Specific Interactions of (+)-Catechin and (-)-Epicatechin with Polymers

Chapter 14

Downloaded by KTH ROYAL INST OF TECHNOLOGY on August 23, 2015 | http://pubs.acs.org Publication Date: November 30, 1987 | doi: 10.1021/bk-1987-0358.ch014

Specific Interactions of (+)-Catechin and (-)-Epicatechin with Polymers that Contain the L-Prolyl Residue 1

Wolfgang R. Bergmann and Wayne L. Mattice

2

1

Department of Polymer Science, University of Akron, Akron, OH 44325 Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803

2

Quantum yields for fluorescence, Q, by (+)-catechin and (-)-epicatechin are sensitive to the nature of the solvent. These compounds and their oligomers form complexes with poly(vinylpyrrolidone). Complex formation is accompanied by a dramatic collapse of the dimensions of the poly(vinylpyrrolidone) chain, and the value of Q changes in the manner expected for transfer of the fluorophore from an aqueous to a nonpolar medium. Much different behavior is seen when poly(L-proline) is substituted for poly(vinylpyrrolidone). Emission by the fluorophores exhibits discrimination between local right- and left-handed helices in the poly(L-proline) chain. P r o t e i n s , n u c l e i c a c i d s , and p o l y s a c c h a r i d e s a r e t h e most w i d e l y known o f t h e polymers t h a t o c c u r i n n a t u r e . Representatives of the p l a n t kingdom p r o d u c e o t h e r t y p e s o f p o l y m e r s w i t h p r o p e r t i e s t h a t are less w e l l understood. A n o t a b l e example i s p r o v i d e d by polymers o f ( + ) - c a t e c h i n and ( - ) - e p i c a t e c h i n , w h i c h a r e found i n a v a r i e t y o f plants. The s t r u c t u r e s o f t h e monomers a r e d e p i c t e d i n F i g u r e 1. Polymers a r e u s u a l l y formed v i a c a r b o n - c a r b o n bonds from C ( 4 ) o f one monomer t o C ( 8 ) o f i t s n e i g h b o r ( 1 , 2 ) . Other l i n k a g e p a t t e r n s , such as C ( 4 ) t o C ( 6 ) , a r e a l s o known ( 3 - 5 ) . The polymers i s o l a t e d from some p l a n t s a r e b r a n c h e d ( 6 ) . W e i g h t - a v e r a g e m o l e c u l a r w e i g h t s a s l a r g e a s 13300 have been measured f o r p e r a c e t y l a t e d samples ( 7 ) · Much l a r g e r polymers a r e p r o d u c e d i n n a t u r e because t h e samples i s o l a t e d have been p o l y d i s p e r s e . Polymers o f ( + ) - c a t e c h i n and ( - ) - e p i c a t e c h i n form comlexes w i t h many m a c r o m o l e c u l e s . The f o r m a t i o n o f complexes w i t h p r o t e i n s i s t h e b a s i s f o r t h e i r u s e by p l a n t s a s a d e f e n s e mechanism ( 8 ) . Most o f t h e p r e v i o u s s t u d i e s o f t h e s e complexes have been performed a t c o n c e n t r a t i o n s t h a t produce p r e c i p i t a t i o n ( 9 - 1 1 ) . The s e n s i t i v i t y w i t h w h i c h f l u o r e s c e n c e c a n be d e t e c t e d p e r m i t s s t u d y o f complex f o r m a t i o n a t c o n c e n t r a t i o n s so low t h a t a l l s p e c i e s remain i n solution (12,13). E x p e r i m e n t a l r e s u l t s r e p o r t e d h e r e show t h a t t h i s technique can a l s o r e v e a l s u b t l e aspects of the complexation 0097-6156/87/0358-0162$06.00/0 © 1987 American Chemical Society

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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process. I n some complexes t h e f l u o r e s c n e c e responds t o a s i m p l e change i n t h e medium s u r r o u n d i n g t h e f l u o r o p h o r e , b u t i n o t h e r c a s e s t h e r e i s a more c o m p l i c a t e d r e s p o n s e t h a t i s i n t i m a t e l y c o n n e c t e d w i t h the conformation o f the macromolecule.


