Mechanism of Autoreduction of Ferric Porphyrins and the Activation of

12 Mechanism of Autoreduction of Ferric Porphyrins and the Activation of

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Coordinated Ligands GERD N. LA MAR and JOHN DEL GAUDIO Department of Chemistry, University of California, Davis, CA 95616 The autoreduction of ferric porphyrins in the presence of certain ligands is accompanied by the formation of ligand radicals, suggesting that these reactions may serve as models for the activation of substrates by peroxidases. NMR and ESR spectroscopy demonstrate that the autoreduction of the dicyanotetraphenylporphinatoferrate(III) yields the dicyano ferrous porphyrin and the cyanide radical by a mechanism thought to involve homolytic bond cleavage. Similar reac tions are observed with n-hexane thiol and piperidine. The reoxidation by molecular oxygen of the ferrous complex of CN or tributyl phosphine leads directly to the low spin ferric complex, as opposed to the expected oxo-dimer. A mechanism involving the formation of the superoxide anion is proposed, suggesting that this reaction may model the hemoprotein activation of molecular oxygen. -

Tron porphyrins constitute the active site of an important class of redox metalloenzymes. This class includes the cytochromes which are in volved in electron transfer processes ( 1 ) ; peroxidases, whose main func tion is to oxidize substrates at the expense of hydrogen peroxide (2); and oxygenases, which catalyze the incorporation of oxygen into sub strates via the activation of molecular oxygen (3). Although the func tions of these enzymes are quite varied, they all cause the iron atom to undergo valency changes during the operation of the enzyme. The mechanism by which an electron is transferred to and from the iron atom is poorly understood. In the case of a simple redox enzyme such as cytochrome c where both sites of the heme iron are ligated by peptide side chains, two path ways have been suggested by which an electron can travel to or from the 207 Raymond; Bioinorganic Chemistry—II Advances in Chemistry; American Chemical Society: Washington, DC, 1977.

208

BIOINORGANIC C H E M I S T R Y

II

i r o n a t o m — a d i r e c t e l e c t r o n t r a n s f e r f r o m the e n z y m e r e d u c t a s e to a n e x p o s e d e d g e of the p o r p h y r i n π c l o u d f o l l o w e d b y a r a p i d transfer to t h e i r o n or, a l t e r n a t i v e l y ( 4 ) , e l e c t r o n transfer v i a a n a x i a l l i g a n d of the iron porphyrin (5).

B o t h reduction mechanisms involve rapid ligand-to-

m e t a l c h a r g e transfer as a c r i t i c a l step i n t h e process.

A r e l a t e d class of

r e d o x h e m o p r o t e i n s , i n c l u d i n g oxygenases, peroxidases, a n d

cytochrome

P 4 5 0 , has o n l y one p o l y p e p t i d e s i d e - c h a i n l i g a n d b o u n d to the h e m e i r o n ,


w i t h t h e sixth site a v a i l a b l e for c o o r d i n a t i n g a substrate w h i c h is to b e activated.

T h u s these e n z y m e s

are i n v o l v e d not o n l y i n o n e - e l e c t r o n

v a l e n c y changes of the i r o n , b u t these changes are also c o u p l e d to the o x i d a t i o n or r e d u c t i o n of the substrates. I n the case of oxygenases a n d cytochrome

P 4 5 0 , this a c t i v a t i o n of

i n v o l v e f o r m a t i o n of

the s u p e r o x i d e

molecular oxygen ion

(6)

by

is t h o u g h t

to

a reaction such

as

R e a c t i o n 1. T h e s u p e r o x i d e i o n as the a c t i v a t e d f o r m of m o l e c u l a r o x y g e n has b e e n s u b s t a n t i a t e d ( 6 ). E-Fe :0

-> E - F e

2

m

+ (V

(1)

Peroxidases p e r o x i d i z e a v a r i e t y of substrates at the expense hydrogen peroxide.

of

T h e s e peroxidases o x i d i z e a m i n e s , A H , for w h i c h 2

p r o d u c t analysis has suggested

a free r a d i c a l i n t e r m e d i a t e ( 2 ) .

c u r r e n t l y a c c e p t e d v a l e n c y changes of the e n z y m e c a n b e b y the scheme i n Reactions 2-5. perox ( F e

111

) +

H 0 2

compound I + A H

2

2

compound I I + A H

The

represented

R e a c t i o n 2 represents the c o n v e r s i o n -> c o m p o u n d I ( F e , p o r )

(2)

compound I I ( F e ) + A H *

(3)

I V

+

I V

2

perox ( F e )

+

m

AH'

(4)

A H ' - » products

(5)

of t h e e n z y m e to a h i g h l y o x i d i z e d state w h i c h is c a p a b l e of one

two-

e l e c t r o n or t w o o n e - e l e c t r o n o x i d a t i o n s ( 7 )

por

(por

+

represents the

p h y r i n r a d i c a l ) . I n R e a c t i o n s 3 a n d 4, a c t i v a t i o n of t h e a m i n e is p r o p o s e d to consist of o n e - e l e c t r o n o x i d a t i o n s of t h e a m i n e . H o w e v e r , the one-electron

o x i d a t i o n of c o o r d i n a t e d l i g a n d s b y i r o n p o r p h y r i n s h a d

not b e e n d e m o n s t r a t e d at the t i m e w e i n i t i a t e d this r e s e a r c h . E S R i n v e s t i g a t i o n s of the o x i d a t i o n of some substrates b y peroxidases h a v e p r o v i d e d direct evidence

f o r t r a n s i e n t free r a d i c a l i n t e r m e d i a t e s ( 8 ,

9).

H o w e v e r , i t has not b e e n d e t e r m i n e d w h e t h e r these substrates c o o r d i n a t e to the i r o n or w h e t h e r t h e y are s e c o n d a r y p r o d u c t s of a c t i v a t i o n b y t h e enzyme.

T h e i d e n t i f i c a t i o n a n d i n v e s t i g a t i o n of m o d e l systems w h i c h

c a n a c t i v a t e substrates b y o n e - e l e c t r o n r e d o x reactions c a n b e to p r o v i d e v a l u a b l e i n s i g h t i n t o the m e c h a n i s m of t h e h e m e

expected

enzymes.

Raymond; Bioinorganic Chemistry—II Advances in Chemistry; American Chemical Society: Washington, DC, 1977.

12.

L A M A R AND D E L GAUDio

209

Ferric Porphyrins and Ligands

S c a t t e r e d reports h a v e a p p e a r e d i n the l i t e r a t u r e i n d i c a t i n g t h a t f e r r i c p o r p h y r i n s c a n a u t o r e d u c e i n solutions c o n t a i n i n g c e r t a i n p o t e n t i a l l i g a n d s (10, 11, 12).

