Ordered Media in Chemical Separations - American Chemical Society

Chapter 16

Computer Imaging of Cyclodextrin Inclusion Complexes R. Douglas Armstrong

Downloaded by UNIV LAVAL on October 5, 2015 | http://pubs.acs.org Publication Date: June 30, 1987 | doi: 10.1021/bk-1987-0342.ch016

La Jolla Cancer Research Foundation, 10901 North Torrey Pines Road, La Jolla, CA 92037

X-ray crystal structures were used for the production of computer projected images of inclusion complexes of structural isomers, enantiomers and diastereomers with a- or β-cyclodextrin. These projections allow for a visual evaluation of the interaction that occurs between various molecules and cyclodextrin, and an understanding of the mechanism for chromatographic resolution of these agents with bonded phase chromatography. The wide i n t e r e s t i n the use o f c y c l o d e x t r i n s as a s e p a r a t i o n medium has l e d t o a number o f u s e f u l a p p l i c a t i o n s . The a b i l i t y o f these molecules to bind other molecules t o form an i n c l u s i o n complex, has p r o v i d e d for their use i n typically difficult separations of enantiomers, diasastereomers, and structural isomers. Through t h e c o u p l i n g o f c y c l o d e x t r i n t o a s o l i d s u p p o r t , such as s i l i c a g e l , a c h r o m a t o g r a p h i c r e s i n can be made, and has been d e v e l o p e d as a u s e f u l c h r o m a t o g r a p h i c p r o c e d u r e . A l t h o u g h i t i s w e l l understood t h a t m o l e c u l e s must be a b l e t o enter the c a v i t y o f the c y c l o d e x t r i n molecule f o r complexation to occur, and t h e r e f o r e , under chromatographic conditions, for r e t e n t i o n t o r e s u l t , t h e d i f f e r e n t i a l b i n d i n g o f two s t e r e o i s o m e r s within the c y c l o d e x t r i n that allows for their differential r e t e n t i o n i s n o t always a p p a r e n t . An u n d e r s t a n d i n g o f t h i s can be o b t a i n e d through t h e use o f t h r e e d i m e n s i o n a l computer graphic imaging o f t h e c r y s t a l s t r u c t u r e o f t h e i n c l u s i o n complex. This review w i l l i l l u s t r a t e examples o f computer p r o j e c t e d models o f i n c l u s i o n complexes o f s t r u c t u r a l isomers ( o r t h o , m e t a , para nitrophenol), enantiomers (dand 1propranolol) and diastereomers [cis and t r a n s .l(p-3-dimethylaminoethoxy-phenylbutene), tamoxifen] in e i t h e r a - or β - c y c l o d e x t r i n . The use o f t h e s e computer p r o j e c t i o n s o f t h e c r y s t a l structures o f these complexes allows f o r the demonstration and p r e d i c t i o n o f t h e chromatographic behavior of these agents on immobilized cyclodextrin.

0097-6156/87/0342-0272$06.00/0 © 1987 American Chemical Society

In Ordered Media in Chemical Separations; Hinze, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

16.

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Computer Imaging of Cyclodextrin Inclusion Complexes