EXPERIMENTAL DETAILS Poly(vinylpyrrolidone), poly(L-proline), (+)-catechin, ( - ) e p i c a t e c h i n , and t r i f l u o r o e t h a n o l were p u r c h a s e d from Sigma C h e m i c a l Co. Two o f t h e f o u r p o l y ( L - p r o l i n e ) samples s t u d i e d c o n t a i n e d an u n i d e n t i f i e d f l u o r e s c e n t m a t e r i a l t h a t was removed w i t h a c t i v a t e d charcoal. A f l u o r e s c e n t compound i n t h e t r i f l u o r o e t h a n o l was removed by d i s t i l l a t i o n . D r . R i c h a r d W. Hemingway k i n d l y s u p p l i e d h i g h l y p u r i f i e d samples o f s y n t h e t i c ( - ) - e p i c a t e c h i n - ( 4 B - 6 ) - ( + ) c a t e c h i n , ( - ) - e p i c a t e c h i n - ( 4 B - 8 ) - ( + ) - c a t e c h i n , and ( - ) - e p i c a t e c h i n (4B-8)-(+)-catechin decaacetate. Dioxane and 1 - p r o p a n o l were o b t a i n e d from A l d r i c h C h e m i c a l C o . F l u o r e s c e n c e measurements were p e r f o r m e d w i t h an SLM 8000 fluorometer. Quantum y i e l d s were o b t a i n e d w i t h q u i n i n e b i s u l f a t e as the standard ( 1 4 , 1 5 ) . E x c i t a t i o n was a t 272-280 nm, and t h e range o f t h e i n t e g r a t i o n u s e d i n t h e d e t e r m i n a t i o n o f 1/1(0) was 285-420 nm. C i r c u l a r d i c h r o i s m measurements were p e r f o r m e d w i t h a J a s c o J-500C s p e c t r o p o l a r i m e t e r . QUANTUM YIELDS FOR FLUORESCENCE Monomers and dimers e x h i b i t maximal e m i s s i o n a t 314-324 nm upon e x c i t a t i o n a t 272-280 nm. Quantum y i e l d s f o r f l u o r e s c e n c e , d e n o t e d by Q, a r e c o l l e c t e d i n T a b l e I . B o t h monomers e x h i b i t i n c r e a s i n g f l u o r e s c e n c e a s t h e s o l v e n t changes from water t o t r i f l u o r o e t h a n o l to dioxane. T h e r e i s no s i g n i f i c a n t d i f f e r e n c e i n t h e e m i s s i o n o f t h e two monomers i n a common s o l v e n t . Dimers show l e s s i n t e n s e e m i s s i o n t h a t do monomers. There i s a f u r t h e r r e d u c t i o n i n emission upon a c e t y l a t i o n o f a l l p h e n o l i c and a l i p h a t i c h y d r o x y l g r o u p s . COMPLEX FORMATION by POLY(VINYLPYRROLIDONE) I n t e g r a t e d f l u o r e s c e n c e i n t e n s i t y i n t h e absence and p r e s e n c e o f an added polymer a r e denoted by l ( 0 ) and I , r e s p e c t i v e l y . The v a l u e s o f 1/1(0) f o r ( + ) - c a t e c h i n and ( - ) - e p i c a t e c h i n i n water a r e enhanced s i g n i f i c a n t l y i n t h e p r e s e n c e o f p o l y ( v i n y l p y r r o l i d o n e ) , as i s shown i n T a b l e I I . T h e r e i s a more d r a m a t i c i n c r e a s e i n e m i s s i o n f o r a mixed dimer than f o r e i t h e r monomer. Previous s t u d i e s (10,11) of t h e i n t e r a c t i o n s w i t h p r o t e i n s have found t h a t t h e s t a b i l i t y o f t h e complexes r i s e s r a p i d l y as one p r o c e e d s from ( + ) - c a t e o h i n o r ( - ) e p i c a t e c h i n t h r o u g h a s e r i e s o f o l i g o m e r s o f i n c r e a s i n g s i z e . The b e h a v i o r o f 1/1(0) i n water shows t h a t ( + ) - c a t e c h i n and ( - ) e p i c a t e c h i n form complexes w i t h p o l y ( v i n y l p y r r o l i d o n e ) i n d i l u t e solution. I t s u g g e s t s t h a t t h e i n t e r a c t i o n s becomes s t r o n g e r when an o l i g o m e r i s s u b s t i t u t e d f o r e i t h e r monomer. Previous studies b a s e d on t h e development o f t u r b i d i t y ( 9 ) o r t h e i n h i b i t i o n o f t h e p r e c i p i t a t i o n o f a r a d i o a c t i v e l y l a b e l l e d t r a c e r p r o t e i n (10) have shown t h a t p o l y ( v i n y l p y r r o l i d o n e ) forms complexes w i t h p r o a n t h o c y a n i d i n s a t much h i g h e r c o n c e n t r a t i o n s .


