
to C u ( I I ) ] .
A f t e r t h e second e l e c t r o n transfer,
h o w e v e r , as i n d i c a t e d i n 16, the d e h y d r o a s c o r b i c a c i d finally f o r m e d is s u c h a w e a k l i g a n d that i t r e a d i l y dissociates
a n d the s i m p l e m e t a l
chelate i n w h i c h the m e t a l i o n is a g a i n i n its l o w e r v a l e n c e state a n d is r e o x i d i z e d b y m o l e c u l a r o x y g e n to regenerate the catalyst 13. I n the first m i x e d l i g a n d c o m p l e x f o r m e d i n the r e a c t i o n m i x t u r e , 14, the c o o r d i n a t e d ascorbate i o n is i n d i c a t e d i n its m o n o p r o t o n a t e d
form.
E x p e r i m e n t a l e v i d e n c e for the d e g r e e of p r o t o n a t i o n of this species w a s o b t a i n e d f r o m the v a r i a t i o n of the c a l c u l a t e d second-order rate constants w i t h h y d r o g e n i o n c o n c e n t r a t i o n i n the l o w p H r a n g e i n w h i c h these reactions w e r e c a r r i e d out.
T h e substrate w a s p r i m a r i l y i n its n e u t r a l
d i p r o t o n a t e d f o r m a n d the e q u i l i b r i u m i n v o l v i n g m i x e d l i g a n d c o m p l e x f o r m a t i o n results i n d i s p l a c e m e n t of one of the t w o protons present o n the e n e - d i o l groups.
T h u s the c o n c e n t r a t i o n of i n t e r m e d i a t e 14
increase as the h y d r o g e n clearly observed
ion concentration
decreases.
i n the plots of the second-order
f u n c t i o n of p H i n F i g u r e 6. and copper-chelate-catalyzed
will
T h i s effect is
rate constants
as a
S i m i l a r effects w e r e o b t a i n e d for b o t h i r o n o x i d a t i o n reactions.
a m e t a l i o n i n a m e t a l chelate c o m p o u n d
S i n c e the t e n d e n c y of
to exist i n its h i g h e r v a l e n c e
state increases w i t h the s t a b i l i t y of the m e t a l chelate c o m p o u n d , a n d i n fact its redox p o t e n t i a l c a n be c a l c u l a t e d f r o m the s t a b i l i t y constant of the m e t a l chelate itself, it w o u l d be expected t h a t i n c r e a s i n g the s t a b i l i t y of the m e t a l c h e l a t e w o u l d decrease its c a t a l y t i c a c t i v i t y for the a u t o x i d a t i o n of ascorbic a c i d , as is seen i n F i g u r e s 4 a n d 5. W h i l e i t is clear t h a t this t r e n d exists, there is n o l i n e a r c o r r e l a t i o n b e t w e e n chelate s t a b i l i t y a n d c a t a l y t i c a c t i v i t y because, i f s u c h a c o r r e l a t i o n existed, it w o u l d n o t
In Ascorbic Acid: Chemistry, Metabolism, and Uses; Seib, P., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
170
ASCORBIC
ACID
h a v e b e e n possible to e l i m i n a t e the dissociative m e c h a n i s m i n v o l v i n g a n alternate c a t a l y t i c route i n v o l v i n g the free a q u o m e t a l i o n i n its h i g h e r valence
state.
It is o b v i o u s
that t h e m o r e stable m e t a l chelates
will
h a v e a l a r g e r n u m b e r of d o n o r groups c o o r d i n a t e d to the m e t a l i o n a n d w i l l h a v e c o o r d i n a t e b o n d s that are m o r e difficult to b r e a k , a process t h a t w o u l d be necessary for the complexes w i t h m u l t i d e n t a t e l i g a n d s i n w h i c h the m e t a l i o n is n e a r l y or f u l l y c o o r d i n a t e d .