P r o b a b l y the best k n o w n e x a m p l e is the f o r m a t i o n

of b i s ( p i p e r i d i n e ) t e t r a p h e n y l p o r p h o r i n a t o i r o n ( I I ), T P P F e the a d d i t i o n of

1 1

( pip ) , 2

by

p i p e r i d i n e to c h l o r o t e t r a p h e n y l p o r p h y r i n a t o i r o n ( I I I ) ,

R e a c t i o n 6. A l t h o u g h this r e a c t i o n w a s first r e p o r t e d i n 1967 (13),

the

pip TPPFe Cl

TPPFe


m

1 1

(pip) 2

(6)

i d e n t i t y of the r e d u c i n g agent a n d the m e c h a n i s m of the r e a c t i o n r e m a i n e d o b s c u r e u n t i l v e r y r e c e n t l y . O u r recent p r e l i m i n a r y c o m m u n i c a t i o n (14)

o n the a u t o r e d u c t i o n of f e r r i c p o r p h y r i n s d e m o n s t r a t e d t h a t

this r e a c t i o n i n v o l v e s r e d u c t i o n of the c o m p l e x b y a p r e s u m a b l y c o o r d i n a t e d l i g a n d , w i t h t h e substrate b e i n g o x i d i z e d to a free r a d i c a l .

The

r a d i c a l s w e r e r e a d i l y d e t e c t e d b y the a p p e a r a n c e of a n E S R s i g n a l d u r i n g the r e a c t i o n i n v o l v i n g c y a n i d e i o n , p i p e r i d i n e , or n - h e x a n e t h i o l as substrates. I n a s m u c h as this r e d o x r e a c t i o n p r o v i d e s a m e t h o d f o r a c t i v a t i n g substrates w h i c h m a y c o o r d i n a t e to i r o n p o r p h y r i n s a n d h e n c e p r o v i d e s some k i n d of a m o d e l for p e r o x i d a s e a c t i v i t y , w e h a v e c o n t i n u e d o u r w o r k o n the r e d o x system most a m e n a b l e to s p e c t r o s c o p i c i n v e s t i g a t i o n , n a m e l y the c y a n i d e i o n o x i d a t i o n . W e h a v e f u r t h e r d i s c o v e r e d that o x i d a t i o n of the r e d u c e d b i s - c y a n o ferrous c o m p l e x b y m o l e c u l a r

oxygen

does not y i e l d the e x p e c t e d o x o - b r i d g e d d i m e r ( 1 5 ) b u t i n s t e a d appears to p r o c e e d b y a m e c h a n i s m that suggests f o r m a t i o n of the s u p e r o x i d e i o n , 0 " . S i n c e this latter process w o u l d represent the o n e - e l e c t r o n r e d u c 2

t i o n of m o l e c u l a r o x y g e n b y a ferrous p o r p h y r i n c o m p l e x

and could

c o n c e i v a b l y s h e d l i g h t o n the m e c h a n i s m of o x y g e n a c t i v a t i o n (6),

the

r e 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 of the a u t o r e d u c e d f e r r i c complexes w a s also i n v e s t i g a t e d s p e c t r o s c o p i c a l l y . T h u s the t w o reactions w h i c h w e p r o p o s e to c l a r i f y are the a u t o reduction (Reaction 7)

( w h e r e Ρ is a g e n e r a l p o r p h y r i n ) a n d the s u b

sequent r e o x i d a t i o n ( R e a c t i o n 8 ). A l t h o u g h a n u m b e r of substrates ( i.e., L

P¥e X->PFe L ? UI

u

PFe L * + 0 n

2

2

2

(7)

+ L

-> P F e L * m

2

(8)

+ 1

a m i n e s , t h i o l s , p h o s p h i n e s , c y a n i d e ) , r e a c t e d a c c o r d i n g to R e a c t i o n 7, as e v i d e n c e d b y the c h a n g e f r o m f e r r i c to ferrous o p t i c a l a n d N M R s p e c t r a , o u r w o r k e m p h a s i z e s the reactions for L =

C N ~ because the

n a t u r e of the complexes i n b o t h o x i d a t i o n states c a n b e d e t e r m i n e d . T h e


210


II

p r e l i m i n a r y d a t a o n t h e m o r e c o m p l e x p i p e r i d i n e r e a c t i o n are p r e s e n t e d o n l y to i n d i c a t e that the a u t o r e d u c t i o n m e c h a n i s m i n v o l v i n g a

one-

e l e c t r o n r e d o x r e a c t i o n a p p e a r s c o m m o n for t h e v a r i o u s substrates. Experimental Materials. T h e T P P F e C l w a s p r e p a r e d a n d p u r i f i e d b y l i t e r a t u r e m e t h o d s (16, 17). O c t a e t h y l p o r p h y r i n w a s a gift f r o m H . H . I n h o f f e n , a n d the h e m i n d i m e t h y l ester [ P P ( I X ) D M E F e C l ] w a s p u r c a s e d f r o m Sigma Chemical Co. T h e solvents u s e d f o r N M R a n d E S R m e a s u r e m e n t s w e r e c o m m e r c i a l sources of d e u t e r a t e d solvents w h i l e s p e c t r o - g r a d e solvents w e r e u s e d f o r v i s i b l e s p e c t r a . Solvents w e r e d r i e d b y s t o r i n g over m o l e c u l a r sieves. T h e amines, tributyl phosphine, n-hexanethiol, N a C N , a n d K C N w e r e o b t a i n e d f r o m s t a n d a r d c o m m e r c i a l sources. T h e C N ~ source i n C D C 1 a n d C D C 1 w a s N B u C N w h i c h was p r e p a r e d b y l i t e r a t u r e methods (18).


