Electrochemical and Spectrochemical Studies of Biological Redox

Electrochemical and Spectrochemical Studies of Biological Redox Components Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SANTA BARBARA on 08/26/15. For personal use only.

29 Electrochemistry and Spectroscopy of Catalytically Active Molybdenum-Catechol Complexes HARRY O. FINKLEA, SUZANNE K. LAHR, and FRANKLIN A. SCHULTZ Florida Atlantic University, Department of Chemistry, Boca Raton, F L 33431

Electrochemical reduction of the cis-dioxo Mo(VI)-catechol complex, MoO (cat) , produces stable mononuclear Mo(V), Mo(IV), and Mo(III) complexes in weakly alkaline aqueous solution. Electrochemistry and spectroscopy are used to characterize these species and the conditions promoting their formation. Their catalytic reactions with several oxo anions are studied. Coordination reactions at sites generated by reductive removal of oxo groups stabilize the reduced monomers and also mediate the redox, spectroscopic, and catalytic properties of the molybdenum center. These results help explain the relationship between molecular structure and redox properties of a mononuclear molybdenum site. Thesefindingsare discussed in relation to the behavior of the molybdenum-containing enzymes (xanthine oxidase, sulfite oxidase, and nitrate reductase) that carry out oxygen-atom transfer reactions. 2-

2

2

'J^he m o l y b d e n u m - c o n t a i n i n g e n z y m e s a l d e h y d e oxidase, sulfite oxidase, x a n t h i n e oxidase, a n d nitrate reductase catalyze reactions i n w h i c h a n o x y g e n a t o m is a d d e d to o r r e m o v e d f r o m t h e substrate m o l e c u l e ( J , 2). I s o l a t i o n o f a c o m m o n l o w - m o l e c u l a r w e i g h t c o f a c t o r f r o m t h e l a s t t h r e e o f t h e s e s p e c i e s (3, 4) p l u s p h y s i c o c h e m i c a l s t u d i e s o f t h e e n z y m e s t h e m s e l v e s (5-17) s u g g e s t a s i m i l a r e n v i r o n m e n t f o r the m o l y b d e n u m a t o m i n these systems. I m p o r t a n t features o f the e n v i r o n m e n t i n c l u d e m o n o n u c l e a r structure w i t h respect to M o , o x o m o l y b d e n u m b o n d i n g , predominantly sulfur ligation i n the re0065-2393/82/0201-0709$06.25/0 © 1982 A m e r i c a n C h e m i c a l Society

BIOLOGICAL REDOX COMPONENTS


710

m a i n d e r o f the c o o r d i n a t i o n sphere, c y c l i n g a m o n g M o ( V I ) , M o ( V ) , a n d M o ( I V ) o x i d a t i o n states d u r i n g e l e c t r o n transfer, a n d M o ( V ) E P R s i g n a l s t h a t are s e n s i t i v e t o t h e b i n d i n g o f s u b s t r a t e s , p r o d u c t s , a n d buffer anions at the m e t a l site. C o n s i s t e n t w i t h w e l l - k n o w n structural f e a t u r e s o f m o l y b d e n u m c o m p l e x e s (18), m o s t l i k e l y t h e M o ( V I ) , M o ( V ) , a n d M o ( I V ) states i n t h e e n z y m e s c o n t a i n t w o , o n e , a n d o n e t e r m i n a l l y b o n d e d oxo groups, respectively, although presence o f one t e r m i n a l s u l f i d o i n p l a c e o f o x o w a s p r o p o s e d for t h e a c t i v e o x i d i z e d f o r m o f x a n t h i n e o x i d a s e (15). Redox chemistry of oxomolybdenum complexes i n both aqueous a n d n o n a q u e o u s m e d i a often is c o m p l i c a t e d b y i r r e v e r s i b l e c h e m i c a l r e a c t i o n s t h a t o c c u r after e l e c t r o n t r a n s f e r a n d l e a d t o p o o r l y c h a r a c t e r i z e d p r o d u c t s . T h e w e l l - c h a r a c t e r i z e d (19-21) c i s - d i o x o M o ( V I ) c a t e c h o l c o m p l e x , M o 0 ( c a t ) " (I) ( H ^ a t = 1 , 2 - d i h y d r o x y b e n z e n e ) , can b e r e d u c e d to stable M o ( V ) , M o ( I V ) , a n d M o ( I I I ) monomers i n 2

2

2

I

a q u e o u s s o l u t i o n . T h i s series o f c o m p l e x e s is a n e f f e c t i v e v e h i c l e for s t u d y i n g t h e r e l a t i o n s h i p b e t w e e n s t r u c t u r a l a n d r e d o x c h e m i s t r y at a m o n o n u c l e a r m o l y b d e n u m center. Important c o m p o n e n t s i n this rela tionship i n c l u d e reversible r e m o v a l of oxo groups u p o n reduction i n o x i d a t i o n state a n d s u b s e q u e n t c o o r d i n a t i o n r e a c t i o n s at t h e s e sites, w h i c h s t a b i l i z e r e d u c e d m o n o m e r i c species. I n a d d i t i o n , the r e d u c e d complexes catalyze the reduction o f a n u m b e r o f oxo anions. W e re v i e w h e r e o u r r e c e n t s t u d i e s (22-24) o n t h e e l e c t r o c h e m i c a l , s p e c t r o scopic, a n d catalytic properties o f aqueous m o l y b d e n u m - c a t e c h o l complexes a n d discuss possible implications for the behavior o f m o l y b d e n u m - c o n t a i n i n g e n z y m e s that c a t a l y z e o x y g e n - a t o m transfer reactions.