T h i s t y p e of
displacement
is i n d i c a t e d s c h e m a t i c a l l y b y f o r m u l a s 13 a n d 13a i n S c h e m e 3 i n w h i c h the m e t a l ions [ C u ( I I ) ] are r e p r e s e n t e d as b e i n g t e t r a - c o o r d i n a t e d . Downloaded by UNIV OF GUELPH LIBRARY on June 5, 2012 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0200.ch007
formation
of
a mixed
ligand complex
r e q u i r e d for
an
The
inner-sphere
electron transfer of the t y p e i n d i c a t e d i n f o r m u l a s 14, 15, a n d 16 w o u l d i n v o l v e d i s p l a c e m e n t of one or m o r e of the c o o r d i n a t e d d o n o r
groups
of the c a r r i e r l i g a n d . It is e x p e c t e d t h a t c o n s i d e r a b l e steric effects w o u l d be associated w i t h s u c h d i s p l a c e m e n t processes a n d t h a t these
effects
w o u l d v a r y i n a v e r y c o m p l e x w a y w i t h the nature of the l i g a n d i n the c a t a l y t i c m e t a l chelate. F i n a l l y it s h o u l d be p o i n t e d out that t h e r e is a n alternate m e c h a n i s m for
the
two
successive
electron
transfer processes i n d i c a t e d
by
the
sequence 14—» 15 - » 16 - » 13 a. I t is q u i t e p o s s i b l e t h a t the ascorbate r a d i c a l a n i o n dissociates f r o m the m i x e d l i g a n d c o m p l e x r e o x i d a t i o n of
the C u ( I )
ion, and recombines
15, p r i o r to
w i t h another
Cu(II)
chelate p r i o r to the final e l e c t r o n transfer step i n d i c a t e d b y 16 —> 17
+
10. T h i s represents a slight m o d i f i c a t i o n of the m e c h a n i s m s i n S c h e m e 3, a n d i n v o l v e s a n alternate b r a n c h for r e a c t i o n sequence 15 - » 1 6 - * T h e m a i n difference b e t w e e n the F e ( I I I ) a n d C u ( I I )
17.
ion-catalyzed
a u t o x i d a t i o n of ascorbic a c i d o n one h a n d , a n d the F e ( I I I )
and C u ( I I )
c h e l a t e - c a t a l y z e d o x i d a t i o n reactions is the l a c k of d e p e n d e n c e of
the
latter systems o n the c o n c e n t r a t i o n of m o l e c u l a r o x y g e n , a n d t h e absence of a t e r n a r y m e t a l s u b s t r a t e - c a r r i e r l i g a n d - d i o x y g e n c o m p l e x as a n i n t e r m e d i a t e i n the p r o p o s e d
reaction mechanism.
T h e absence of s u c h a n
i n t e r m e d i a t e m a y b e c o n s i d e r e d to be d u e at least i n p a r t to the o c c u p a t i o n of a l l or n e a r l y a l l the c o o r d i n a t i o n positions of t h e m e t a l i o n b y the c a r r i e r l i g a n d , thus c r o w d i n g relatively weak monodentate
out the d i o x y g e n ,
w h i c h is at best a
l i g a n d . It s h o u l d also be n o t e d that the
p r o p o s e d m e c h a n i s m i n S c h e m e 3 i n v o l v e s t w o successive electron t r a n s fers r a t h e r t h a n a single i o n i c t y p e t w o - e l e c t r o n shift of the t y p e i n d i c a t e d i n S c h e m e 2. over
It is not i n t e n d e d at this stage to f a v o r one
the other; b o t h s h o u l d be
considered
alternatives.
mechanism A
factor
in
ascorbate a u t o x i d a t i o n t h a t m a y not a p p l y to m a n y other substrates is the resonance
s t a b i l i z a t i o n of t h e one-electron
the ascorbate r a d i c a l a n i o n . oxidation products
are s t a b i l i z e d b y resonance
factors, the one-electron
oxidation intermediate,
I n cases w h e r e one-electron
transfer process m a y
r e d u c t i o n of
or other c o n s t i t u t i o n a l be
favored
over
e l e c t r o n r e d o x reactions.
In Ascorbic Acid: Chemistry, Metabolism, and Uses; Seib, P., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
two-
7.
