Photoinduced Electron Transfer Across Peptide Spacers - Advances in

5 Photoinduced Electron Transfer Across Downloaded by NORTH CAROLINA STATE UNIV on December 24, 2012 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1990-0226.ch005

Peptide Spacers Leonardo A . Cabana and K i r k S. Schanze

1

Department of Chemistry, University of F l o r i d a , Gainesville, FL 32611

A series of molecules was prepared in which a metal complex center, [(bpy)Re(I)(CO) (pyr)] (bpy is 2,2'-bipyridine, pyr is 4-aminopyr idine), is linked to a 4-(N,N-dimethylamino)benzoate unit (DMAB) by a covalent bridge consisting of 0, 1, and 2 L-proline units (compounds 0, 1, and 2, respectively). In these molecules, electron transfer from DMAB to Re is initiated by photoexcitation of the Re --> bpy metal-to-ligand charge-transfer (MLCT) excited state. +

3

(bpy)Re DMAB --> I• • •

(bpy )Re -

DMAB --> (bpy )Re MLCT

II•

•

•

-

I•

•

•

DMAB

+

Electron-transfer kinetics were studied in two solvents by monitoring the MLCT emission. Rate constants for intramolecular electron transfer (k ) for 0, 1, and 2 in CH OH at 20 °C are 9.8 X 10 , 5.3 X 10 , and 5.6 X 10 s , respectively. The temperature dependence of k T also was determined in CH OH. The results are consistent with a nonadiabatic, long-range electron-transfer mechanism. Emission-decay kinetics of 2 in CH CN suggest the presence of two conformational isomers with dramatically different electron-transfer rates. The emission data is supported by the observation of two conformationalisomers in the C NMR spectrum of 2. The strong dependence of rate on conformation for 2 is consistent with a through— ET

6

7

3

5

-1

E

3

3

13

space mechanism for electron transfer.

THE MECHANISM OF ELECTRONT -RANSFER (ET) REACTIONS

has b e e n s t u d i e d b y u s i n g p h o t o c h e m i s t r y for m a n y years. E a r l y k i n e t i c studies focused 1

Address correspondence to this author. 0065-2393/90/0226-0101$07.00/0 © 1990 American Chemical Society

In Electron Transfer in Biology and the Solid State; Johnson, M., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

102

E L E C T R O N TRANSFER IN BIOLOGY A N D T H E SOLID STATE

o n b i m o l e c u l a r q u e n c h i n g o f p h o t o e x c i t e d c h r o m o p h o r e s b y e l e c t r o n donors or acceptors (quenchers). T h r o u g h these studies a significant a m o u n t o f information has b e e n o b t a i n e d c o n c e r n i n g the r e l a t i o n s h i p b e t w e e n the d r i v i n g force ( A G ) a n d the rate (fc ) for e n d o t h e r m i c a n d w e a k l y e x o t h e r m i c E T reactions (1-8).

Downloaded by NORTH CAROLINA STATE UNIV on December 24, 2012 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1990-0226.ch005

E X

EX

H o w e v e r , a disadvantage o f b i m o l e c u l a r systems is that the rate of the i n t r i n s i c E T step is not d i r e c t l y o b t a i n e d from the o b s e r v e d q u e n c h i n g rate because the c h r o m o p h o r e a n d q u e n c h e r m u s t diffuse together to f o r m a n e n c o u n t e r c o m p l e x p r i o r to E T (8). T h i s l i m i t a t i o n is severe for strongly e x o t h e r m i c reactions because diffusion is the r a t e - d e t e r m i n i n g step for q u e n c h i n g i n this case ( J , 4). I n a d d i t i o n , n o i n f o r m a t i o n is available c o n c e r n i n g the structure o f the e n c o u n t e r c o m p l e x d u r i n g E T for b i m o l e c u l a r reactions. T o o v e r c o m e the disadvantages of b i m o l e c u l a r systems, an i n c r e a s i n g n u m b e r o f studies have focused o n p h o t o i n d u c e d i n t r a m o l e c u l a r E T i n co valently l i n k e d c h r o m o p h o r e - q u e n c h e r ( C - Q ) c o m p o u n d s (9-30). I n C - Q systems, an e l e c t r o n d o n o r o r acceptor is covalently attached to a c h r o m o p h o r e to a l l o w m e a s u r e m e n t o f k w i t h o u t the c o m p l i c a t i o n of diffusion effects. I n these systems, i t is possible i n p r i n c i p l e to m e a s u r e k d i r e c t l y , e v e n i n cases w h e r e the rate is exceedingly r a p i d . T h i s m e a s u r e m e n t has b e c o m e possible t h r o u g h advances i n fast t i m e - r e s o l v e d m e t h o d s for the study o f the kinetics o f p h o t o i n d u c e d processes (31, 32). ET