m

3

2

2

4

Spectroscopic Measurements. N M R S P E C T R A . T h e N M R s p e c t r a w e r e r e c o r d e d at 2 9 8 ° C w i t h a J E O L - P S 1 0 0 F T N M R spectrometer o p e r a t i n g at 99.5 M H z . T h e a u t o r e d u c t i o n s w e r e d o n e d i r e c t l y i n the N M R t u b e , g e n e r a l l y w i t h 0 . 4 - m l samples of 1 0 - m M i r o n p o r p h y r i n solutions c o n t a i n i n g a 2 0 - 1 0 0 m o l a r excess of the substrate. S a m p l e s w e r e r o u tinely prepared i n a nitrogen atmosphere. A n a e r o b i c samples w e r e p r e p a r e d b y d e g a s s i n g t h e solvent b y t h r e e f r e e z e - t h a w cycles w h i l e solids a n d N M R tubes w e r e degassed b y p l a c i n g u n d e r v a c u u m , t h e n s t o r i n g i n a n i t r o g e n a t m o s p h e r e . If o x y g e n is not c a r e f u l l y e x c l u d e d , the ferrous p o r p h y r i n o b t a i n e d b y the a u t o r e d u c t i o n m a y b e r a p i d l y o x i d i z e d to the o x o - b r i d g e d d i m e r . A p h o t o c h e m i c a l c o n t r i b u t i o n to the a u t o r e d u c t i o n r e a c t i o n w a s i n v e s t i g a t e d b y p r e p a r i n g t w o i d e n t i c a l N M R samples of T P P F e ( CN) i n d r y D M S O c o n t a i n i n g excess C N " ; one s a m p l e w a s s h i e l d e d f r o m a n d the other exposed to the n o r m a l fluorescent l i g h t i n the l a b o r a t o r y . T h e N M R w e r e r e c o r d e d , a n d i t w a s n o t e d t h a t the e x p o s e d s a m p l e a u t o r e d u c e d a b o u t 5 0 % faster. U n d e r the c o n d i t i o n s of the e x p e r i m e n t , t h e r m a l effects o n the e x p o s e d s a m p l e are e x p e c t e d to b e n e g l i g i b l e . T h e ferrous p o r p h y r i n s w e r e r e o x i d i z e d b y i n t r o d u c i n g o x y g e n i n t o the N M R t u b e of the a u t o r e d u c e d s a m p l e . T o detect w a t e r as a p r o d u c t of the r e o x i d a t i o n of T P P F e ( C N ) ~ , it was necessary to c o m p l e t e l y e x c l u d e the p o s s i b i l i t y of a t m o s p h e r i c contact ( d r y D M S O r a p i d l y a b sorbs w a t e r f r o m the a t m o s p h e r e ) . T h e s e e x p e r i m e n t s w e r e d o n e i n a n N M R t u b e fitted w i t h a g r o u n d glass stopcock. T h i s a l l o w e d the a d d i t i o n of o x y g e n i n t o the N M R t u b e b y v a c u u m l i n e t e c h n i q u e s , c o m p l e t e l y e l i m i n a t i n g a t m o s p h e r i c contact. E x p e r i m e n t s d o n e o n b l a n k s of d r y D M S O showed no water peak. 1 1 1

2

1 _

1

1 1

E S R SPECTRA. E P R spectrometer. i n the E S R c a v i t y t i o n as the a b o v e E S R a n d N M R of

2

2

T h e E S R spectra w e r e r e c o r d e d w i t h a V a r i a n E - 4 T h e autoreductions a n d reoxidations were followed g e n e r a l l y w i t h 0 . 1 - m l samples of t h e same c o n c e n t r a N M R samples. I n the case of T P P F e (CN) the the a u t o r e d u c t i o n w e r e r e c o r d e d o n the same s a m p l e 1 1 1


2

1 _

12.

211


L A M A R A N D D E L G AUDIO

b y s u p p o r t i n g the E S R t u b e i n a n N M R t u b e , thus v e r i f y i n g that t h e E S R s i g n a l arose as t h e a u t o r e d u c t i o n p r o c e e d e d . V I S I B L E S P E C T R A . T h e v i s i b l e s p e c t r a w e r e o b t a i n e d w i t h a C a r y 14 r e c o r d i n g s p e c t r o p h o t o m e t e r b y s t a n d a r d t e c h n i q u e s . T h e s p e c t r u m of TPPFe (CN)2 i n D M S O c o n t a i n i n g excess C N ~ w a s o b t a i n e d u n d e r o x y g e n to i n s u r e t h a t a u t o r e d u c t i o n h a d not o c c u r r e d . T h e s p e c t r u m of T P P F e ( C N ) ~ was obtained on a sample whose N M R spectrum h a d indicated complete reduction. i n

1 _


1 1

2

Results and

2

Discussion

Autoreduction

of Fe(III)

Porphyrins.

OXIDATION

O F PIPERIDINE.

T h e a u t o r e d u c t i o n of T P P F e C l w i t h neat p i p e r i d i n e is r a p i d , h o w e v e r t h e r e a c t i o n rate c a n b e d e c r e a s e d b y d i l u t i o n w i t h C D C 1

3

or

DMSO.

T h e N M R s p e c t r u m of T P P F e C l i n C D C 1 , o n a d d i t i o n of p i p e r i d i n e , 3

shows resonances consistent w i t h the p r e s e n c e of h i g h s p i n F e ( I I I ) low spin T P P F e observed.

1 1

(Pip) . 2

A low spin T P P F e

I t is l i k e l y t h a t t h e l a t t e r c o m p l e x

1 1 1

(Pip)

2

and

species w a s

r a p i d l y reduces

at

not room

temperature. 15.8 G

Figure 1. ESR signal obtained by adding piperidine to a CDCl solution of TPPFeCl 3

Q=2.006

T h e r e d u c t i o n w a s c a r r i e d out i n a n E S R c a v i t y a n d f o u n d to accompanied

b y the appearance

be

of a s t r o n g E S R s i g n a l e x h i b i t i n g a

t r i p l e t s t r u c t u r e i n d i c a t i v e of a n a m i n e r a d i c a l ( F i g u r e 1 ) .

T h i s suggests

t h a t the r e d u c t i o n of the i r o n p o r p h y r i n p r o c e e d s b y a o n e - e l e c t r o n o x i d a t i o n of a c o o r d i n a t e d p i p e r i d i n e . H o r s e r a d i s h p e r o x i d a s e activates a n i l i n e (2),

a n d the a c t i v a t i o n is t h o u g h t to o c c u r v i a t h e g e n e r a t i o n of

a n i l i n e r a d i c a l , ( C H N H · ). 6

5

T h e r e is a s i m i l a r i t y b e t w e e n

an

Reaction 4

of t h e p e r o x i d a s e scheme, w h i c h i n t h e case of a n i l i n e w o u l d i n v o l v e t h e o n e - e l e c t r o n o x i d a t i o n of a n i l i n e , p e r h a p s b y c o o r d i n a t i o n to a n F e ( I V )


212


porphyrin, compared

w i t h R e a c t i o n 7 w h i c h most l i k e l y i n v o l v e s

II

the

o n e - e l e c t r o n o x i d a t i o n of p i p e r i d i n e c o o r d i n a t e d to a n F e ( I I I ) p o r p h y r i n . T h e p i p e r i d i n e r e a c t i o n is not c o m p l e t e l y c h a r a c t e r i z e d . T h e c o n d i tions for the g e n e r a t i o n of the r a d i c a l i n F i g u r e 1 h a v e not b e e n c l e a r l y defined, a n d i n t e r m e d i a t e s o b s e r v e d d u r i n g the r e d u c t i o n h a v e n o t b e e n i d e n t i f i e d . T h e s e difficulties h a v e b e e n o v e r c o m e i n the c y a n i d e system, w h i c h p r o v e d to b e m o r e a m e n a b l e to spectroscopic i n v e s t i g a t i o n .


O X I D A T I O N O F C Y A N I D E I O N . I n the case of C N " as substrate, i d e n t i f i c a t i o n of reactants a n d p r o d u c t s KCN

p r o v e d feasible.

A d d i t i o n of

excess

to a 1 0 - m M T P P F e C l s o l u t i o n i n D M S O gives the l o w s p i n f e r r i c

b i s - c y a n i d e c o m p l e x , R e a c t i o n 9. T h e N M R of T P P F e ( C N ) T P P F e C l + 2 C N - ^± T P P F e ( C N ) previously characterized (19) s p e c t r u m is t i m e d e p e n d e n t ,

2

2

1 _

has b e e n

+ CI"

1 _

a n d is i l l u s t r a t e d i n A of F i g u r e 2.