171
Chelates of Ascorbic Acid
MARTELL
Ascorbic Acid Oxidase A s c o r b i c a c i d oxidase is a c t i v a t e d b y C u ( I I ) i o n a n d is b e l i e v e d to f u n c t i o n i n a m a n n e r s i m i l a r to the m e c h a n i s m i n d i c a t e d i n S c h e m e
3
i n v o l v i n g a series of t w o successive one-electron transfer steps i n w h i c h the a s c o r b i c
a c i d is o x i d i z e d to
a n i n t e r m e d i a t e free
radical anion
c o o r d i n a t e d at the active site i n a m a n n e r s i m i l a r to t h a t i n d i c a t e d b y f o r m u l a 15. I n the e n z y m i c system i t seems l i k e l y that t h e i n t e r m e d i a t e ascorbate free r a d i c a l w o u l d r e m a i n c o o r d i n a t e d to the m e t a l i o n i n b o t h
Downloaded by UNIV OF GUELPH LIBRARY on June 5, 2012 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0200.ch007
Cu(I) and
and C u ( I I )
reassociation
forms, r a t h e r t h a n u n d e r g o steps
that w o u l d
t e n d to
successive
greatly
dissociation
slow
down
the
e n z y m i c r e a c t i o n process. I n this respect the e n z y m i c m e c h a n i s m
may
differ f r o m that suggested i n S c h e m e 3 for m e t a l chelate catalysis of t h e a u t o x i d a t i o n of ascorbic a c i d , since there is no e v i d e n c e i n t h e latter case t h a t the w e a k C u ( I )
intermediate complex
w o u l d h o l d together
e n o u g h f o r the r e o x i d a t i o n step to o c c u r (see
discussion later).
long Since
the m e t a l i o n w o u l d r e m a i n b o u n d to t h e active site of the e n z y m e , its reoxidation by
molecular
oxygen
may
very
w e l l occur
through
the
f o r m a t i o n of a d i o x y g e n c o m p l e x , thus p r o v i d i n g a n a d d i t i o n a l r e a c t i o n i n t e r m e d i a t e . A s m e n t i o n e d before, the free r a d i c a l ascorbate a n i o n is r e s o n a n c e - s t a b i l i z e d a n d its f o r m a t i o n i n these systems i n a p p r e c i a b l e q u a n t i t i e s therefore seems reasonable.
A n o t h e r p r o b a b l e difference
be
t w e e n the e n z y m i c m e c h a n i s m a n d the m e c h a n i s m suggested i n S c h e m e 3 for m e t a l chelate catalysis is the d i s p l a c e m e n t of some of the l i g a n d d o n o r groups f r o m the c o o r d i n a t i o n sphere of the m e t a l i o n that w o u l d b e r e q u i r e d for the f o r m a t i o n of t h e m i x e d l i g a n d complexes i n w h i c h the e l e c t r o n transfer process takes p l a c e .
Such displacement
reactions
w o u l d greatly s l o w d o w n a n d i n h i b i t the r e a c t i o n rate a n d i t is b e l i e v e d t h a t the e n z y m e c e r t a i n l y w o u l d e l i m i n a t e s u c h r e a c t i o n b a r r i e r s b y the d e s i g n of a c o o r d i n a t i o n sphere t h a t w o u l d m a k e a v a i l a b l e at least t w o l a b i l e c o o r d i n a t i o n sites of C u ( I I ) for c o m b i n a t i o n w i t h the substrate.
Rate Laws for Metal-Ion- and Metal-Chelate-Catalyzed Autoxidation of Ascorbic Acid O n the basis of k i n e t i c d a t a of the t y p e i l l u s t r a t e d i n F i g u r e s 1, 2, a n d 3 the rate l a w of a u t o x i d a t i o n of a s c o r b i c a c i d is represented
by
E q u a t i o n 1, i n v o l v i n g a t h i r d - o r d e r rate constant a n d a r e a c t i o n rate that is first o r d e r oxygen.
i n substrate m o n o a n i o n ,
metal ion, and
molecular
Since the r a t e - d e t e r m i n i n g step i n v o l v e s e l e c t r o n transfer i n a
ternary complex, third-order behavior involves two pre-equilibria, E q u a tions 2 a n d 3, for the f o r m a t i o n of the t e r n a r y c o m p l e x , a n d a f i n a l s l o w step, E q u a t i o n 4, i n w h i c h t h e c o o r d i n a t e d e l e c t r o n r e d u c t i o n to h y d r o g e n p e r o x i d e .