Computer P r o j e c t e d I n c l u s i o n

Complexes

S t r u c t u r a l Isomers Cyclodextrin bonded phases have been demonstrated to be p a r t i c u l a r l y adept i n r e s o l v i n g s t r u c t u r a l isomers [I) such as t h e o r t h o , meta and para forms o f n i t r o p h e n o l (Figure 1). These m o l e c u l e s can be s e p a r a t e d on columns o f e i t h e r Β o r α - c y c l o d e x t r i n , the r e s p e c t i v e b i n d i n g c o n s t a n t s ( a t jaH 10) f o r α-CD i n c l u s i o n c o m p l e x a t i o n are 200, 500, and 2439 M respectively for ortho, meta and p a r a n i t r o p h e n o l [I). F i g u r e 2 i l l u s t r a t e s the computer p r o j e c t i o n o f each o f t h e s e complexes based on t h e i r x - r a y c r y s t a l structures (2,3,4). The p i c t u r e s i l l u s t r a t e a s i d e view o f t h e complex with the f r o n t o f t h e c y c l o d e x t r i n c u t away i n o r d e r t o see the degree of penetration of the complexed molecule. The p a r a - n i t r o p h e n o l ( r e d m o l e c u l e in F i g u r e 2 a ) , i s found to complex deep i n the c y c l o d e x t r i n c a v i t y , w i t h t h e n i t r o - p o r t i o n o f t h e molecule in potential position to interact with the lower 6-hydroxyl atoms o f the α - c y c l o d e x t r i n . The deep and c e n t e r e d penetration of para-nitrophenol also allows for excellent i n t e r a c t i o n o f the phenol ring with the n o n p o l a r cyclodextrin cavity. This is in contrast to the m e t a - n i t r o p h e n o l (green molecule, Figure 2b), which also significantly enters the cyclodextrin cavity, but to a lesser degree than does the para-nitrophenol. T h i s a l l o w s f o r a reduced i n t e r a c t i o n with the α - c y c l q d e x t r i n , and the lower b i n d i n g c o n s t a n t (500 compared to 2439 M f o r the p a r a - n i t r o p h e n o l ) . The o r t h o - n i t r o p h e n o l ( y e l l o w m o l e c u l e , F i g u r e 2 c ) , which e x h i b i t s the lowest b i n d i n g constant with α - c y c l o d e x t r i n at 200 Μ , i s c l e a r l y i l l u s t r a t e d to have the l e a s t p e n e t r a t i o n and c o m p l e x a t i o n i n the α - c y c l o d e x t r i n c a v i t y . The l o c a t i o n o f t h e n i t r o group when i n the o r t h o p o s i t i o n b l o c k s t h e m o l e c u l e from e n t e r i n g the c y c l o d e x t r i n c a v i t y , as o b s e r v e d from the c l o s e n e s s o f t h e van d e r Waal s ' r a d i i . The computer p r o j e c t e d i n c l u s i o n complexes n i c e l y demonstrate the r e a s o n s f o r t h e v a r i a b l e b i n d i n g c o n s t a n t s o f t h e s e s t r u c t u r a l i s o m e r s , and a r e very p r e d i c t i v e o f t h e i r r e s u l t i n g chromatographic b e h a v i o r . There have been s e v e r a l r e p o r t s which have demonstrated the chromatographic separation of o r t h o , meta and p a r a structural isomers on B - c y c l o d e x t r i n m a t r i c e s ( 5 ) . One common f e a t u r e o f these separations is t h a t the o r d e r o f r e t e n t i o n ( g r e a t e s t to l o w e s t ) f a l l s i n the o r d e r o f para > o r t h o > m e t a , a l t h o u g h the o r t h o and meta isomers e l u t e v e r y c l o s e to one a n o t h e r . This i s in c o n t r a s t t o what i s o b t a i n e d , as i l l u s t r a t e d above f o r n i t r o p h e n o l , w i t h α - c y c l o d e x t r i n , where the b i n d i n g a f f i n i t y f o r the meta isomer is greater than the o r t h o . In an attempt t o understand this anomaly, t h e complex o f o r t h o o r m e t a - n i t r o p h e n o l i n B - c y c l o d e x t r i n was m o d e l e d . As e x p e c t e d from the much g r e a t e r c a p a c i t y o f t h e β-cyclodextrin (composed of 7 glucose units) compared to the alpha-cyclodextrin (composed of 6 glucose units), the small n i t r o p h e n o l m o l e c u l e s both e a s i l y f i t i n t o the B - c y c l o d e x t r i n . The b l o c k e d e n t r y problem t h a t the o r t h o - n i t r o p h e n o l e x h i b i t e d i n the α - c y c l o d e x t r i n , does not e x i s t i n the B - c y c l o d e x t r i n , and a l l o w s for even g r e a t e r complexation than what is observed for meta-nitrophenol (binding constants of 357 M" for


273


2

Figure

Nitrophenol

1.

(OH) meta

(OH) ortho

OH (para)

Ν0 3

2

Trans-tamoxifen

OjCr 2

2

3 2

CH -CH -N(CH )

S t r u c t u r e s o f d i f f e r e n t i s o m e r s r e s o l v e d by bonded phase c y c l o d e x t r i n chromatography.

Propranolol

Ο — CH2-CH-NH-CH(CH )

OH


23

m

GO

η > r

2 η χ m

>

m α

m Ό

*J

α m

Ν)

16.

ARMSTRONG

ortho-nitrophenol

Computer Imaging of Cyclodextrin Inclusion Complexes compared

to

147

M

for

meta-nitrophenol,

ref.