(9) The

c o n v e r t i n g f r o m t h a t t y p i c a l of l o w s p i n

F e ( I I I ) to one c h a r a c t e r i s t i c of d i a m a g n e t i c F e ( I I ) , Ε i n F i g u r e 2.

The

e l e c t r o n i c s p e c t r u m of the i n i t i a l a n d final s o l u t i o n is s h o w n i n F i g u r e 3.

PPM

Ho

—>

Figure 2. Proton NMR traces showing the autoreduction of TPPFe (CN)t ' in DMSO at 25°C. (A) TPPFe^CN)^-; (B-D) increasing reduc tion; (E) final product TPPFe (CN) \ 111

1

u

2

2



12.

L A MAR AND D E L GAUDio

213


S p e c t r u m B , the final s o l u t i o n , is also c h a r a c t e r i s t i c of a n F e ( I I )

por-

p h y r i n ( 2 0 ) a n d verifies that r e d u c t i o n of the i r o n has o c c u r r e d . I n the case of the N M R spectra, the peaks m o v e w i t h a n average resonance o b s e r v e d f o r each p o s i t i o n ( B , C , a n d D of F i g u r e 2 ) .

The

o b s e r v a t i o n of a n a v e r a g e d c h e m i c a l shift i n d i c a t e s t h a t the same e n t i t y exists i n b o t h o x i d a t i o n states a n d establishes the p r o d u c t of the r e d u c t i o n to b e the ferrous b i s - c y a n o c o m p l e x

(i.e., R e a c t i o n 1 0 ) .

R a p i d electron

e x c h a n g e averages the t w o s p e c t r a a c c o r d i n g to R e a c t i o n 11. T h e c h e m i c a l shift difference gives a l o w e r l i m i t to t h e rate of e l e c t r o n withfc > >

exchange,

1.5 X 1 0 s e c . 4

TPPFe(CN)

2

TPPFe" (CN)

2

1

" + e"-» T P P F e ( C N )

2

" + TPPFe*

TPPFe

1 1 1

1 1 1

(CN)

2

1

2

"

"

2

(10)

^ ki

(CN), - + TPPFe* 1

T h e p r e p a r a t i o n of the i r o n p o r p h y r i n T P P F e C l - d

2 0

1 1

(CN)

2

2

"

( 2 1 ) , w h e r e the

p h e n y l groups are c o m p l e t e l y d e u t e r a t e d , has a l l o w e d the f u r t h e r c h a r a c t e r i z a t i o n of t h e b i s - c y a n o F e ( I I ) s p e c t r u m of T P P F e

1 1 1

(CN)

2

1 _

and Fe(III)

porphyrins.

I n the

i n D M S O , the resonance at + 1 5 . 4 5 p p m

u p f i e l d f r o m T M S is v e r i f i e d as t h e p y r r o l e H . C o m p a r i n g t h e s p e c t r u m of T P P F e

1 1

(CN)

2

2

" and T P P F e

1 1

( C N ) ~-d 2

2

2 0

shows the p e a k at

-7.82

p p m f r o m T M S to b e the p y r r o l e H , a n d i n t e g r a t i o n i n d i c a t e s t h a t t h e


214


II


4 G

L

s

9 = 2,003

Figure 4.

The ESR spectrum observed during the autoreduction of (CN), ' in DMSO

TPPFe

111

1

l o w field s h o u l d e r at —7.85 p p m contains e i g h t p r o t o n s , consistent w i t h its assignment as t h e p h e n y l ortho p r o t o n s . T h e n a t u r e of the r e d u c i n g agent w a s p r o v i d e d b y E S R spectrosc o p y . B y c a r r y i n g out the r e d u c t i o n of T P P F e ( C N ) the c a v i t y of a n E S R spectrometer, reduction proceeded.

2

1 _

in D M S O within

a s t r o n g s i g n a l a p p e a r e d as the

T h i s c o m p l e x s i g n a l , F i g u r e 4, is i d e n t i c a l to t h a t

p r e v i o u s l y r e p o r t e d d u r i n g the a n o d i c o x i d a t i o n of t e t r a p h e n y l a r s o n i u m c y a n i d e i n D M S O ( 2 2 ) a n d has b e e n i n t e r p r e t e d to represent the c y a n i d e t e t r a m e r , A . T h e p r o p o s e d m e c h a n i s m of f o r m a t i o n of this t e t r a m e r is r e p r o d u c e d i n R e a c t i o n 12. T h u s the c y a n i d e r a d i c a l ' C N is b e i n g p r o d u c e d d u r i n g the r e d u c t i o n of the f e r r i c p o r p h y r i n , a n d t h e o v e r a l l r e a c t i o n consistent w i t h b o t h the N M R a n d E S R d a t a c a n b e w r i t t e n as R e a c t i o n 13. "NC CN"

CN-

C N - -> ( C N ) , ->

(NC) C=N2

î-le

CN-

CN"

\

CN-

-> -le

/

/ C = N

.NC

-> ( C N ) * -

(12)

2

TPPFe

1 1 1

(CN), " + 1

C N " -> T P P F e 1

1 1

(CN)

2

2

" +

-CN


(13)

12.

L A M A R AND DEL GAUDio

FACTORS INFLUENCING T H E AUTOREDUCTION OF T P P F E

(CN)

1 1 1

2

1

'.

R i g o r o u s e x c l u s i o n of m o l e c u l a r

Anaerobic Nature of the Reduction. oxygen

215


shows t h a t t h e r e d u c t i o n p r o c e e d s a n a e r o b i c a l l y .

T h i s result

excludes the p o s s i b i l i t y t h a t t h e r a d i c a l s o b s e r v e d d u r i n g t h e a u t o r e d u c t i o n are b e i n g p r o d u c e d b y s o m e f o r m of a c t i v a t e d o x y g e n . Autocatalytic

Nature of the Reduction.

T h e data i n F i g u r e 5 show

t h e r e d u c t i o n rate to increase w i t h t i m e , i n d i c a t i n g t h a t t h e r e d u c t i o n is


autocatalytic.

T h i s is most l i k e l y c a u s e d b y t h e r e d u c t i o n of the f e r r i c

p o r p h y r i n b y the i n t e r m e d i a t e r a d i c a l s g e n e r a t e d b y R e a c t i o n 12, w h i c h are e x p e c t e d to b e m o r e p o t e n t r e d u c i n g agents t h a n t h e c y a n i d e i o n . H e n c e a n y d e t a i l e d i n t e r p r e t a t i o n of t h e rates w i l l b e severely l i m i t e d . The

Solvent Effects.

been observed i n C D C N 3

DMSO

autoreduction and D M S O ,

is solvent

dependent,

b u t not i n C D C 1

3

and

having CD C1 . 2

2

is the m o r e s u i t a b l e solvent, for s o l u b i l i t y reasons, a n d t h e f o l -

2

CL CL CO

Time , hrs.