Experimental F u l l details o f experimental procedures are p r o v i d e d i n References 22-24. C o m p l e x I was formed b y a d d i n g N a M o 0 4 * 2 H 0 to a 50-fold or greater excess of catechol (20). Voltammetric experiments were conducted at a h a n g i n g mercury drop electrode o f 0.022 c m area. D c polarographic experi ments were c o n d u c t e d w i t h a c a p i l l a r y h a v i n g characteristics of m = 1.82 m g/s 2

2

2

29.

FINKLEA ET A L .

711

Molybdenum-Catechol Complexes

a n d t = 4.3 s at - 0 . 8 V i n 1 M K C 1 a n d 0.15 M H c a t ( p H 9.4). Spectroelectrochemistry was done i n a transparent thin-layer c e l l c o n t a i n i n g an amalga mated g o l d m i n i g r i d w o r k i n g electrode. T h e c e l l design was similar to that of A n d e r s o n et a l . (25). Ε P R spectra were recorded at room temperature w i t h a Varian V-4502 instrument. Samples were prepared b y c o n t r o l l e d potential re duction at a mercury p o o l cathode and transferred anaerobically to the Ε P R c e l l on a S c h l e n k l i n e . T h e monomeric M o ( V ) content o f the solution was established b y t w o means: (1) measurement o f the voltammetric peak current for M o ( V ) —> M o ( V I ) oxidation f o l l o w i n g c o n t r o l l e d potential reduction, a n d (2) d o u b l e integration of the M o ( V ) Ε P R signal intensity a n d comparison to that observed for a standard solution o f K M o ( C N ) . A g r e e m e n t b e t w e e n the two techniques was ± 2 0 % . Unless otherwise noted, potentials are reported vs. the saturated c a l o m e l reference electrode ( S C E ) .


2

3

8

Results and Discussion A q u e o u s E l e c t r o c h e m i s t r y at p H 3 . 5 - 7 . T h e c y c l i c v o l t a m m o g r a m i n F i g u r e 1 is r e p r e s e n t a t i v e o f t h e e l e c t r o c h e m i c a l b e h a v i o r o f M o 0 ( c a t ) " i n w e a k l y acidic solution. Peak current magnitudes, peak p o t e n t i a l separations, a n d c o n t r o l l e d - p o t e n t i a l c o u l o m e t r i c e x p e r i m e n t s c o n f i r m the successive transfer o f one a n d t w o electrons, i n a g r e e m e n t w i t h t h e i n i t i a l p o l a r o g r a p h i c s t u d y o f t h i s c o m p l e x (26). T h u s , r e d u c t i o n e n s u e s f r o m M o ( V I ) —» M o ( V ) a n d M o ( V ) —> M o ( I I I ) . T h e p H d e p e n d e n c e s o f the v o l t a m m e t r i c h a l f - w a v e potentials i n d i cate the u p t a k e o f t w o protons i n e a c h e l e c t r o n transfer step a n d c o n v e r s i o n o f a n o x o g r o u p to a c o o r d i n a t e d w a t e r m o l e c u l e : 2

2

2

2

Mo0 (cat) " + 2 H 2

2

+

+ e " τ± M o O ( H 0 ) ( c a t ) -

MoO(H 0)(cat) " + 2 H 2

2

2

+

2

+ 2 e ~ τ± M o ( H 0 ) ( c a t ) 2

2

2

(E

1/2

)i

(1)

(E

1 / 2

)

(2)

C o o r d i n a t i o n o f a d d i t i o n a l l i g a n d s or b u f f e r c o m p o n e n t s t h e s e a q u o sites a n d is d e t e c t a b l e

2

occurs

at

f r o m shifts i n (£1/2)1 a n d (£1/2)2

w i t h c h a n g e s i n t h e i r c o n c e n t r a t i o n s . T h u s , s p e c i e s s u c h as L = p y r i d i n e or i m i d a z o l e f o r m m o n o a d d u c t s

i n t h e M o ( V ) state a n d m o n o

and

b i s a d d u c t s i n t h e M o ( I I I ) state. M o O ( H 0 ) ( c a t ) - + L 9. O n e - e l e c t r o n , c o n t r o l l e d - p o t e n t i a l r e d u c t i o n o f M o 0 ( c a t ) ~ i n a s o l u t i o n buffered w i t h excess c a t e c h o l produces the cyclic voltammogram s h o w n i n F i g u r e 2 A . T h e anodic p e a k at —0.56 V c o r r e s p o n d s t o o x i d a t i o n o f M o ( V ) m o n o m e r , p r e s e n t at 8 0 - 9 0 % o f t h e t o t a l M o c o n c e n t r a t i o n u n d e r t h e s e c o n d i t i o n s , a n d t h a t at - 0 . 2 0 V t o o x i d a t i o n o f t h e r e s i d u a l M o ( V ) , p r e s e n t as d i m e r . 2


2

2

VA .O-Mo-0

.

) 4 8 1.7 ~5

x x x x

2

βι (Μ-')

10' 10 10 10

2

/3 '"(M- ) 2

1 x 10 3 χ 10

4

4

4

l a

Estimated from EPR signal intensities.

2 x 10

2

6 χ 10 2 x 10

5

e


714


A.

B.

nm Figure 2. Cyclic volt ammo gram s at 50 mV/s (A,D), EPR spectra (B,E), and visible absorption spectra (C,F) of Mo(V) complexes produced by one-electron controiled-potential reduction of Mo0 (cat) ~ at pH >9. Solution conditions (A-C): 2 mM Mo, 0.15 M H cat, 1 M KCl, pH 9.4; (D-F): 2 mM Mo, 0.1 M H cat, 1 M NH Cl, and 1 M NH , pH 9.6. 2

2

2

2

2

2

4

29.

FINKLEA ET A L .