ET

Spacer Structures I n m a n y early C - Q systems, the c h r o m o p h o r e a n d q u e n c h e r w e r e attached v i a flexible m e t h y l e n e chains (9-11). S o m e i n f o r m a t i o n was o b t a i n e d w i t h these systems r e g a r d i n g the effect o f relative o r i e n t a t i o n of the d o n o r a n d acceptor o n fc (9). H o w e v e r , because o f the flexibility o f the m e t h y l e n e spacers, little i n f o r m a t i o n was o b t a i n e d c o n c e r n i n g the effect o n E T o f sep aration distance or the m o l e c u l a r structure of the spacer. M o r e r e c e n t l y , attention has t u r n e d to C - Q systems, i n w h i c h the c h r o m o p h o r e a n d q u e n c h e r are attached b y r i g i d spacers i n an effort to p r o v i d e d e t a i l e d i n formation about structural factors that c o n t r o l E T reaction rates. I n these studies c h r o m o p h o r e a n d q u e n c h e r sites are h e l d together b y r i g i d organic spacers, p e p t i d e s , a n d p r o t e i n matrices (14, 15, 1 9 - 2 2 , 2 7 , 2 8 , 33-37). EX

T h e w e l l - d e f i n e d spacer structure makes i t possible to d e t e r m i n e fc u n d e r conditions i n w h i c h the separation distance b e t w e e n a n d the r e l a t i v e orientation of the d o n o r a n d acceptor are k n o w n . W h e n the r i g i d spacer systems are u s e d , it b e c o m e s possible to address several questions of interest: EX

1. W h a t is the effect of distance o n the rate for E T ? 2. H o w does the m o l e c u l a r a n d electronic structure o f the spacer affect the rate for long-distance E T ?


5.

CABANA & SCHANZE

3. H o w does k

E1

103

Peptide Spacers

d e p e n d o n the relative o r i e n t a t i o n o f the d o n o r

and acceptor? I n a d d i t i o n to addressing these questions, studies o f p h o t o i n d u c e d

intra

m o l e c u l a r E T across r i g i d spacers have p r o v i d e d e x p e r i m e n t a l e v i d e n c e for the M a r c u s i n v e r t e d r e g i o n (13) a n d have l e d to t h e d e v e l o p m e n t o f s y n t h e t i c systems that m i m i c t h e p r i m a r y events i n photosynthesis (14, 15,


17, 18, 38). S t r u c t u r a l a n d k i n e t i c studies o f proteins i n v o l v e d i n b i o l o g i c a l redox reactions (e.g., t h e photosynthetic reaction c e n t e r p r o t e i n c o m p l e x ,

cyto

c h r o m e c, b l u e c o p p e r proteins) (39-45) have l e d to the r e a l i z a t i o n that t h e p r o t e i n - b o u n d redox sites are f r e q u e n t l y separated b y 5 - 2 0 A w h e n E T occurs. I n m a n y cases t h e transferring e l e c t r o n " t u n n e l s " across a n i n t e r v e n i n g p r o t e i n m a t r i x (45, 46). T h i s r e a l i z a t i o n has l e d to a n i n c r e a s i n g n u m b e r o f e x p e r i m e n t a l a n d theoretical investigations focused o n u n d e r standing t h e role o f the i n t e r v e n i n g p r o t e i n i n m e d i a t i n g long-range E T . O n e e x p e r i m e n t a l approach to this p r o b l e m has b e e n to s t u d y E T i n s t r u c t u r a l l y w e l l - d e f i n e d native a n d m o d i f i e d proteins (33-37, 40-42,

45-47).

H o w e v e r , a disadvantage of this approach is that i t is difficult to systematically v a r y s t r u c t u r a l elements i n t h e p r o t e i n - b a s e d systems. A n alternative approach is to study i n t r a m o l e c u l a r E T i n systems i n w h i c h a d o n o r a n d acceptor are separated b y synthetic p e p t i d e spacers. T h i s a p p r o a c h was p i o n e e r e d b y I s i e d , w h o s t u d i e d t h e r m a l l y activated E T b e t w e e n m e t a l centers i n several systems that use a series o f r i g i d o l i g o - L p r o l i n e p e p t i d e spacers (48-50). W i t h this m e t h o d , t h e separation distance b e t w e e n t h e redox sites c a n b e systematically v a r i e d to a l l o w i n v e s t i g a t i o n o f t h e distance d e p e n d e n c e o f fc

ET

across a w e l l - d e f i n e d p e p t i d e

spacer.

O l i g o p r o l i n e p e p t i d e s are w e l l - s u i t e d for u s e as spacers because i n p r o t i c solvents t h e p e p t i d e c h a i n adopts a h e l i c a l f o r m that exists p r e d o m i n a n t l y i n a single conformation a n d undergoes i s o m e r i z a t i o n at a c o m p a r a t i v e l y slow rate. T h i s fact has b e e n established i n N M R spectroscopic

experiments

(51-54) a n d t h r o u g h studies of e n d - t o - e n d F o r s t e r excited-state e n e r g y t r a n s fer across oligoprolines r a n g i n g from 6 to 10 a m i n o a c i d u n i t s (55, 56).