Figure 5. The effect of CN~ concentration on the auto reduction rate of a 10-mM solution of TPPFe (CN) ~ in DMSO. A = 0.19 M CN~; Ο = 0.38 M CN : The increasing rate with time indicates that the autoreduc tion is also autocatalytic. III

1

2

1


216

BIOINORGANIC

lowing

discussions

a p p l y to t h e a u t o r e d u c t i o n

of

CHEMISTRY

TPPFe(CN)

2

1

II

" in

D M S O unless stated o t h e r w i s e . Effect of Light. thermal pathway,

T h e a u t o r e d u c t i o n occurs i n t h e d a r k , that is b y a a n d f u r t h e r discussions

are b a s e d

o n l y o n results

o b t a i n e d w i t h t h e e x c l u s i o n of l i g h t . H o w e v e r , l i g h t accelerates t h e rate of a u t o r e d u c t i o n . Effect of Water. T h e r e d u c t i o n rate i n c r e a s e d as t h e c o n c e n t r a t i o n


of w a t e r w a s decreased.

F o r e x a m p l e , r e m o v i n g w a t e r ( ~ 50 m M ) to

levels w h e r e i t is n o t o b s e r v e d b y N M R i n c r e a s e d t h e r e d u c t i o n rate a l m o s t three f o l d .

C y a n i d e c o o r d i n a t e d to f e r r i c p o r p h y r i n s acts as a

hydrogen-bond acceptor towards water ( 2 3 ) . S u c h a n interaction w o u l d m a k e t h e c o o r d i n a t e d c y a n i d e m o r e difficult to o x i d i z e (24)

a n d hence

w o u l d decrease t h e r e a c t i o n rate. Cyanide Ion Concentration.

Increasing the cyanide i o n concentra

t i o n speeds u p t h e a u t o r e d u c t i o n , as s h o w n i n F i g u r e 5. B e c a u s e of t h e a u t o c a t a l y t i c n a t u r e of the r e d u c t i o n , t h e exact c y a n i d e i o n d e p e n d e n c e has n o t y e t b e e n d e f i n e d . A l s o i t is n o t y e t clear w h e t h e r t h e c y a n i d e i o n is i n v o l v e d m e c h a n i s t i c a l l y i n t h e a u t o r e d u c t i o n .

T h e cyanide i o n con

c e n t r a t i o n c a n affect t h e o b s e r v e d rate v i a t h e a u t o c a t a l y t i c m e c h a n i s m or b y c o m p e t i n g w i t h t h e c o m p l e x e d c y a n i d e i o n f o r h y d r o g e n b o n d i n g w i t h trace a m o u n t s of w a t e r .

50 J Ο

Ο

S 30 Ο Ο Ο Ο

10

Ο Ο

—ι

f

1—

3

5 Time, hr&

Figure 6. The reduction rate of various bis-cyano porphyrins. Δ = OEPFe't'iCN), -, • = PP(IX)DMEFe (CN) ~, Ο = TPPFe (CN)^1

III

1

2

111


12.

L A MAR

Ferric

AND D E L G A U D i o

Porphyrin

Basicity.

Porphyrins

and

217

Ligands

T h e r e d u c t i o n rate is d e c r e a s e d s i g n i f i c a n t l y as

the p o r p h y r i n is m a d e m o r e b a s i c ( F i g u r e 6 ) . T h e b i s - c y a n i d e

complex

of f e r r i c o c t a e t h y l p o r p h y r i n , the m o r e b a s i c p o r p h y r i n , is r e d u c e d the slowest w h i l e the b i s - c y a n i d e c o m p l e x of f e r r i c T P P , the least b a s i c ( 1 6 ) , is r e d u c e d the fastest.

I n c r e a s i n g the p o r p h y r i n b a s i c i t y places

more

e l e c t r o n d e n s i t y o n the i r o n , m a k i n g i t m o r e difficult to a c c e p t another electron.


Effect of Axial Ligand.

I n a d d i t i o n to c y a n i d e i o n a n d p i p e r i d i n e ,

a u t o r e d u c t i o n w a s o b s e r v e d for the f o l l o w i n g p o t e n t i a l l i g a n d s — p r i m a r y , secondary,

a n d tertiary amines; p y r i d i n e ; n-hexanethiol; a n d

tributyl

phosphine. M E C H A N I S M OF T H E AUTOREDUCTION OF T P P F E

1 1 1

(CN)

2

1 _

The

.

trans

fer of a n e l e c t r o n f r o m a c y a n i d e i o n to the F e ( I I I ) c a n o c c u r b y at least three m e c h a n i s m s . ( 1 ) A n outer sphere o x i d a t i o n of free C N " b y T P P F e 1

1 1 1

(CN)

2

1 _

,

w h i c h is m e c h a n i s t i c a l l y d e s c r i b e d b y R e a c t i o n 13. T h e e l e c t r o c h e m i c a l o x i d a t i o n of c y a n i d e i o n i n a c e t o n i t r i l e occurs at potentials m o r e p o s i t i v e t h a n + 0 . 5 v o l t vs. S C E ( 2 2 ) . Fe

1 1

^ CN)

2

1 _

T h e e l e c t r o c h e m i c a l r e d u c t i o n of

TPP

i n a c e t o n i t r i l e occurs at a p o t e n t i a l of — 0 . 5 v o l t vs. S C E

( 2 5 ) . T h u s a n outer sphere m e c h a n i s m is c o n s i d e r e d u n l i k e l y . ( 2 ) A n u c l e o p h i l i c attack o n the c o o r d i n a t e d

cyanide by a

free

c y a n i d e , R e a c t i o n 14, w h i c h y i e l d s d i r e c t l y one of the precursors of the c y a n i d e t e t r a m e r i n R e a c t i o n 12. T h i s m e c h a n i s m is consistent w i t h the Ν C « - : C N P Fe

1 1 1

->

Ρ Fe

C Ν

1 1

C Ν I I

+

[CN:CN]"

(14) Ν

c

ex_ rapid

> ρ Fe C Ν

1 1

effect of the p o r p h y r i n b a s i c i t y o n the r e a c t i o n rate, as w e l l as w i t h the c y a n i d e i o n d e p e n d e n c e . H o w e v e r , as m e n t i o n e d a b o v e , the c y a n i d e i o n d e p e n d e n c e m a y result f r o m other causes.

T h e m e c h a n i s m is i n c o n

sistent w i t h the effect of w a t e r . H y d r o g e n b o n d i n g of w a t e r to a c o o r d i n a t e d c y a n i d e s h o u l d e n h a n c e the rate b y m a k i n g the c y a n i d e m o r e sus c e p t i b l e to n u c l e o p h i l i c attack.