The

m o n o m e r - d i m e r d i s t r i b u t i o n is c o n f i r m e d b y q u a n t i t a t i v e


715 mea

s u r e m e n t o f t h e i n t e n s i t y o f t h e M o ( V ) E P R s i g n a l t h a t is o b s e r v e d for the r e d u c e d solution ( F i g u r e 2 B ) . T h e p H a n d catechol

dependences

o f the M o ( V I ) - M o ( V ) p o t e n t i a l u n d e r these conditions i n d i c a t e that


t h e e l e c t r o d e h a l f - r e a c t i o n is Mo0 (cat) 2

2 2

- + Hzcat + H

+

2

+ e " *± M o O ( H c a t ) ( c a t ) " 2

( E 1,2)1'

+ H 0 2

9 5 , 9 7

T h u s , t h e E P R s i g n a l at g = 1.933, w h i c h h a s a p l i n g constant

of A =

45

4

x

10" cm

_ I

(6)

M o hyperfine cou

, is a s s i g n e d to t h e

species

2

M o O ( H c a t ) ( c a t ) " . T h e v i s i b l e s p e c t r u m o f this species r e c o r d e d i n a 2

t h i n - l a y e r s p e c t r o e l e c t r o c h e m i c a l c e l l e x h i b i t s a s h o u l d e r at ~ 4 6 5 n m 1

(21,500 c m " ) w i t h e = 2400 M ^ e m " t h i s e n e r g y is c h a r a c t e r i s t i c o f M o 0

3

1

+

( F i g u r e 2 C ) . A n a b s o r p t i o n at complexes

(18).

Virtually i d e n t i c a l electrochemical behavior a n d no change

in

( £ 1 / 2 ) 1 ' is o b s e r v e d i n b u f f e r s c o n t a i n i n g N H o r a m i n e s i n a d d i t i o n to 3

catechol ( F i g u r e 2 E ) . H o w e v e r , u n d e r these conditions, a s m a l l n e w E P R s i g n a l a p p e a r s at g = 1.943 ( F i g u r e 2 D ) , a n d t h e v i s i b l e a b s o r p tion b a n d intensifies a n d b e c o m e s better defined w i t h X nm

and € = 2600 M

_ 1

cm

- 1

m a x

= 470-475

(Figure 2F). These spectroscopic

changes

a r e a t t r i b u t e d to a s m a l l e x t e n t o f s u b s t i t u t i o n , e s t i m a t e d to b e 3 - 5 % from E P R signal intensities, b y R N H 2

MoO(Hcat)(cat) " + R N H 2

2

2

for H e a t " o n t h e M o ( V ) c e n t e r .

τ± M o O ( R N H ) ( c a t ) " + H e a t " 2

(7)

2

T h e r e d o x a n d coordination b e h a v i o r o f the M o ( V I ) - M o ( V ) c o u p l e at p H > 9 is d e p i c t e d i n S c h e m e I I . S t a b i l i z a t i o n o f m o n o m e r i c M o ( V ) is a c h i e v e d b y catechol Oi

v

firm

coordination o f monodentate, 4

= 1.7 x 1 0 M

_ 1

monoprotonated

, T a b l e I) at t h e site g e n e r a t e d b y r e m o v a l 2

of one oxo group from M o 0 ( c a t ) " . D i s s o c i a t i o n o f the 2

f r o m H ^ a t [pK

2

first

proton

= 9.23 (27)] assists t h i s c o o r d i n a t i o n . A p p r o x i m a t e l y

a

1 0 - 2 0 % o f t h e M o ( V ) e x i s t s as a d i m e r u n d e r t h e s e c o n d i t i o n s . A t p H < 7 a s m a l l a m o u n t o f m o n o m e r i c M o ( V ) is d e t e c t e d i n e q u i l i b rium

with

Mo 0 (cat) (H 0) 2

4

2

2

2 2

~ (22),

and

this

finding

suggests

a

M o ( V ) ?± 2 M o ( V ) e q u i l i b r i u m t h a t is s h i f t e d l a r g e l y to t h e r i g h t at 2

h i g h p H a n d t o t h e l e f t at l o w p H . The

M o ( V I ) - M o ( V ) - c a t e c h o l c o u p l e is t h e first e x a m p l e o f r e v e r

sible electrochemical conversion between M o 0

2 + 2

and M o 0

3

+

centers

y i e l d i n g stable, characterizable products. I f the monodentate l i g a n d L r e m a i n s at t h e s i t e o f t h e d i s p l a c e d o x o g r o u p cis Mo0 (cat) 2

pair

2 2

to M o = 0 ,

then

" a n d M o O ( L ) ( c a t ) " constitute a structurally related redox

analogous

2

to

the

compounds

Mo0 (tox)2 2

and

MoOCl(tox)2


716

HON

X>—Mo—0

Mo—0 Electrochemical and Spectrochemical Studies of Biological Redox Components Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SANTA BARBARA on 08/26/15. For personal use only.