Photoinduced Intramolecular ET W e i n i t i a t e d a series o f investigations i n w h i c h w e a p p l y t h e t e c h n i q u e o f p h o t o i n d u c e d i n t r a m o l e c u l a r E T to study t h e k i n e t i c s o f E T across a series of r i g i d o l i g o p r o l i n e spacers. T h e s e studies w e r e d e s i g n e d to o b t a i n q u a n titative i n f o r m a t i o n c o n c e r n i n g t h e distance d e p e n d e n c e o f k

ET

across p e p

tides for the p h o t o c h e m i e a l l y activated process. O n e goal is to p r o v i d e data that w i l l allow a d i r e c t c o m p a r i s o n o f the distance d e p e n d e n c e for a p h o t o i n d u c e d E T w i t h t h e t h e r m a l l y activated E T processes s t u d i e d b y I s i e d . It is h o p e d that s u c h comparisons w i l l l e n d i n s i g h t c o n c e r n i n g the m e c h a n i s m


104

E L E C T R O N TRANSFER IN BIOLOGY A N D T H E SOLID STATE


for electron t u n n e l i n g across spacers that have s t r u c t u r a l characteristics i n c o m m o n w i t h redox proteins. I n the p e p t i d e - l i n k e d e l e c t r o n - d o n o r - a c c e p t o r systems d e s c r i b e d i n this chapter (see structures), the photoexcited Re(I) m e t a l c o m p l e x acts as an e l e c t r o n acceptor a n d the d i m e t h y l a m i n o b e n z o a t e ( D M A B ) m o i e t y acts as an e l e c t r o n donor. T h e Re(I) c o m p l e x was chosen p r i m a r i l y because it has a w e l l - c h a r a c t e r i z e d m e t a l - t o - l i g a n d charge-transfer ( M L C T ) e x c i t e d state that is l u m i n e s c e n t , r e l a t i v e l y l o n g - l i v e d , a n d a strong oxidant (24, 25, 57, 58). T h e D M A B donor was chosen for the relative ease w i t h w h i c h it can b e i n c o r p o r a t e d i n t o the p e p t i d e synthesis a n d because the a m i d o l i n k a g e that b i n d s the d o n o r to the p e p t i d e c h a i n restricts d y n a m i c m o t i o n .

0-m :

R = H

0:

R = NMe

2

1- m :

n=l,R = H

1:

n = l,R = NMe

2- m :

n = 2, R = H

2:

n = 2, R = N M e 2

2

I n c o m p o u n d s 0-2, photoexcitation of the Re(I) c o m p l e x i n t o the M L C T excited state initiates i n t r a m o l e c u l a r E T from the D M A B d o n o r to the m e t a l center. (bpy)Re - • ' D M A B — 1

(bpy-)Re 11

• - D M A B - ^ (bpy-)Re 1

•-DMAB

MLCT


+

(1)

5.

CABANA & SCHANZE

105

Peptide Spacers

w h e r e b p y is 2 , 2 ' - b i p y r i d i n e . T h e rate constant for E T i n 0-2

has b e e n

e x a m i n e d i n two solvents b y b o t h steady-state a n d t i m e - r e s o l v e d fluores cence spectroscopy.

T h e results indicate that the rate of E T is strongly

d e p e n d e n t u p o n the n u m b e r of p e p t i d e spacers. I n a d d i t i o n , t e m p e r a t u r e d e p e n d e n c e e x p e r i m e n t s indicate that E T i n 0-2 is nonadiabatic a n d thus suggest that long-range E T occurs i n each case.


Experimental Procedures The peptide ligands and metal complexes were prepared by standard methods (25, 27). Each complex was purified by repeated column chromatography. The structures of the complexes were confirmed by *H and C N M R spectroscopy and by elemental analysis. Cyclic voltammetry was carried out on a voltammograph (Bioanalytical Systems C V - 2 ) . Experiments were carried out in a two-compartment cell i n which Pt disk working electrodes and Pt wire auxiliary electrodes were separated from the reference electrode (saturated calomel, S C E ) by a medium-porosity glass frit. Tetraethylammonium perchlorate (TEAP) (Kodak, recrystallized) was used at a concentration of 0.1 M as supporting electrolyte. Solvents used i n emission experiments were Kodak spectroquality. In all ex periments, sample concentrations were —10 M , with optical densities at 400 nm ~0.10. Stern-Volmer experiments demonstrated that this concentration was suflficiendy low to preclude bimolecular quenching. Temperature control during the luminescence experiments was maintained to within ± 1 °C by using a recirculating bath (Hakke D3). Steady-state luminescence spectra were obtained on a spectro photometer (Spex Industries F-112A). Quantum-yield measurements were made relative to an emission actinometer consisting of Zn(II)-[5,10,15,20-tetraphenylporphyrin] in air-saturated benzene (emission quantum yield,

Photoinduced Electron Transfer Across Peptide Spacers - Advances in

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