A l s o the o b s e r v e d

photochemical

en

h a n c e m e n t of the rate w o u l d not b e e x p e c t e d w i t h a m e c h a n i s m i n v o l v i n g n u c l e o p h i l i c attack. T h u s , a l t h o u g h this m e c h a n i s m cannot b e e l i m i n a t e d at this t i m e , it is c o n s i d e r e d u n l i k e l y . ( 3 ) I n t r a m o l e c u l a r o n e - e l e c t r o n transfer w i t h s u b s e q u e n t

dissocia

t i o n (i.e., h o m o l y t i c b o n d c l e a v a g e ) , as d e s c r i b e d b y R e a c t i o n 15. T h i s


218


Ν C P F e C Ν

m

- * P F e C Ν

n

+

-CN (15) N C ^PFe C Ν

CNrapid


II

1 1

m e c h a n i s m is consistent w i t h the effect of the p o r p h y r i n b a s i c i t y o n the r e a c t i o n a n d , m o r e i m p o r t a n t l y , c a n b e e x p e c t e d to b e enhanced.

photochemically

A l t h o u g h the role of the c y a n i d e i o n c o n c e n t r a t i o n is u n c l e a r

i n this m e c h a n i s m a n d c o u l d b e i n v o l v e d i n the a u t o c a t a l y t i c m e c h a n i s m , as w e l l as i n c o m p e t i t i o n w i t h trace a m o u n t s of w a t e r , as i n d i c a t e d a b o v e , h o m o l y t i c b o n d cleavage c a n d e s c r i b e the a u t o r e d u c t i o n r e a c t i o n for the v a r i e t y of substrates i n v e s t i g a t e d to date. O u r results i n d i c a t e t h a t the a u t o r e d u c t i o n cannot o c c u r b y a c o n v e n t i o n a l o u t e r sphere m e c h a n i s m b e c a u s e of the gross m i s m a t c h of t h e e l e c t r o c h e m i c a l potentials. E x p e r i m e n t a l d a t a a v a i l a b l e at this t i m e are consistent w i t h h o m o l y t i c i r o n - c a r b o n b o n d cleavage w h i c h m a y or m a y not i n v o l v e a s i m u l t a n e o u s n u c l e o p h i l i c attack o n the c o o r d i n a t e d c y a n i d e . T h e h o m o l y t i c m e t a l - c a r b o n b o n d cleavage m a y serve as a m o d e l s i m i l a r processes r e p o r t e d for v i t a m i n Βχ O T H E R SYSTEMS.

Alkyl Thiols.

2

for

(26).

T h e r e d u c t i o n of h e m i n w i t h ethane-

t h i o l has b e e n suggested to o c c u r b y a free r a d i c a l m e c h a n i s m o n t h e basis of

product

analysis

(11).

The

r e a c t i o n of

n-hexanethiol

with

T P P F e C l i n D M S O c a r r i e d out i n the E S R c a v i t y gives rise to the s i g n a l i l l u s t r a t e d i n F i g u r e 7. T h e s e are p r e l i m i n a r y results, a n d the s p e c t r u m is of p o o r q u a l i t y a n d p r o b a b l y reflects some s a t u r a t i o n f r o m the

low

steady-state c o n c e n t r a t i o n of the r a d i c a l . N e v e r t h e l e s s the s i g n a l o n l y appears d u r i n g the a u t o r e d u c t i o n of the i r o n p o r p h y r i n a n d a g a i n i n d i cates that the a u t o r e d u c t i o n occurs b y a free r a d i c a l p a t h w a y . Pyridines,

Amines, and Phosphines.

W e have not observed an E S R

s i g n a l d u r i n g the a u t o r e d u c t i o n w i t h the a b o v e substrates.

With

the

p y r i d i n e s a n d p h o s p h i n e s so f a r i n v e s t i g a t e d , the r e d u c t i o n p r o b a b l y has b e e n too s l o w to generate a p p r e c i a b l e concentrations of r a d i c a l species. T h e a u t o r e d u c t i o n is m u c h faster w i t h p r i m a r y a n d s e c o n d a r y

amines,

h o w e v e r the r a d i c a l s p r o d u c e d are most l i k e l y too s h o r t - l i v e d to detect w i t h o u r present m e t h o d s . W e are i n v e s t i g a t i n g the a p p l i c a b i l i t y of r a p i d flow a n d spin trapping techniques. A l t h o u g h t h e m e c h a n i s t i c details of the a b o v e systems h a v e not yet b e e n c l a r i f i e d , o u r results suggest that the a u t o r e d u c t i o n of f e r r i c p o r p h y r i n s b y a free r a d i c a l p a t h w a y that most l i k e l y i n v o l v e s t h e h o m o l y t i c



12.

LAMAR

AND D E L GAUDio

219


— 100 G

·'

I g3 2 0 0

Figure 7. The ESR signal obtained upon the addition of n-hexanethiol to a CDCl solution of TPPFeCl 3

cleavage of t h e i r o n substrate b o n d is a g e n e r a l f e a t u r e of

Fe(III)

porphyrin chemistry. M o d e l systems f o r t h e a c t i v a -

Reoxidation of Fe(II) Porphyrins.

t i o n of m o l e c u l a r o x y g e n v i a c o o r d i n a t i o n to a n F e ( I I ) p o r p h y r i n h a v e not b e e n r e p o r t e d because of the r a p i d i r r e v e r s i b l e a u t o o x i d a t i o n of t h e F e ( I I ) t o the F e ( I I I ) o x o - b r i d g e d d i m e r ( C ) ( R e a c t i o n 1 6 ) . PFe

11

Since the

(L) ^± L + P F e L n

2

I (B)

> PFe 0

X I I

0Fe P

(16)

n i

2

(C)

rate of this r e a c t i o n is s u p p r e s s e d b y excess l i g a n d , L, a n i n t e r m e d i a t e five-coordinated

Fe(II)

complex

( B ) is i m p l i c a t e d ( 2 7 ) . S t e r i c a l l y

h i n d e r e d F e ( I I ) p o r p h y r i n s h a v e b e e n d e s i g n e d (27, 28) w h i c h p r e v e n t t h e f o r m a t i o n of t h e o x o - b r i d g e d d i m e r , m o s t l i k e l y b y i n t e r f e r i n g w i t h the b i m o l e c u l a r r e a c t i o n b e t w e e n t h e F e ( I I ) o x y g e n a d d u c t a n d a s e c o n d F e ( I I ) p o r p h y r i n . T h e h i n d e r e d p o r p h y r i n s h a v e b e e n s h o w n to r e v e r s i b l y b i n d m o l e c u l a r o x y g e n a n d b e h a v e as s u i t a b l e m o d e l

compounds

for t h e a c t i v e site of m y o g l o b i n a n d h e m o g l o b i n . H o w e v e r , to date these m o d e l s h a v e not b e e n a b l e t o a c t i v a t e m o l e c u l a r o x y g e n . C Y A N I D E C O M P L E X E S . W e h a v e i n v e s t i g a t e d t h e o x i d a t i o n of T P P Fe

1 1

(CN)

2

2

" i n D M S O w i t h m o l e c u l a r o x y g e n . T h e effect of i n t r o d u c i n g

m o l e c u l a r o x y g e n i n t o a 1 0 - m M D M S O s o l u t i o n of T P P F e ( I I ) ( C N )

2

2

"

c o n t a i n i n g excess K C N ( p r e p a r e d b y a l l o w i n g c o m p l e t e a n a e r o b i c a u t o r e d u c t i o n of the f e r r i c c o m p l e x ) is s h o w n i n F i g u r e 8. T h e i n i t i a l species


220


II

"DMSO- dg


pyrrole H

Figure 8. Proton NMR traces showing the reoxidation by oxy gen of TPPFe (CN) ~ in DMSO at 25°C. (A) reduced complex; (B-D) traces with in creasing time after addition of oxygen. n

2

2

-4

PPM

Ο

H.