ίό)

RNhU

NHoR

9 (23).

l/°

.0—Mo-0

molybdenum-cate

( H t o x = 8 - m e r c a p t o q u i n o l i n o l ) , r e c e n t l y c h a r a c t e r i z e d (28). S u b s t i t u t i o n r e a c t i o n s at t h i s site affect t h e r e d o x , s p e c t r o s c o p i c , a n d c a t a l y t i c p r o p e r t i e s o f t h e M o ( V ) c e n t e r . T h e r e f o r e , t h i s c o o r d i n a t i o n p o s i t i o n is s i m i l a r i n s e v e r a l r e s p e c t s to t h e a n i o n - b i n d i n g site o f t h e M o ( V ) state o f x a n t h i n e o x i d a s e , s u l f i t e o x i d a s e , a n d n i t r a t e r e d u c t a s e (9, J O , 1517). F u r t h e r a s p e c t s o f t h i s r e l a t i o n s h i p a r e d i s c u s s e d i n t h e c o n c l u d i n g section o f this chapter. GENERATION O F MO(IV) A N D MO(III) SPECIES. A t h i g h p H t h e character o f the s e c o n d M o 0 ( c a t ) ~ r e d u c t i o n w a v e changes from a t w o - e l e c t r o n M o ( V ) —> M o ( I I I ) s t e p t o o n e i n w h i c h a n e n s u i n g c h e m i c a l r e a c t i o n l e a d s to f o r m a t i o n o f a n i n t e r m e d i a t e f o r m a l l y i n t h e M o ( I V ) o x i d a t i o n state. T h i s b e h a v i o r is i l l u s t r a t e d i n F i g u r e 3 for a p H 7.7 e t h y l e n e d i a m i n e b u f f e r . T h e 0.2 V / s e x p e r i m e n t s h o w s t h e e x p e c t e d t w o - e l e c t r o n r e d u c t i o n o f M o ( V ) at - 0 . 9 5 V. 2

2

2

+

29.

FiNKLEA ET AL.

717

Molybdenum-Catechol Complexes 2

MoO(Hcat)(cat) - + 2 H

+

2

2

+ 2e~ 9. F i r m b i n d i n g o f l i g a n d s at sites g e n e r a t e d b y r e m o v a l o f o x o groups stabilizes the r e d u c e d m o n o m e r s against d i m e r i z a t i o n . T h e structural changes that ensue w h e n l i g a n d s r e p l a c e oxo groups also

720



c a u s e s u c c e s s i v e e l e c t r o n transfers to b e a c c o m p l i s h e d m o r e e a s i l y . A s a r e s u l t , t h e f o u r o x i d a t i o n states, M o ( V I ) t h r o u g h M o ( I I I ) , c a n b e g e n e r a t e d as s t a b l e m o n o m e r i c s p e c i e s w i t h i n a r a n g e o f a b o u t 450 m V . Electrocatalytic Reduction of Oxo Anions. T h e ability of molyb d e n u m t o c a t a l y z e c h e m i c a l l y (36-38) a n d e l e c t r o c h e m i c a l l y (39-41) the r e d u c t i o n o f oxo anions l e d u s to e x a m i n e t h e r e a c t i v i t y o f s u c h species d u r i n g e l e c t r o c h e m i c a l reduction o f m o l y b d e n u m - c a t e c h o l c o m p l e x e s . I n f o r m a t i o n o n the c o m p o s i t i o n a n d structure o f e l e c t r o d e r e a c t i o n p r o d u c t s o f t h e s e c o m p l e x e s c a n t h e n s e r v e as a b a s i s f o r u n d e r s t a n d i n g t h e features that c o n t r o l c a t a l y t i c r e a c t i v i t y at a m o n o n u c l e a r m o l y b d e n u m site. F i g u r e 5 shows d c p o l a r o g r a p h i c a n d c y c l i c v o l t a m m e t r i c traces o f p H 9.4 c a t e c h o l s o l u t i o n s c o n t a i n i n g : n i t r i t e i o n , M o ( V I ) , a n d M o ( V I ) p l u s n i t r i t e i o n . C a t a l y t i c r e d u c t i o n o f n i t r i t e o c c u r s i n t w o r e g i o n s : (1) at p o t e n t i a l s b e t w e e n - 0 . 7 a n d - 1 . 0 V , w h e r e M o ( V I ) i s r e d u c e d t o M o ( V ) , a n d (2) at p o t e n t i a l s i n t h e v i c i n i t y o f - 1 . 1 V , w h e r e M o ( V ) i s r e d u c e d to M o ( I V ) b y t h e s e q u e n c e o f Reactions 8 a n d 9 p l u s t h e r e v e r s e o f R e a c t i o n 10. W h e n t h e p o t e n t i a l i s s w e p t t o m o r e n e g a t i v e values, t h e current d i m i n i s h e s a n d defines a m a x i m u m . I n a c y c l i c v o l t a m m e t r i c e x p e r i m e n t (left s i d e o f F i g u r e 5 C ) t h e c u r r e n t a l s o d e fines a c a t h o d i c m a x i m u m w h e n s w e p t i n t h e p o s i t i v e d i r e c t i o n f r o m p o t e n t i a l s m o r e n e g a t i v e t h a n —1.1 V . T h e s e o b s e r v a t i o n s a r e c h a r a c teristic o f a catalytic c h e m i c a l reaction interposed b e t w e e n sequential c h a r g e t r a n s f e r s t e p s (42). F i g u r e 5 d e m o n s t r a t e s t h a t b o t h t h e M o ( V ) a n d M o ( I V ) o x i d a t i o n states a r e e f f e c t i v e i n t h e c a t a l y t i c r e d u c t i o n o f nitrite i o n ; that M o ( I V ) is m o r e reactive than M o ( V ) ; a n d that t h e M o ( I I I ) is e i t h e r i n a c t i v e or s u b s t a n t i a l l y less r e a c t i v e t h a n M o ( I V ) a n d Mo(V). O t h e r substrates r e d u c e d b y t h e M o ( I V ) - a n d M o ( V ) - c a t e c h o l c o m p l e x e s are c h l o r a t e a n d b r o m a t e i o n ( T a b l e I I ) . I n b o t h cases t r a c e s s i m i l a r to F i g u r e 5 C are r e c o r d e d i n the presence o f these ions, a n d the s e q u e n c e o f r e a c t i v i t y is M o ( I V ) > M o ( V ) > M o ( I I I ) . B e c a u s e the p o tentially more o x i d i z i n g nitrate a n d perchlorate anions are i n a c t i v e , the feature r e s p o n s i b l e for catalytic a c t i v i t y appears to b e t h e n u c l e o p h i l i c character o f the substrate i m p a r t e d b y t h e u n s h a r e d p a i r o f e l e c t r o n s o n its c e n t r a l a t o m . T h e k i n e t i c s o f the M o ( V ) - n i t r i t e reaction w e r e s t u d i e d b y c y c l i c v o l t a m m e t r y a n d d c p o l a r o g r a p h y . R e s u l t s c o n f o r m to t h e b e h a v i o r o f a c a t a l y t i c E C m e c h a n i s m (43, 44), a l t h o u g h t h e c o m p l e t e m e c h a n i s m p r o b a b l y is m o r e c o m p l i c a t e d . T h e rate c o n s t a n t s (k ) r e p o r t e d i n T a b l e I I s h o u l d n o t b e c o n s i d e r e d a c c u r a t e for t h i s r e a s o n , b u t t h e i r r e l a t i v e values a r e u s e f u l i n d e t e r m i n i n g q u a l i t a t i v e aspects o f the mechanism. U n d e r pseudo first-order conditions [nitrite i o n a n d obs