o x y g e n undergoes
a two-
A s pointed out b y T a q u i K h a n
a n d M a r t e l l ( 9 ) , the h y d r o g e n p e r o x i d e r a p i d l y d i s a p p e a r s a n d is n o t
In Ascorbic Acid: Chemistry, Metabolism, and Uses; Seib, P., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
172
ASCORBIC
detected
i n t h e final r e a c t i o n m i x t u r e . T h e h y d r o g e n p e r o x i d e
ACID
formed
m a y be r e c o n v e r t e d to o x y g e n a n d w a t e r t h r o u g h t h e c a t a l a s e - l i k e a c t i o n of the c u p r i c i o n or of some of its complexes.
" Cu
fc
+
2 +
CuHA
Downloaded by UNIV OF GUELPH LIBRARY on June 5, 2012 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0200.ch007
[ ~n m]
=
H A " ^± C u H A +
+
HL
0
^
2
(i)
Cu2
(2)
+
CuHA0
2
(3)
+
slow
CuHACV + The
H
metal-chelate-catalyzed
dependent
> Cu
+
+ A +
2 +
H 0 2
a u t o x i d a t i o n of
(4)
2
ascorbic
o n the c o n c e n t r a t i o n of m o l e c u l a r o x y g e n .
a c i d is
not
O n the basis of
the d a t a i l l u s t r a t e d i n F i g u r e s 4, 5, a n d 6 the rate expression
suggested
for this t y p e of r e a c t i o n is g i v e n b y E q u a t i o n 5, w h i c h i n d i c a t e s that t h e d i s a p p e a r a n c e of the m o n o a n i o n of t h e substrate is first o r d e r i n b o t h m e t a l chelate a n d the ascorbate m o n o a n i o n .
T h u s the r e a c t i o n sequences
i n d i c a t e d b y E q u a t i o n s 6, 7, a n d 8 consist of a p r e - e q u i l i b r i u m i n v o l v i n g t h e f o r m a t i o n of a m i x e d l i g a n d m e t a l - c a r r i e r l i g a n d - s u b s t r a t e m o n o anion complex.
F o l l o w i n g the r a t e - d e t e r m i n i n g e l e c t r o n transfer r e a c t i o n
w i t h i n the m i x e d l i g a n d c o m p l e x of the m e t a l c o m p l e x
( E q u a t i o n 7 ) , the l o w e r v a l e n c e
is r a p i d l y r e o x i d i z e d i n s o l u t i o n b y
o x y g e n , thus r e g e n e r a t i n g t h e catalyst.
form
molecular
H A represents a m u l t i d e n t a t e n
l i g a n d , a n d H L is a s c o r b i c a c i d . 2
-
C u
d
[
ii (2-n A
^ " L
) +
=
]
+
fc[HL"]
[CuA " ( 2
n ) +
]
(5)
HL"— Cu (A) (HL) " n
a
(6)
n ) +
slow
Cu (A) (HL) -» n
( 1
) +
>CuW ""'* +
L" +
1
H
(7)
+
fast
Free radicals
>A + CuA< - > , 2
n
+
0
H 0 2
R e c e n t l y J a m e s o n a n d B l a c k b u r n (14,15,16) alternate m e c h a n i s m for the c o p p e r - c a t a l y z e d
have
complex
an
ascorbic (17)
of
a n i o n , a n d the s u b s e q u e n t f o r m a t i o n of a n i n t e r m e d i a t e
peroxo type C u ( I I ) - d i o x y g e n - a s c o r b a t e d a t a suggested
suggested
a u t o x i d a t i o n of
a c i d , i n v o l v i n g the f o r m a t i o n of a b i n u c l e a r C u ( I I ) t h e ascorbate
(8)
2
2
complex
(18).
Their kinetic
a v a r i e t y of rate b e h a v i o r d e p e n d i n g o n t h e n a t u r e of
the s u p p o r t i n g electrolyte.
F o r m u l a 17, w h i c h w a s p o s t u l a t e d for n i t r a t e
In Ascorbic Acid: Chemistry, Metabolism, and Uses; Seib, P., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
7.