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Table I .

Quantum Y i e l d s f o r F l u o r e s c e n c e ( E x c i t a t i o n a t 272-280 nm)

Compound

Solvent

Q

(+)-Catechin

Water Trifluoroethanol Dioxane Water Dioxane Dioxane Dioxane Dioxane

0.11 0.19 0.30 0.10 0.29 0.05 0.07 0.03


(-)-Epicatechin (-)-Epicatechin-(4B-6)- (+)-catechin (-)-Epicatechin-(4B-8)-(+)-catechin (-)-Epicatechin-(4B-8)-(+)-catechin (Decaacetate)

Table I I .

1/1(0) i n Water C o n t a i n i n g P o l y ( v i n y l p y r r o l i d o n e ) a t a M o l a r Monomer C o n c e n t r a t i o n o f 0.009

Fluorophore (+)-Catechin (-)-Epicatechin (-)-Epicatechin-(4B-6)-(+)-catechin

M o l a r i t y χ 10 4.7 4.7 0.9

1/1(0) 1.31 1.24 2.45

The sample o f p o l y ( v i n y l p y r r o l i d o n e ) u s e d i n t h e s e experiments has an i n t r i n s i c v i s c o s i t y o f 1.72 d l / g i n water a t 30° C . In the p r e s e n c e o f 0.0014 M ( + ) - c a t e c h i n , t h e i n t r i n s i c v i s c o s i t y f a l l s t o 0.13 d l / g . Since i t i s the (+)-catechin that induces the c o l l a p s e o f t h e p o l y ( v i n y l p y r r o l i d o n e ) c h a i n , i t i s assumed t h a t t h i s m o l e c u l e f i n d s i t s e l f i n an environment t h a t i s n o t c o m p l e t e l y aqueous, b u t i n s t e a d h a s a h i g h l o c a l c o n c e n t r a t i o n o f p o l y v i n y l p y r r o l i d o n e ) segments. The s o l v e n t dependence o f t h e quantum y i e l d f o r f l u o r e s c e n c e , r e p o r t e d i n T a b l e I , shows t h a t Q i n c r e a s e s as ( + ) - c a t e c h i n i s t r a n s f e r r e d from water t o d i o x a n e . C o l l a p s e o f the p o l y ( v i n y l p y r r o l i d o n e ) c h a i n about ( + ) - c a t e c h i n t h e r e f o r e produces an i n c r e a s e i n Q, w h i c h i s r e s p o n s i b l e f o r 1/1(0) > 1 i n T a b l e I I . COMPLEX FORMATION WITH POLY(L-PROLINE) P r e c i p i t a t i o n s t u d i e s a t c o m p a r a t i v e l y h i g h c o n c e n t r a t i o n s have shown t h a t p r o a n t h o c y a n i d i n s form complexes w i t h p o l y ( L - p r o l i n e ) as w e l l as w i t h p o l y ( v i n y l p y r r o l i d o n e ) ( 9 , 1 0 ) . Figure 2 d e p i c t s the r e p e a t u n i t s o f p o l y ( v i n y l p y r r o l i d o n e T "and p o l y ( L - p r o l i n e ) . The L p r o l y l u n i t c o n t a i n s one fewer methylene and h a s t h e r e m a i n i n g atoms r e a r r a n g e d so t h a t t h e amide and five-membered r i n g become p a r t s o f t h e backbone o f t h e c h a i n . The r e s u l t s r e p o r t e d i n t h e p r e c e e d i n g s e c t i o n might l e a d t o t h e e x p e c t a t i o n t h a t a d d i t i o n o f p o l y ( L p r o l i n e ) t o an aqueous s o l u t i o n o f one o f t h e f l u o r o p h o r e s would p r o d u c e 1/1(0) > 1. T h i s e x p e c t a t i o n i s n o t r e a l i z e d . Poly(Lp r o l i n e ) w i t h M = 60000 does n o t produce 1/1(0) > 1. Instead i t quenches t h e f l u o r e s c e n c e . F o r example, t h e v a l u e s o f 1/1(0) f o r 3 . 6 χ 10~5 M ( + ) - c a t e c h i n i n w a t e r a r e 0 . 7 5 and 0 · 4 1 , r e s p e c t i v e l y ,