FINKLEA ET A L .


29.

ι -0.5


ι

ι

I 1 -1.0 E ( V o l t s vs. S C E ) ι

ι

1

1

1

721

1 -1.5

Figure 5. Dc polarographic and 20 mV/s cyclic voltammetric traces of 0.15 M H cat, 1 M KCl, pH 9.4 solutions containing A , 0.2 M KN0 , S = 1 μΑ; B, 1 mM Mo(Vl), S = 5 μΑ for dc polarography, S = 2 μΑ for cyclic voltammetry; and C, 1 mM Mo(VI) + 0.2 M KN0 , S = 10 μΑ. 2

2

2


722


Table II. Substrates Catalytically R e d u c e d by M o ( V ) - and Mo(IV)-Catechol Complexes

b

Substrate

Mo(V)

NCV C10 " Rr0 " NO3" C10 ~

2.5 x 1 0 1.5 x 1 0 2.0 x 1 0

3

3

Mo(IV) 4 x 10 6 x 10 3 x 10

1

1

3

2

2

6

N o reaction N o reaction

4

a

Observed second-order rate constant in the presence of excess substrate and excess constant catechol concentration, as defined by Equation 15. Determined by cyclic voltammetry from theory for a catalytic reaction following reversible charge transfer (43 ). Values are uncorrected for reaction stoichiometry and subject to limitations of accuracy discussed in the text. 0.15 M H cat, 1 M KC1, pH 9. Estimated as described in footnote a by correcting total current of the maximum near -1.1 V for catalytic and diffusionlimited current owing to Mo(V) and dividing by diffusion-limited current expected for a one-electron transfer. 2

6

c a t e c h o l p r e s e n t i n e x c e s s o v e r M o ( V I ) ] , t h e c a t a l y t i c r e a c t i o n r a t e is d i r e c t l y p r o p o r t i o n a l to t h e m o l y b d e n u m a n d n i t r i t e c o n c e n t r a t i o n s a n d i n v e r s e l y p r o p o r t i o n a l to the c o n c e n t r a t i o n o f m o n o p r o t o n a t e d c a t e c h o l ( H e a t " ) . T h e r e is n o p H d e p e n d e n c e o t h e r t h a n t h a t a t t r i b u t a b l e to t h e p r o t o n a t i o n o f H e a t " . W h e n e x c e s s N 0 ~ is r e a c t e d w i t h MoO(Hcat)(cat) ~, spectroscopic and electrochemical measurements i n d i c a t e t h a t M o 0 ( c a t ) ~ is r e g e n e r a t e d q u a n t i t a t i v e l y . T h e s e find i n g s are c o n s i s t e n t w i t h t h e m e c h a n i s m g i v e n as f o l l o w s i n w h i c h n i t r i t e i o n first r e p l a c e s H e a t " i n a r a p i d l y e s t a b l i s h e d e q u i l i b r i u m , 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 s t e p a n d o n e or m o r e s u b s e q u e n t r e a c tions that p r o d u c e M o 0 ( c a t ) ~ . 2

2

2

2

2

2

2

2

2

2

+

M o 0 ( c a t ) - + Hzcat + H 2

2

2

+ e~ «± M o O ( H c a t ) ( c a t ) " + H 0 2

2

MoO(Hcat)(cat) " + N 0 " ^ 2

MoO(N0 )(cat)

2

2

2 2

2

" + Heat"

(11) (12)

fc-i 2

MoO(N0 )(cat) 2

MoO(ONO)(cat)

2

2 2

-

(13)

r.d.s. 2

M o O ( O N O ) ( c a t ) " —» M o 0 ( c a t ) 2

2

2 2

" + products

(14)

Consistent w i t h s i m i l a r catalytic reactions o f M o ( V ) i n nonaqueous s o l v e n t s (36, 3 7 ) , t h e r a t e - d e t e r m i n i n g s t e p is p r e s u m e d to i n v o l v e r e a r r a n g e m e n t o f c o o r d i n a t e d N 0 " f r o m a n i t r o g e n - b o n d e d to a n o x y g e n - b o n d e d s p e c i e s . I f t h i s c h a n g e is a c c o m p l i s h e d so t h a t a n o x y g e n a t o m o f N 0 ~ b e c o m e s l o c a t e d cis to t h e o x o g r o u p o n M o ( V ) , t h e n 2

2

FINKLEA ET A L .

29.