MARTELL
0
Downloaded by UNIV OF GUELPH LIBRARY on June 5, 2012 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0200.ch007
173
Chelates of Ascorbic Acid
H
CHOHCH OH 2
I n i t i a l b i n u c l e a r complex, ( C u H L )
2
2 +
, 17
I n t e r m e d i a t e d i o x y g e n c o m p l e x , C u H L 0 C u H L , 18 2
m e d i a , seems to suggest the rate i n d i c a t e d b y E q u a t i o n 9.
Thus
the
r e a c t i o n sequence suggested consists of t w o p r e - e q u i l i b r i a , E q u a t i o n s 10 a n d 11, w h i c h r e s u l t i n the f o r m a t i o n d i o x y g e n c o m p l e x , 18. transfer
(Equation
of
the dinuclear peroxo
T h i s is f o l l o w e d b y a r a t e - d e t e r m i n i n g
12),
w h i c h results i n the f o r m a t i o n
of
type
electron two
free
r a d i c a l s that are c o n v e r t e d to final p r o d u c t s i n s u b s e q u e n t steps t h a t seem to be p a r t i a l l y rate d e t e r m i n i n g . I n c h l o r i d e m e d i a [ a n d p r o b a b l y i n the
^
2
=
J
fc'[Cu ][HL-][0 ] 2+
2CuHL — +
(CuHL)
2
+
2 +
0
(CuHL)
2
(10)
2 +
2
^± C u H L 0 C u H L
2
(9)
1/2
2
(11)
2 +
slow
CuHL0 CuHL 2
2 +
>LCu0 H- + 2
Cu
2 +
+
L-
(12)
free r a d i c a l s - » products
(13)
p r e s e n c e of other anions that c o o r d i n a t e C u ( I I ) ] t h e c h l o r i d e i o n seems to p a r t i c i p a t e i n b i n u c l e a r c o m p l e x f o r m a t i o n , a n d f u r t h e r
complicates
the kinetics.
In Ascorbic Acid: Chemistry, Metabolism, and Uses; Seib, P., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
174
ASCORBIC A C I D
T h e h a l f - o r d e r d e p e n d e n c e of t h e rate of a u t o x i d a t i o n of
ascorbic
a c i d o n m o l e c u l a r o x y g e n that w a s f o u n d b y B l a c k b u r n a n d J a m e s o n a n d that w a s the basis for t h e i r suggestion of b i n u c l e a r i n t e r m e d i a t e s 17 a n d 18, is q u i t e i n t e r e s t i n g , e s p e c i a l l y i n v i e w of the fact t h a t p e r o x o b r i d g e d d i o x y g e n complexes analogous to 18 h a v e also b e e n o b s e r v e d for c o b a l t d i o x y g e n c o m p l e x systems (17).
A s n o t e d a b o v e , the
first-order
dioxygen
d e p e n d e n c e o b s e r v e d b y T a q u i K h a n a n d M a r t e l l ( 9 ) undergoes a t r a n s i t i o n at l o w o x y g e n concentrations to l o w e r order d e p e n d e n c e a n d
finally
z e r o - o r d e r d e p e n d e n c e as i n d i c a t e d i n F i g u r e 3. T h u s it m a y v e r y w e l l Downloaded by UNIV OF GUELPH LIBRARY on June 5, 2012 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0200.ch007
b e that at l o w o x y g e n concentrations b i n u c l e a r complexes
of the k i n d
o b s e r v e d b y B l a c k b u r n a n d J a m e s o n b e c o m e the r e a c t i o n i n t e r m e d i a t e s for a u t o x i d a t i o n of ascorbic
a c i d , r e s u l t i n g i n the o b s e r v e d
d e p e n d e n c e of the r e a c t i o n rates o n o x y g e n c o n c e n t r a t i o n . h a n d , b i n u c l e a r complexes
half-order
O n the other
of the t y p e i l l u s t r a t e d , 17 a n d 18, t e n d to
f o r m o n l y i n solutions i n w h i c h the m e t a l i o n c o n c e n t r a t i o n is at least m o d e r a t e l y h i g h . T h e r e f o r e i n solutions i n w h i c h t h e c a t a l y t i c species, either the C u ( I I ) or F e ( I I I ) ions, or t h e i r m e t a l chelates, are present at v e r y l o w concentrations the f o r m a t i o n of b i n u c l e a r c o m p l e x i n t e r m e d i a t e s is not v e r y l i k e l y .