»

(TMS)

( A i n F i g u r e 8 ) is c h a r a c t e r i s t i c of t h e F e ( I I ) p o r p h y r i n , a n d t h e p r e s ence of a n excess of K C N guarantees the b i s - c y a n i d e c o m p l e x .

O n reac

t i o n w i t h m o l e c u l a r o x y g e n a l l p e a k p o s i t i o n s shift u p h e l d to p o s i t i o n s c h a r a c t e r i s t i c of t h e i n i t i a l F e ( I I I ) h e m i c h r o m e ( B , C , a n d D i n F i g u r e 8).

A g a i n the o b s e r v a t i o n of a v e r a g e d c h e m i c a l shifts i n d i c a t e s that t h e

same species exists i n b o t h o x i d a t i o n states w i t h r a p i d e l e c t r o n e x c h a n g e a v e r a g i n g t h e s p e c t r a a n d establishes t h e o x i d a t i o n p r o d u c t t o b e bis-cyano F e ( I I I ) TPPFe

1 1

c o m p l e x , R e a c t i o n 17. (CN)

f o l l o w e d to c o m p l e t i o n

2

2

" +

0

(about

2

-> T P P F e 80%

T h i s r e a c t i o n has n o t 1 1 1

(CN), " +

completion)

1

0

2

( ? )

"

since t h e

a u t o r e d u c t i o n is c o m p e t i t i v e w i t h t h e o x i d a t i o n process.

the been (17)

anaerobic

I n t h e case of


12.

PP(IX)DMEFe (CN) n

2

2

221


L A M A R A N D D E L GAUDIO

~ , since the f e r r i c f o r m autoreduces s l o w e r , the

o x i d a t i o n w i t h m o l e c u l a r o x y g e n i n D M S O is m o r e r a p i d a n d c a n c a r r i e d to c o m p l e t i o n .

be

T h e N M R s p e c t r u m o b t a i n e d for the o x i d i z e d

p r o d u c t is i d e n t i c a l to that of the w e l l c h a r a c t e r i z e d P P ( I X ) D M E F e (CN)

2

n l

-

a n d f u r t h e r establishes the p r o d u c t of the o x i d a t i o n as the b i s -

1 -

cyano F e

1 1 1

species.

T h e N M R d a t a i n F i g u r e 8 also i n d i c a t e that w a t e r is a n o x i d a t i o n


product.

T h i s is the o n l y o x y g e n - c o n t a i n i n g species that has as yet b e e n

identified.

T h e source of the p r o t o n i n the w a t e r has not b e e n

estab-

l i s h e d ; h o w e v e r the o x i d a t i o n of the p h e n y l - d e u t e r a t e d p o r p h y r i n T P P Fe

1 1

( C N ) ~-d o i n D M S O - d 2

2

6

also gave a w a t e r peak, i n d i c a t i n g t h a t at

least some of the protons w e r e o b t a i n e d f r o m the p y r r o l e p o s i t i o n .

Dur-

i n g these experiments p r e c a u t i o n w a s t a k e n to p r e v e n t a n y contact w i t h the a t m o s p h e r e .

S i n c e most of the i n t e n s i t y of the p y r r o l e p e a k is s t i l l

present at the e n d of a n a u t o r e d u c t i o n , r e o x i d a t i o n , a n d a s e c o n d a u t o r e d u c t i o n c y c l e , w e suggest that p r o t o n o b s t r a c t i o n f r o m the p y r r o l e s is o n l y a m i n o r p a t h w a y for d e a c t i v a t i o n of the a c t i v a t e d o x y g e n . S i n c e the p r o d u c t of the o x i d a t i o n is the bis c y a n i d e F e ( I I I )

species,

w h i c h is a g a i n r e d u c e d b y the excess c y a n i d e , the r e a c t i o n c a n b e c y c l e d , as r e p r e s e n t e d b y the f o l l o w i n g s c h e m e : H 0 2

T h e presence of the s u p e r o x i d e i o n has not b e e n c o n f i r m e d .

The

E S R s p e c t r u m of the s u p e r o x i d e a n i o n i n f r o z e n D M S O is k n o w n . S a m ples of r e d u c e d

porphyrins have been

f r o z e n i m m e d i a t e l y after

the

i n t r o d u c t i o n of m o l e c u l a r o x y g e n , h o w e v e r a s u p e r o x i d e a n i o n s i g n a l has not been observed. our

It m a y b e that the 0 " i o n is too short l i v e d u n d e r 2

experimental conditions

to b e

g e n e r a t i n g the T P P F e ( I I I ) ( C N )

2

1 _

observed.

S i n c e the

o x i d a t i o n is

species, w h i c h is t h e n r e d u c e d

by

t h e excess C N " present, the c y a n i d e t e t r a m e r E S R s i g n a l ( R e a c t i o n 12) 1

appears. I n t e r e s t i n g l y , s u p e r i m p o s e d o n this is a t h r e e - l i n e s i g n a l ( F i g u r e 9 ) w h i c h is not seen d u r i n g t h e o r i g i n a l r e d u c t i o n of T P P F e ( I I I ) ( C N ) ~ . 2

2

T h e h y p e r f i n e s p l i t t i n g of t h e t r i p l e t is 1.5 gauss. H y p e r f i n e s p l i t t i n g i n t h e c y a n i d e r a d i c a l , o b t a i n e d b y u v i r r a d i a t i o n of H C N i n a n a r g o n m a t r i x , w a s r e p o r t e d as 4.6 gauss a n d is t h o u g h t to b e s u r p r i s i n g l y s m a l l (29).

H y p e r f i n e s p l i t t i n g i n t h e N C O r a d i c a l i n the gas p h a s e

r e p o r t e d as a b o u t 19 gauss ( 3 0 ) . not been determined.

was

T h e o r i g i n of the t h r e e - l i n e s i g n a l has

T h e a d d i t i o n of a D M S O s o l u t i o n of K 0

2

solu-

b i l i z e d w i t h the d i c y c l o h e x y l - 1 8 - c r o w n - 6 c y c l i c ether to D M S O s a t u r a t e d w i t h K C N ( a n d m o r e d i l u t e solutions ) d i d n o t r e p r o d u c e the t r i p l e t n o r



222


Figure 9.