723

a f a v o r a b l e s i t u a t i o n e x i s t s for r a p i d e l e c t r o n t r a n s f e r a n d r e g e n e r a t i o n 2

of M o 0 ( c a t ) ~ . Because b o n d i n g o f nitrite initially through nitrogen 2

2

w o u l d o c c u p y the

final

o c t a h e d r a l c o o r d i n a t i o n site cis to M o = 0 , a

possible means o f a c h i e v i n g this rearrangement


mediate

is t h r o u g h t h e i n t e r

structure: Ο

I Ο Ν

II/jo ^O—Mo—Ο

III

A h e p t a c o o r d i n a t e d , p e n t a g o n a l b i p y r a m i d a l s t r u c t u r e s i m i l a r to I I I was

observed

in Mo(VI)-benzohydroxamate complexes

o x y g e n a n d n i t r o g e n - b o n d e d h y d r o x y l a m i d e as a n η

2

containing

ligand

(45).

A n a p p r o p r i a t e rate l a w for t h e m e c h a n i s m d e s c r i b e d b y R e a c t i o n s 1 1 - 1 4 is

r

a

t

e

=

_ d[Mo(V)]

=

WMo(V)][substrate]

dt fc-i[Hcar] + k ~ ^i^2[Mo(V)j[substrate] _ , . . -, = [Hcaf] N>bs[Mo(V)][ substrate]

β

2

h

Values

o f the

observed

second-order

r

v

f

Α

Λ

Ί

Γ

/ 1 K

rate constant,

Zc bs 0

=

v (15)

kik /k^ 2

t

[ H c a t ~ ] , s u b j e c t to t h e l i m i t a t i o n s a l r e a d y d i s c u s s e d , a r e l i s t e d i n T a b l e I I for t h e v a r i o u s s u b s t r a t e s . D e t a i l e d m e c h a n i s t i c s t u d i e s w e r e

not

c o m p l e t e d for C 1 0 ~ a n d B r 0 " , b u t t h e s e i o n s are e x p e c t e d to r e a c t 3

3

w i t h M o ( V ) i n a m a n n e r s i m i l a r to N 0 " . 2

T h e n i t r o g e n - c o n t a i n i n g p r o d u c t o f R e a c t i o n 14 is e x p e c t e d t o b e n i t r i c o x i d e . H o w e v e r , N O is n o t d e t e c t e d , a n d a d d i t i o n a l r e d o x r e a c tions

apparently

occur

following

Reaction

14.

Titrations

2

MoO(Hcat)(cat) " w i t h nitrite indicate a M o ( V ) : N 0 2

2

of

" stoichiometry

o f b e t w e e n 2 a n d 3 : 1 . T h u s , n i t r o u s o x i d e , d i n i t r o g e n , or a m i x t u r e o f t h e s e gases m a y b e t h e u l t i m a t e r e a c t i o n p r o d u c t s . Q u a n t i t a t i v e s t u d y o f t h e c a t a l y t i c r e a c t i o n s o f M o ( I V ) is c o m p l i c a t e d b y t h e fact t h a t t h e s e p r o c e s s e s a r e p r e c e d e d b y r e a c t i o n s i n v o l v i n g M o ( V ) . H o w e v e r , the pattern o f reactivity o b t a i n e d from the r e l a t i v e rate constants i n T a b l e II i n d i c a t e s that c o o r d i n a t i o n b y a n u cleophilic

s u b s t r a t e a l s o is a r e q u i r e m e n t

for c a t a l y s i s b y m o l y b -


724

d e n u m ( I V ) . A n i m p o r t a n t q u e s t i o n is w h e t h e r t h e g r e a t e r r e a c t i v i t y o f 2

this species results from s p e c i a l features a t t r i b u t a b l e to the d

config

2

uration o f M o ( I V ) , i f the f o r m a l i s m M o ( I V ) ( c a t ) " is correct, or to t h e 3

occurrence

o f u n p a i r e d electron density o n t h e ligands, i f the

for

2

m a l i s m M o ( I I I ) ( c a t ) ( S Q ) " is c o r r e c t . D e t a i l s o f t h e c a t a l y t i c r e a c t i o n s Electrochemical and Spectrochemical Studies of Biological Redox Components Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SANTA BARBARA on 08/26/15. For personal use only.

2

o f t h i s o x i d a t i o n state a r e u n d e r

investigation to help answer

this

question. I m p l i c a t i o n s for B e h a v i o r o f O x o m o l y b d e n u m E n z y m e s . A l t h o u g h c a t e c h o l l i g a n d a n d its o x o m o l y b d e n u m c o m p l e x e s are r e m o t e from d i r e c t b i o l o g i c a l r e l e v a n c e , several features that are a p p a r e n t from t h e r e d o x a n d catalytic b e h a v i o r o f these m o n o n u c l e a r species merit discussion. T h e M o ( V I ) - M o ( V ) c o u p l e is a n e x a m p l e o f r e v e r s i b l e r e d o x c o n version between M o 0 and M o 0 centers, structures that rea sonably m i g h t b e e x p e c t e d to b e present i n t h e M o ( V I ) a n d M o ( V ) states o f t h e e n z y m e s . P r e v i o u s e l e c t r o c h e m i c a l i n v e s t i g a t i o n s c o n d u c t e d i n nonaqueous solvents have encountered irreversible b e havior almost exclusively w h e n reduction o f M o ( V I ) 0 species or o x i d a t i o n o f M o ( V ) 0 s p e c i e s is a t t e m p t e d (46-50). T h u s , a p r o t i c e n v i r o n m e n t facilitates r e v e r s i b l e e x c h a n g e o f one oxo g r o u p w i t h i n the Mo0 -Mo0 c o u p l e . T h e a q u o c o o r d i n a t i o n site g e n e r a t e d b y t h i s r e d u c t i o n is the l o c u s o f s u b s t i t u t i o n reactions that c a n alter the redox, spectroscopic, a n d catalytic properties o f the M o ( V ) center. T h e s e ob s e r v a t i o n s p a r a l l e l t h e b e h a v i o r o f t h e M o ( V ) states o f x a n t h i n e oxidase, sulfite oxidase, a n d nitrate reductase, w h e r e b i n d i n g o f sub strates, p r o d u c t s , o r b u f f e r a n i o n s at t h e m e t a l site affects t h e r e d o x p o t e n t i a l s (10, 51, 52) a n d M o ( V ) E P R s i g n a l s (6, 1 0 , 15-17) o f t h e enzymes. 2 +