U n d e r s u c h c o n d i t i o n s , catalysis b y
mononuclear
complexes of the types i n d i c a t e d i n Schemes 2 a n d 3 w o u l d seem to be favored. It is i n t e r e s t i n g to note that the + 3
o x i d a t i o n state of c o p p e r has
b e e n i n v o k e d b y J a m e s o n a n d B l a c k b u r n (15,16) d i o x y g e n complexes.
i n the f o r m a t i o n of
A l t h o u g h this is a n a t t r a c t i v e i d e a i n v i e w of recent
investigations r e p o r t e d b y M a r g e r u m et a l . (18),
the f o r m a t i o n of stable
C u ( I I I ) complexes i n aqueous s o l u t i o n w o u l d r e q u i r e c o o r d i n a t i o n w i t h l i g a n d s h a v i n g v e r y s p e c i a l properties.
H e r e a g a i n i t seems
somewhat
u n l i k e l y that a p p r e c i a b l e concentrations of C u ( I I I ) complexes are f o r m e d i n these r e a c t i o n systems.
O n the other h a n d , C u ( I I I ) m a y b e i n v o k e d
for e x p l a i n i n g the stabilities of C u ( I I ) - d i o x y g e n
complexes
of the t y p e
i l l u s t r a t e d i n S c h e m e 2, a n d 18. T h u s t h e d i o x y g e n c o m p l e x i n t e r m e d i a t e s f o r m e d i n trace a m o u n t s i n these r e a c t i o n systems m a y b e
considered
to i n v o l v e o x i d a t i o n of c o p p e r to a n i n t e r m e d i a t e o x i d a t i o n state b e t w e e n C u ( I I ) and C u ( I I I ) .
Role of Ascorbic Acid in a Mono-oxygenase Model (U den friend's System) R e c e n t l y ascorbic a c i d has b e e n assigned a significant f u n c t i o n i n the reactions i n v o l v i n g o x y g e n i n s e r t i o n b y m o d e l oxygenase a n d peroxidase systems i n w h i c h f e r r i c i o n or a f e r r i c chelate is c o n s i d e r e d to b e t h e catalyst.
A l t h o u g h reaction mechanisms
involved ascorbic
acid merely
suggested
b y earlier workers
as a r e d u c t a n t to c o n v e r t
Fe(III)
to
F e ( I I ) , w h i c h w o u l d t h e n i n t u r n i n t e r a c t w i t h the o x i d a n t , H a m i l t o n
In Ascorbic Acid: Chemistry, Metabolism, and Uses; Seib, P., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
7.
175
Chelates of Ascorbic Acid
MARTELL
suggested that ascorbate or s i m i l a r reductants s u c h as c a t e c h o l are
(11)
i n v o l v e d i n the f o r m a t i o n of a t e r n a r y c o m p l e x i n w h i c h the m e t a l i o n is c o o r d i n a t e d s i m u l t a n e o u s l y to the r e d u c t a n t a n d the o x i d a n t . T h e r e a c t i o n sequences o r i g i n a l l y suggested for o x y g e n i n s e r t i o n i n a substrate s u c h as s a l i c y c l i c a c i d i n U d e n f r i e n d ' s S y s t e m (19-22) w h i c h molecular oxygen
is the o x i d a n t a n d a n i r o n chelate
in
s u c h as
F e ( I I ) - E D T A is the catalyst a n d ascorbic a c i d is the r e d u c t a n t , is i n d i c a t e d b y E q u a t i o n s 14-18.
I n this r e a c t i o n sequence o x y g e n i n s e r t i o n
is c o n s i d e r e d to o c c u r b y d i r e c t r e a c t i o n of the a r o m a t i c c o m p o u n d H0 Downloaded by UNIV OF GUELPH LIBRARY on June 5, 2012 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0200.ch007
2
with
a n d O H - free r a d i c a l s , a n d the ascorbate r e d u c t a n t m e r e l y serves
the p u r p o s e of r e g e n e r a t i n g the F e ( I I ) chelate as i n d i c a t e d b y E q u a t i o n s 16 a n d 17.