14. B E R G M A N N A N D M A T T I C E

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a t L - p r o l y l c o n c e n t r a t i o n s o f 0.026 and 0 . 1 0 M. The Q i n T a b l e I do not permit r a t i o n a l i z a t i o n o f the quenching a b i l i t y of p o l y ( L p r o l i n e ) as a " s o l v e n t " e f f e c t . I t therefore i s pertinent to i n q u i r e whether t h e r e might be a more s p e c i f i c i n t e r a c t i o n o f ( + ) catechin with poly(L-proiine). The t e s t f o f s p e c i f i c i t y e x p l o i t s t h e a b i l i t y o f t h e p o l y ( L p r o l i n e ) c h a i n t o e x i s t i n e i t h e r o f two d i f f e r e n t c o n f o r m a t i o n s . Some t h i r t y y e a r s ago t h e s e c o n f o r m a t i o n s were c h a r a c t e r i z e d by t h e a n a l y s i s of the x-ray s c a t t e r i n g patterns of p o l y ( L - p r o l i n e ) i n the solid state. The c o n f o r m a t i o n known a s Form I i s a r i g h t - h a n d e d h e l i x w i t h 3.33 residues per t u r n , a t r a n s l a t i o n along the h e l i x a x i s o f 1.9 A / r e s i d u e , and a c i s c o n f i g u r a t i o n a t a l l amide bonds (16). The c o n f o r m a t i o n o f Form I I i n t h e s o l i d s t a t e i s a l e f t handed h e l i x w i t h 3 . 0 0 r e s i d u e s p e r t u r n , a t r a n s l a t i o n a l o n g t h e h e l i x a x i s o f 3 . 1 2 A / r e s i d u e , and a t r a n s c o n f i g u r a t i o n a t t h e amide bonds ( 1 7 , 1 8 ) . D i l u t e s o l u t i o n s i n s o l v e n t s s u c h as w a t e r , a c e t i c a c i d , and t r i f l u o r o e t h a n o l s u p p o r t a l o c a l s t r u c t u r e w i t h a s t r o n g s i m i l a r i t y t o Form I I ( 1 9 ) , b u t t h e r e i s s u f f i c i e n t f l e x i b i l i t y so t h a t a p o l y ( L - p r o l i n e ) c h a i n o f h i g h m o l e c u l a r w e i g h t behaves i n t h e s e s o l v e n t s a s a random c o i l w i t h a c h a r a c t e r i s t i c r a t i o o f 1 4 20 ( 2 0 ) . I n a s u i t a b l y chosen mixed s o l v e n t s y s t e m , s u c h as a c e t i c a c i d and 1 - p r o p a n o l ( 2 1 ) , a s h a r p r e v e r s i b l e t r a n s i t i o n between Form I and Form I I c a n be a c h i e v e d w i t h a s m a l l change i n s o l v e n t composition. Measurements o f o p t i c a l a c t i v i t y p r o v i d e a c o n v e n i e n t way t o f o l l o w t h e t r a n s i t i o n i n d i l u t e s o l u t i o n . There are l a r g e changes i n o p t i c a l a c t i v i t y because Forms I and I I a r e h e l i c e s o f