3

+

2

2

2

+

3

+

2

For m o l y b d e n u m - c a t e c h o l complexes, the M o ( V I ) - M o ( V ) redox p o t e n t i a l is m e d i a t e d b y t h e effect o f c o o r d i n a t i o n R e a c t i o n s 3 a n d 6 o n e l e c t r o d e R e a c t i o n 1. T h i s p o t e n t i a l shifts i n t h e p o s i t i v e d i r e c t i o n f a v o r i n g M o ( V I ) —» M o ( V ) r e d u c t i o n w h e n a l i g a n d is b o u n d i n p l a c e o f a n o x o g r o u p . S i m i l a r shifts a r e n o t e d w h e n p r o d u c t s or b u f f e r a n i o n s are b o u n d i n t h e M o ( V ) states o f t h e e n z y m e s (51, 52). C o o r d i n a t i o n r e a c t i o n s at o x o sites n o t o n l y f a v o r t h e a t t a i n m e n t o f l o w e r o x i d a t i o n states i n m o l y b d e n u m c o m p l e x e s , b u t a l s o s t a b i l i z e m o n o m e r i c s p e c i e s a g a i n s t d i m e r i z a t i o n . C o n s e q u e n t l y , at p H > 9 i t is p o s s i b l e t o generate m o n o m e r i c M o ( V I ) , M o ( V ) , M o ( I V ) , a n d M o ( I I I ) complexes w i t h i n a range o f ca. 4 5 0 m V . T h e structural a n d c o m p o s i t i o n a l c h a n g e s that o c c u r d u r i n g e l e c t r o n transfer l e a d to a c o m p r e s s i o n o f r e d o x states, a n i m p o r t a n t f e a t u r e o f m o l e c u l e s t h a t m u s t c a r r y o u t m u l t i e l e c t r o n reactions. Spectroscopic properties o f M o O ( H c a t ) ( c a t ) " indicate that the s i x t h c o o r d i n a t i o n site o n t h i s c o m p l e x h a s a d d i t i o n a l f e a t u r e s i n 2

2

FINKLEA ET A L .

29.


725

c o m m o n w i t h t h e M o ( V ) a n i o n - b i n d i n g site o f o x o m o l y b d e n u m e n z y m e s . T h i s c o n d i t i o n is e x e m p l i f i e d b y the a p p e a r a n c e o f a n e w E P R 1

s i g n a l a n d intensification o f the 2 1 , 5 0 0 - c m " absorption b a n d w h e n a n a m i n e is s u b s t i t u t e d for H e a t " . E x c h a n g e o f l i g a n d s i n t h e c o o r d i n a t i o n 3


sphere o f M o 0

+

c o m p l e x e s frequently leads to changes i n these spec

t r o s c o p i c p r o p e r t i e s (18,53-56). B e c a u s e o f t h e r e v e r s i b l e c h a r a c t e r o f t h e M o ( V I ) - M o ( V ) c o n v e r s i o n , t h i s s u b s t i t u t i o n m o s t l i k e l y is c o n f i n e d to a n e q u a t o r i a l site cis t o M o = 0 i n t h e c o m p l e x e s M o O ( L ) ( c a t ) 2 ~ (for n

L = H e a t " , η = 2 ; for L = R N H ,

p y r i d i n e , i m i d a z o l e , η = 1). A l

2

though rearrangement

o f L t o t h e s i t e trans t o M o = 0 m a y o c c u r , n o

e v i d e n c e supports this p o s s i b i l i t y , a n d the process m a y b e b l o c k e d b y c h e l a t i o n o f c a t e c h o l at t h i s p o s i t i o n . B i n d i n g o f a n u c l e o p h i l i c s u b s t r a t e at t h e e x c h a n g e a b l e c o o r d i n a 2

tion site o n M o O ( H c a t ) ( c a t ) " appears t o b e a p r e r e q u i s i t e 2

to the

catalytic reduction o fN 0 " , C 1 0 " , and B r 0 " b y M o ( V ) . F a i l u r e to 2

3

3

r e d u c e nitrate c l e a r l y contrasts w i t h the e n z y m a t i c b e h a v i o r , b u t this d i s c r e p a n c y c a n b e attributed i n part to solvent, b e c a u s e nitrite is a m o d e r a t e l y s t r o n g a n d n i t r a t e a v e r y w e a k n u c l e o p h i l e i n w a t e r (57). Reduction

o f N 0 " b y other 3

Mo(V) complexes

was observed i n

n o n a q u e o u s s o l v e n t s (36-38), w h e r e c o o r d i n a t i o n o f t h i s a n i o n is m o r e favored. T h e contrasting behavior of N 0

3

" and N 0

2

" i n different sol

v e n t m e d i a s u p p o r t s a p r e v i o u s s u g g e s t i o n (36) t h a t t h e m o l y b d e n u m center at t h e a c t i v e site o f nitrate r e d u c t a s e e x p e r i e n c e s a p r e d o m i nantly nonaqueous A

final

environment.