The ESR signal observed during the oxidation of in DMSO by oxygen

II

TPPFe (CN) ~ n

2

2

w a s t h e s t r o n g E S R s i g n a l ( R e a c t i o n 1 2 ) , i n d i c a t i v e of C N f o r m a t i o n , observed. O T H E R SYSTEMS. A

s e c o n d case w h e r e t h e o x i d a t i o n

of

an

p o r p h y r i n does n o t g i v e t h e b r i d g i n g o x o - d i m e r is t h e system in C D C 1

3

w i t h t h e s t r o n g field l i g a n d P ( n - B u ) l n

C l in CDC1

3

TPPFe

1 1

as t h e a x i a l base ( 3 1 ) .

3

A s m e n t i o n e d a b o v e , t h e a d d i t i o n of excess P ( n - B u ) t i o n of T P P F e

Fe(II)

3

to a 1 0 - m M s o l u

causes t h e a u t o r e d u c t i o n of t h e p o r p h y r i n .

O n a d d i t i o n of m o l e c u l a r o x y g e n , t h e p o r p h y r i n peaks shift u p f i e l d w i t h a n a v e r a g e d c h e m i c a l shift, i n d i c a t i n g t h e f o r m a t i o n of a l o w s p i n F e ( I I I ) complex.

A l t h o u g h this system has n o t y e t b e e n f u l l y c h r a c t e r i z e d , i t is

clear t h a t t h e o x i d a t i o n does n o t give t h e b r i d g i n g o x o - d i m e r . The

autooxidation

of t h e b i s - p i p e r i d i n e c o m p l e x

o b t a i n e d b y t h e a n a e r o b i c a u t o r e d u c t i o n of T P P F e

I H

TPPFe

1 1

(pip ) ,

C l in CDC1

p i p e r i d i n e (as w i t h other a m i n e s ) , y i e l d s , as e x p e c t e d ,

2

3

with

the bridging

oxo-dimer. MECHANISM

OXYGEN.

O F OXIDATION O F T P P F E

W e have

considered

1 1

(CN)

2

2

~

three m e c h a n i s m s

WITH

MOLECULAR

to account

for the

u n u s u a l l a c k of d i m e r f o r m a t i o n i n the a u t o o x i d a t i o n . ( 1 ) A n outer sphere o x i d a t i o n of t h e F e ( I I ) , R e a c t i o n 18.

0

2

Ν C + P Fe C Ν

( I I )

Ν C -» Ρ F e C N

m

+ 0

2

( ? )

"


(18)

12.

T h e o x i d a t i o n of F e ( I I )

223


L A M A R AND D E L GAUDio

p o r p h y r i n s is t h o u g h t to i n v o l v e a

five-

c o o r d i n a t e i n t e r m e d i a t e , R e a c t i o n 16. H o w e v e r , w i t h c y a n i d e i o n as a x i a l base i t m a y b e p o s s i b l e to transfer a n e l e c t r o n f r o m t h e F e ( I I ) to m o l e c u lar o x y g e n b y a n e l e c t r o n transfer t h r o u g h t h e c o o r d i n a t e d c y a n i d e i o n w i t h o u t t h e f o r m a t i o n of a n i r o n - o x y g e n b o n d . A l t h o u g h this m e c h a n i s m appears a t t r a c t i v e f o r t h e c y a n i d e i o n , t h e o x i d a t i o n of t h e p h o s p h i n e c o m p l e x most l i k e l y occurs b y t h e same m e c h a n i s m , a n d e l e c t r o n transfer


t h r o u g h t h e c o o r d i n a t e d p h o s p h i n e appears m u c h less l i k e l y . outer-sphere m e c h a n i s m is c o n s i d e r e d u n l i k e l y .

Thus an

A c o n s i d e r a t i o n of t h e

s t a n d a r d p o t e n t i a l , E ° , for R e a c t i o n 18 appears to suggest that t h e F e ( I I ) p o r p h y r i n is a n insufficiently s t r o n g r e d u c i n g agent ( b y p e r h a p s 0.2 v o l t ) to r e d u c e o x y g e n to 0 ~ b y a n outer-sphere m e c h a n i s m .

H o w e v e r , E°

2

v a l u e s are r e f e r e n c e d to t h e standard-state c o n c e n t r a t i o n s of 1 M . U n d e r our e x p e r i m e n t a l c o n d i t i o n s t h e c o n c e n t r a t i o n of t h e r e a c t i v e species 0 ~ 2

is e x p e c t e d to b e q u i t e s m a l l , a n d c o n s i d e r a t i o n of t h e N e r n s t e q u a t i o n suggests t h a t t h e p o t e n t i a l f o r t h e r e a c t i o n c o u l d b e c o n s i d e r a b l y l a r g e r t h a n t h e s t a n d a r d E° v a l u e . T h u s a n outer sphere m e c h a n i s m c a n n o t b e e l i m i n a t e d o n t h e basis of t h e s t a n d a r d p o t e n t i a l E°.

T h i s a r g u m e n t is

d i s c u s s e d m o r e t h o r o u g h l y i n R e f . 32. ( 2 ) F o r m a t i o n a n d cleavage of the o x o - b r i d g e d d i m e r , R e a c t i o n s 19 a n d 20. Ν C Ρ F e + 0·> -> Ρ F e C Ν 1 1

i n

CN'

PFe

n i

OFe

i n

P + CN"

OFe P

(19)

Ν C > Ρ Fe C Ν

(20)

m

1 1 1

T h e o x o - b r i d g e d d i m e r is c l e a v e d w i t h excess K C N i n D M S O , R e a c t i o n 20, i.e., u n d e r o u r e x p e r i m e n t a l c o n d i t i o n s , a l t h o u g h t h e rate is v e r y s l o w ( 3 1 ) . T h e p o s s i b i l i t y of t h e o x o - b r i d g e d d i m e r as a n i n t e r m e d i a t e can

b e d i s c o u n t e d b y c o n s i d e r i n g the f o l l o w i n g e x p e r i m e n t .

of P P ( I X ) D M E F e

I I X

(CN)

2

2

A sample

- i n D M S O saturated w i t h K C N was a l l o w e d

to a u t o r e d u c e a n a e r o b i c a l l y u n t i l a b o u t 5 0 % h a d b e e n r e d u c e d , A i n F i g u r e 10. A n e q u i v a l e n t a m o u n t of t h e d i m e r [ P P ( I X ) D M E F e ] 0 m

2

w a s a d d e d i n t h e absence of o x y g e n ( Β i n F i g u r e 10 ). T h e n o x y g e n w a s i n t r o d u c e d , c a u s i n g t h e o x i d a t i o n of t h e P P ( I X ) D M E F e ( C N ) n

oxidation proceeded, m e t h y l resonances

as e v i d e n c e d b y t h e d o w n f i e l d m o v e m e n t

2

2

".

As

of t h e

( C a n d D i n F i g u r e 1 0 ) , t h e s p e c t r u m of t h e o x o -


224

BIOINORGANIC

CHEMISTRY

II


—-DMSO-d

Mechanism of Autoreduction of Ferric Porphyrins and the Activation of

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