o p p o s i t e handedness.

The l e f t p a n e l i n F i g u r e 3 d e p i c t s t h e

r e v e r s i b l e t r a n s i t i o n t h a t i s d e t e c t e d by c i r c u l a r d i c h r o i s m measurements i n m i x t u r e s o f t r i f l u o r o e t h a n o l and 1 - p r o p a n o l . Form I I i s the only conformation present i n t r i f l u o r e t h a n o l . A solution o f p o l y ( L - p r o l i n e ) i n 35 : 65 t r i f l u o r o e t h a n o l : 1 - p r o p a n o l e x h i b i t s the same c i r c u l a r d i c h r o i s m p a t t e r n as does a s o l u t i o n where t h e s o l v e n t i s pure t r i f l u o r o e t h a n o l . However, f u r t h e r a d d i t i o n o f 1p r o p a n o l produces a d r a m a t i c change i n t h e c i r c u l a r d i c h r o i s m . At 20 : 80 t r i t l u o r o e t h a n o l : 1 - p r o p a n o l t h e c i r c u l a r d i c h r o i s m p a t t e r n i s t h a t c h a r a c t e r i s t i c o f Form I . Data i n F i g u r e 3 do n o t e x t e n d beyond 10 : 90 t r i f l u o r o e t h a n o l : 1 - p r o p a n o l because o f t h e low s o l u b i l i t y of p o l y ( L - p r o l i n e ) i n 1-propanol.

The

r i g h t p a n e l i n F i g u r e 3 d e p i c t s 1/1(0) f o r ( + ) - c a t e c h i n i n

m i x t u r e s o f t r i f l u o r o e t h a n o l and 1 - p r o p a n o l t h a t c o n t a i n p o l y ( L p r o l i n e ) a t a c o n c e n t r a t i o n o f 0 . 9 0 mg/ml. In pure t r i f l u o r o e t h a n o l 1/1(0) i s s l i g h t l y l e s s than o n e . A t t h e o p p o s i t e extreme o f s o l v e n t c o m p o s i t i o n , 1/1(0) i s g r e a t e r than one and i n c r e a s e s o v e r a period of several days. The g r e a t e s t s e n s i t i v i t y o f 1/1(0) t o s o l v e n t c o m p o s i t i o n o c c u r s n e a r 25 : 75 t r i f l u o r o e t h a n o l : 1p r o p a n o l , w h i c h i s where t h e Form I - Form I I i n t e r c o n v e r s i o n i s d e t e c t e d by c i r c u l a r d i c h r o i s m . Measurements o f t h e f l u o r e s c e n c e o f ( + ) - c a t e c h i n i n t h e mixed s o l v e n t s , b u t i n t h e absence o f p o l y ( L p r o l i n e ) , show no u n u s u a l dependence o f e m i s s i o n on s o l v e n t . We c o n c l u d e t h a t ( + ) - c a t e c h i n c a n d i s c r i m i n a t e between t h e two Forms o f p o l y ( L - p r o l i n e ) . The e m i s s i o n from ( + ) - c a t e c h i n becomes v e r y much s t r o n g e r i f t h e l o c a l s t r u c t u r e o f t h e p o l y ( L - p r o l i n e ) c h a i n i s a right-handed h e l i x with 3.33 residues per t u r n , a t r a n s l a t i o n o f 1.9 A / r e s i d u e , and c i s amide b o n d s . In c o n t r a s t , the