Fe -EDTA +
0
n
COO
+
2
H 0 -> F e - E D T A + m
2
COO"
OH" +
COO"
(+OH-)
2
Fe -EDTA + m
Fe -EDTA + m
H A + O H " -» F e - E D T A + n
2
HA- +
H A - - f O H " -> F e - E D T A + A + n
Fe -EDTA + H 0 n
(14)
2
COO"
(+H 0 ) 2
H0 -
2
2
-> F e - E D T A + m
OH- +
H 0
(16)
2
H 0
(17)
2
OH"
(18)
O n the basis of the facts t h a t o x y g e n i n s e r t i o n occurs p r e f e r e n t i a l l y at the ortho a n d para positions r e l a t i v e to the a c t i v a t i n g h y d r o x y l g r o u p o n the a r o m a t i c r i n g , a n d that t h e r e a c t i o n d i d not seem to i n v o l v e free r a d i c a l s , H a m i l t o n (11)
suggested a n i o n i c m e c h a n i s m i n v o l v i n g a t e r n a r y
ascorbate m e t a l d i o x y g e n c o m p l e x of the t y p e i l l u s t r a t e d b y 19 i n S c h e m e 4. I n t h e p r o p o s e d m e c h a n i s m a c o n c e r t e d shift of e l e c t r o n p a i r s results i n t h e i n s e r t i o n of a n o x y g e n a t o m i n t o the substrate a n d a c o n c o m i t a n t t w o - e l e c t r o n r e d u c t i o n of ascorbate to d e h y d r o a s c o r b i c a c i d . A n alternate e l e c t r o n transfer sequence is, of course, possible i n v o l v i n g t w o
single
e l e c t r o n transfer steps a n d the f o r m a t i o n of a n i n t e r m e d i a t e c o m p l e x i n w h i c h the ascorbate a n i o n r a d i c a l is c o o r d i n a t e d to t h e m e t a l i o n .
The
m e c h a n i s m p r o p o s e d i n S c h e m e 4 is s o m e w h a t m o r e s a t i s f y i n g t h a n t h e p r e v i o u s l y r e c o m m e n d e d free r a d i c a l m e c h a n i s m ( E q u a t i o n 1 4 - 1 8 )
In Ascorbic Acid: Chemistry, Metabolism, and Uses; Seib, P., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
be-
Downloaded by UNIV OF GUELPH LIBRARY on June 5, 2012 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0200.ch007
176
ASCORBIC
| CH OH
CH.OH Scheme 4.
ACID
20
2
Proposed mechanism for Fe(II)-catalyzed oxygen insertion (UdenfrieruTs system).
cause i t assigns a m o r e i m p o r t a n t r o l e to t h e m e t a l i o n that is a n essential catalyst i n these e n z y m e m o d e l systems. A similar mechanism illustrated i n Scheme b y H a m i l t o n ( I I ) for peroxidase
model
5 has b e e n
systems
suggested
(22) i n w h i c h the
o x i d a n t is h y d r o g e n p e r o x i d e , t h e r e d u c t a n t is a s c o r b i c a c i d o r a n a n a l o gous reagent s u c h as c a t e c h o l , a n d t h e c a t a l y t i c m e t a l i o n is F e ( I I I ) . I n this system t h e i n t e r m e d i a t e t e r n a r y c o m p l e x undergoes a n i n t r a m o l e c u l a r e l e c t r o n transfer i n v o l v i n g fission of t h e o x y g e n - o x y g e n
b o n d of t h e
p e r o x i d e l i g a n d to g i v e w a t e r a n d a n i n t e r m e d i a t e c o m p l e x t h a t essentially i n v o l v e s c o o r d i n a t i o n of a t o m i c o x y g e n w i t h F e ( I I I ) .
This intermediate
is s o m e w h a t s t a b i l i z e d b y several resonance f o r m s (21a-c) i n w h i c h some n e g a t i v e c h a r g e is seen to reside o n t h e o x y g e n a t o m , thus a c c o u n t i n g f o r its a b i l i t y to r e m a i n briefly c o o r d i n a t e d w i t h t h e m e t a l i o n . I n s e r t i o n of a t o m i c o x y g e n i n t o a n a p p r o p r i a t e substrate results i n r e g e n e r a t i o n of
In Ascorbic Acid: Chemistry, Metabolism, and Uses; Seib, P., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
Downloaded by UNIV OF GUELPH LIBRARY on June 5, 2012 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0200.ch007
7.