point

is a p o s s i b l e e x p l a n a t i o n o f t h e oxidation-state

changes a n d r e d o x - p o t e n t i a l separations

o b s e r v e d for t h e o x o m o l y b

d e n u m e n z y m e s . T a b l e III illustrates that a c o m m o n feature o f nitrate reductase,

sulfite oxidase, a n d xanthine

Mo(VI)-Mo(V)

and Mo(V)-Mo(IV)

o x i d a s e is c l o s e l y

redox

steps.

spaced

Although

the

m o l y b d e n u m - c a t e c h o l complexes p r o v i d e a n example o f structural c h a n g e s t h a t l e a d t o c l o s e l y s p a c e d r e d o x states, t h e w e i g h t o f e x p e r i m e n t a l e v i d e n c e is that t h e o x o m o l y b d e n u m enzymes c y c l e a m o n g M o ( V I ) , M o ( V ) a n d M o ( I V ) rather than M o ( V I ) t h r o u g h M o ( I I I ) oxida t i o n states (2), a n d t h a t t h e l o w e r o x i d a t i o n states p r o b a b l y r e t a i n at l e a s t o n e t e r m i n a l l y b o n d e d o x o g r o u p (11, 13, 14). T h u s , t h e r e d o x sequence i n these e n z y m e s p r o b a b l y is best represented b y M o 0 Mo0

3 +

, and M o 0

2

+

2 + 2

,

centers.

A r e a c t i o n s e q u e n c e for l i n k i n g t h e s e t h r e e r e d o x states i n t o a n e l e c t r o n transfer s e q u e n c e s i m i l a r to that e x p e c t e d i n t h e e n z y m e s is p r o p o s e d i n S c h e m e I I I . T h e s c h e m e is c o n s t r u c t e d b y c o m b i n i n g t h e reversible proton-dependent

Mo0

2 2

+

-Mo0

3

+

c o u p l e d e m o n s t r a t e d for

o c t a h e d r a l c o m p l e x e s i n this w o r k w i t h recent reports o f r e v e r s i b l e Mo0

3

+

-Mo0

2

+

e l e c t r o c h e m i s t r y o b s e r v e d u n d e r a p r o t i c c o n d i t i o n s for

square p y r a m i d a l complexes c o n t a i n i n g either a single tetradentate


726 0

. —M J o — L |« * L V

/

X

2

H

2

e

Τ/χ

~

H 0

L—Mo—L


2

0

v « / L—Mo—L L

L/Î -X

V

ll/

L

L— Mo—L' L/ Scheme

III.

Reaction sequence Mo0 -Mo0 3+

Table III.

2+

,v«/ L—Mo—L L

L/ 2+

3+

for linking Mo0 -Mo0 redox couples. 2

and

Redox Potentials of O x o m o l y b d e n u m E n z y m e s

N i t r a t e r e d u c t a s e ( p H 7) S u l f i t e o x i d a s e ( p H 7) X a n t h i n e oxidase ( p H 8) (native) X a n t h i n e o x i d a s e ( p H 8) ( d e s u l f o ) M o - c a t e c h o l ( p H 8)

Mo(VI)/Mo(V)

Mo(V)/Mo(IV)

Ref.

+220 +38 -355 -440 -480

+180 -163 -355 -480 —

58 11 51 51

Note: Values are in millivolts vs. NHE. Ε ii2 of Reaction 1 extrapolated to pH 8. a

a m i n o t h i o l a t e l i g a n d (59) o r f o u r m o n o d e n t a t e t h i o p h e n o l a t e l i g a n d s (60). A c h a n g e i n c o o r d i n a t i o n n u m b e r is r e q u i r e d b e t w e e n t h e M o ( V I ) a n d M o ( I V ) states. T h i s c h a n g e is a c h i e v e d b y r e a r r a n g e m e n t o f m o n o d e n t a t e l i g a n d X t o t h e p o s i t i o n trans t o M o = 0 i n t h e M o ( V ) state. I n t h i s l o c a t i o n t h e M o - X b o n d w o u l d b e w e a k e n e d or p o s s i b l y b r o k e n b y t h e trans i n f l u e n c e o f t h e o x o g r o u p (61, 6 2 ) . T h e p r e s e n c e o f o n e less l i g a n d o n t h e m e t a l w o u l d c a u s e t h e p o t e n t i a l o f t h e M o ( V ) - M o ( I V ) c o u p l e to shift i n the p o s i t i v e d i r e c t i o n a n d b e c o m e more n e a r l y e q u a l to the M o ( V I ) - M o ( V ) potential. I n a d d i t i o n , a sub strate b i n d i n g site b e c o m e s a v a i l a b l e i n t h e l o w e r o x i d a t i o n states. A

29.

FiNKLEA ET AL.


727

crucial undeveloped element in this model is control of proton avail ability. Protons are required for reversibility of the M o 0 -Mo0 conversion, but must be excluded to preserve reversibility of the M o 0 - M o 0 conversion and to prevent reduction to a lower oxida tion state. The enzymes may control this feature by means of ligandbound hydrogen atoms (63). In addition, considerable ligand flexibility is required to accommodate the change in coordination number with change in oxidation state. Satisfying these requirements promises to be a challenging aspect in the development of more refined models for the oxomolybdenum enzymes. 2 +

3 +

2


3 +

2 +

Acknowledgments Support of this research by the National Science Foundation under Grant No. C H E 80-20442 is gratefully acknowledged. The authors also thank J. W. McDonald, W. E . Newton, and G . D . Watt of the Charles F. Kettering Research Laboratory, Yellow Springs, Ohio, for their help in obtaining the E P R results described. Literature

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Electrochemical and Spectrochemical Studies of Biological Redox

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