166

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F i g u r e 1. C o v a l e n t s t r u c t u r e s and numbering system f o r ( + ) c a t e c h i n ( R l = OH, R2 = H, R3 = d i h y d r o x y p h e n y l ) and ( - ) e p i c a t e c h i n ( R l = H, R2 = OH, R3 = d i h y d r o x y p h e n y l ) .

-CH-CH 2

F i g u r e 2. proline).

Repeating u n i t s

-N — CH —

C-

in poly(vinylpyrrolidone)

VOLUME %

and p o l y ( L -

l-PROPANOL

F i g u r e 3. L e f t p a n e l : Mean r e s i d u e e l l i p t i c i t y a t 210 nm f o r p o l y ( L - p r o l i n e ) i n m i x t u r e s o f t r i f l u o r o e t h a n o l and 1-propanol. R i g h t p a n e l : 1/1(0) f o r ( + ) - c a t e c h i n i n m i x t u r e s of t r i f l u o r o e t h a n o l and 1-propanol t h a t c o n t a i n p o l y ( L - p r o l i n e ) a t a c o n c e n t r a t i o n o f 0.90 mg/ml. Measurements o f 1/1(0) were performed a t 0, 3, 7, 12, and 20 days a f t e r p r e p a r a t i o n o f the s o l u t i o n s . 1/1(0) i n c r e a s e s w i t h time a t the h i g h e s t 1-propanol concentrations.



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(-h)-Catechin and (-)-Epicatechin

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e m i s s i o n i s s l i g h t l y quenched i f t h e l o c a l s t r u c t u r e i s a l e f t handed h e l i x w i t h 3 . 0 0 r e s i d u e s p e r t u r n , a t r a n s l a t i o n o f 3 . 1 2 A p e r r e s i d u e , and t r a n s amide b o n d s . T h e o r i g i n o f t h e time dependence o f t h e s t e a d y s t a t e e m i s s i o n h a s n o t y e t been i d e n t i f i e d . The s p e c i f i c i t y o f t h e i n t e r a c t i o n o f ( + ) - c a t e c h i n w i t h p o l y ( L p r o l i n e ) i s not s u r p r i s i n g i n view o f the b i o l o g i c a l f u n c t i o n o f p o l y m e r s o f ( + ) - c a t e c h i n and ( - ) - e p i c a t e c h i n . The presence o f t h e s e polymers i n c e r t a i n p l a n t s makes them an u n a t t r a c t i v e f o o d source f o r animals. A molecular basis f o r this p r o t e c t i v e action may l i e i n t h e a b i l i t y o f p o l y m e r s o f ( + ) - c a t e c h i n and ( - ) e p i c a t e c h i n t o i n t e r a c t s t r o n g l y w i t h mammalian s a l i v a r y p r o t e i n s t h a t have an u n u s u a l l y h i g h c o n t e n t o f t h e L - p r o l y l r e s i d u e ( 1 0 , 2 2 ) . ACKNOWLEDGMENTS T h i s r e s e a r c h was s u p p o r t e d b y N a t i o n a l S c i e n c e F o u n d a t i o n r e s e a r c h g r a n t DMB 85-00338. We thank D r . R i c h a r d W. Hemingway f o r t h e samples o f t h e d i m e r s .

LITERATURE CITED

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March 13, 1987


Specific Interactions of (+)-Catechin and (-)-Epicatechin with Polymers

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