177
Chelates of Ascorbic Acid
MARTELL
Scheme 5.
Proposed mechanism for a model peroxidase system.
t h e F e ( I I I ) c o m p l e x 6 of the r e d u c i n g l i g a n d .
R e d u c t i o n of
Fe(III)
b y the ascorbate l i g a n d is p r e v e n t e d b y t h e o x i d a n t , h y d r o g e n p e r o x i d e , to regenerate
the o r i g i n a l r e a c t i v e t e r n a r y c o m p l e x ,
21.
Hamilton's
m e c h a n i s m for these p e r o x i d a s e m o d e l reactions is of interest because of its s i m i l a r i t y
to
the
reactions
o c c u r r i n g i n catalase
and
peroxidase
e n z y m i c systems i n w h i c h resonance forms of the i r o n p o r p h y r i n r i n g system s t a b i l i z e a t w o - e l e c t r o n o x i d a n t i n t e r m e d i a t e b e l i e v e d to i n v o l v e a coordinated oxygen atom.
In Ascorbic Acid: Chemistry, Metabolism, and Uses; Seib, P., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
178
ASCORBIC ACID
Downloaded by UNIV OF GUELPH LIBRARY on June 5, 2012 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0200.ch007
Literature Cited 1. Smith, R. M.; Martell, A. A. "Critical Stability Constants"; Plenum: New York, 1977. 2. Hvoslef, J. Acta Crystallogr., Sect. B 1974, 30, 2711. 3. Hughes, D. L. J. Chem. Soc., Dalton Trans. 1973, 2209. 4. Hvoslef, J. Acta Crystallogr., Sect. B 1969, 25, 2214. 5. Kriss, E. E. Russ. J. Inorg. Chem. 1978, 23(7), 1004. 6. Evtushenko, N. P.; Yatsimirskii, K. B.; Kriss, E. E.; Kurbatova, G. T. Zh. Neorg. Kim. 1977, 22, 1543. 7. Kriss, E. E.; Kurbatova, G. T.; Kuts, V. S.; Prokopenko, V. P. Zh. Neorg. Khim. 1976, 21, 2978. 8. Taqui Khan, M. M.; Martell, A. E. J. Am. Chem. Soc. 1967, 89, 4176. 9. Ibid., 7104. 10. Dekker, A. O.; Dickinson, R. G. J. Am. Chem. Soc. 1940, 62, 2165. 11. Hamilton, G. A. Adv. Enzymol. Delat. Subj. Biochem. 1969, 32, 55. 12. Taqui Khan, M. M.; Martell, A. E. J. Am. Chem. Soc. 1968, 90, 6011. 13. Ibid., 1969, 91, 4468. 14. Jameson, R. F.; Blackburn, N. J. J. Inorg. Nucl. Chem. 1975, 37, 809. 15. Jameson, R. F.; Blackburn, N. J. J. Chem. Soc., Dalton Trans. 1976, 534. 16. Ibid., 1596. 17. McLendon, G.; Martell, A. E. Coord. Chem. Rev. 1976, 19, 1. 18. Margerum, D. W.; Chellappa, K. L.; Bossu, F. P.; Burce, G. S. J. Am. Chem. Soc. 1975, 97, 6894. 19. Udenfried, S.; Clark, C. T.; Axelrod, J.; Brodie, B. B. J. Biol. Chem. 1954, 208, 731. 20. Brodie, B. B.; Axelrod, J.; Shore, P. A.; Udenfriend, S. J. Biol. Chem. 1954, 208, 741. 21. Mason, H. S.; Onoprienko, I.; Buhler, E. Biochim. Biophys. Acta 1957, 24, 225. 22. Taqui Khan, M. M.; Martell, A. E. "Homogeneous Catalysis by Metal Complexes: Activation of Small Inorganic Molecules"; Academic: New York, 1974; Vol. 1, p. 151. RECEIVED for review January 22, 1981. ACCEPTED May 11, 1981.
In Ascorbic Acid: Chemistry, Metabolism, and Uses; Seib, P., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.