DEnantiomers Potentially, one of the most valuable applications of cyclodextrin as an analytical tool is its use in resolving e n a n t i o m e r i c compounds, t h o s e compounds which a r e m i r r o r images o f each other. This is an important concern with synthetic p h a r m a c e u t i c a l s , which a r e o f t e n produced as e n a n t i o m e r s . In most c a s e s , both i s o m e r s can have p h y s i o l o g i c a l a c t i v i t y , a l t h o u g h o n l y one actually has capacity to produce the d e s i r e d therapeutic action. The i n a c t i v e isomer w i l l o f t e n c o n t r i b u t e t o host t o x i c i t y o r o t h e r u n d e s i r e d a c t i o n s which can l i m i t the e f f e c t i v e n e s s o f the a c t i v e isomer. The a b i l i t y o f c y c l o d e x t r i n to r e s o l v e many t y p e s o f e n a n t i o m e r s i s o f o b v i o u s b e n e f i t , and has been demonstrated f o r a number o f r e l e v a n t p h a r m a c e u t i c a l s ( 6 ) . One i m p o r t a n t drug that is synthesized as b o t h d and 1 enantiomers, which can be resolved using immobilized 3-cyclodextrin, is propranolol (Figure 1). Under standard chromatographic conditions (see ref. 6) , t h e d - p r o p r a n o l o l is r e t a i n e d much l o n g e r than i s the 1 - p r o p r a n o l o l . The r e s p e c t i v e i n c l u s i o n complex o f each isomer in 3 - c y c l o d e x t r i n i s i l l u s t r a t e d i n the computer p r o j e c t i o n s i n F i g u r e 3 a and b. This i l l u s t r a t e s t h a t t h e r e i s no d i f f e r e n c e between d and 1 p r o p r a n o l o l i n t h e i r actual complexation within the 3-cyclodextrin cavity, as the n a p t h o l r i n g s o f each compound assume the e x a c t same placement w i t h i n the 3 - c y c l o d e x t r i n . However, v e r y i m p o r t a n t d i f f e r e n c e s e x i s t from the p o i n t o f the c h i r a l carbon on the a l i p h a t i c s i d e chain. In c o n t r a s t to the o r t h o , m e t a , p a r a s t r u c t u r a l isomers, which were shown t o r e s o l v e because o f t h e i r r e s p e c t i v e a b i l i t i e s t o be complexed w i t h i n the c y c l o d e x t r i n c a v i t y , f o r e n a n t i o m e r i c r e s o l u t i o n , t h e u n i d i r e c t i o n a l 2- and 3 - h y d r o x y l groups l o c a t e d a t t h e mouth o f t h e c y c l o d e x t r i n c a v i t y appear t o be i n t e g r a l for chiral recognition. In the models i l l u s t r a t e d i n F i g u r e 3, t h e van d e r Waals' r a d i i a r e shown f o r o n l y t h e s e 2- and 3 - h y d r o x y l groups o f the 3 - c y c l o d e x t r i n , along with the secondary amine o f t h e propranolol molecules. The h y d r o x y ! group a t t a c h e d to the c h i r a l carbon of propranolol i s i n the same p o s i t i o n f o r the d and 1 i s o m e r s , and i s p l a c e d f o r o p t i m a l hydrogen bonding to a 3 - h y d r o x y l o f the c y c l o d e x t r i n . Important d i f f e r e n c e s a r e o b s e r v e d , however, between the d and 1 forms with r e s p e c t t o t h e i r s e c o n d a r y amine group. In the d - p r o p r a n o l o l complex, t h e n i t r o g e n is ideally s i t u a t e d f o r hydrogen bonding with both a 2- and 3 - h y d r o x y l group on the 3 - c y c l o d e x t r i n , e x h i b i t i n g bond d i s t a n c e s o f 3.3 and 2 . 8 Â . The amine o f t h e 1 - p r o p r a n o l o l however, i s l e s s f a v o r a b l y s i t u a t e d f o r hydrogen b o n d i n g , w i t h d i s t a n c e s o f 3.8 and 4 . 5 Â to the c l o s e s t 2- and 3 - h y d r o x y l groups o f t h e 3 - c y c l o d e x t r i n which a r e too g r e a t f o r hydrogen b o n d i n g . The gap between the van d e r W a l l s ' r a d i i o f t h e s e atoms i s c l e a r l y seen i n the 1- p r o p r a n o l o l model (Figure 3 b ) , whereas t h e van d e r Waals* r a d i i i n the d - p r o p r a n o l o l a r e v e r y c l o s e l y a s s o c i a t e d with t h o s e f o r the 2- and 3 - h y d r o x y l groups on the 3-cyclodextrin (Figure 4a). With t h e a b i l i t y t o form a d d i t i o n a l hydrogen b o n d s , t h e d - p r o p r a n o l o l e x h i b i t s a s t r o n g e r b i n d i n g with t h e 3 - c y c l o d e x t r i n , and i s t h e r e b y r e t a i n e d longer


275

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ORDERED MEDIA IN CHEMICAL SEPARATIONS


under bonded phase c h r o m a t o g r a p h i c c o n d i t i o n s ( 6 ) . T h e r e f o r e , two parameters a r e i m p o r t a n t f o r c h i r a l r e s o l u t i o n : 1. t h e a b i l i t y o f t h e compound to form an i n c l u s i o n complex w i t h i n the c y c l o d e x t r i n c a v i t y , which p r o v i d e s f o r r e t e n t i o n o f t h e compound, and 2. t h e i n t e r a c t i o n o f t h e p o r t i o n o f the m o l e c u l e c o n t a i n i n g the c h i r a l c a r b o n with t h e u n i d i r e c t i o n a l 2- and 3 - h y d r o x y l groups on the cyclodextrin. Diastereomers The a b i l i t y o f c y c l o d e x t r i n to r e s o l v e s t e r e o i s o m e r s i s v e r y r e a d i l y a p p l i e d f o r the s e p a r a t i o n o f d i a s t e r e o m e r s , such as the c i s - and t r a n s - g e o m e t r i c isomers ( 6 , 7 ) . S i m i l a r t o both o f t h e examples p r e s e n t e d a b o v e , r e s o l u t i o n o f g e o m e t r i c isomers a p p e a r s t o r e s u l t from both the l e v e l o f i n c l u s i o n complex f o r m e d , as w e l l as the l e v e l o f i n t e r a c t i o n o f t h e m o l e c u l e with the 2- and 3 - h y d r o x y l groups o f t h e c y c l o d e x t r i n . T h i s can be i l l u s t r a t e d w i t h the s y n t h e t i c a n t i e s t r o g e n tamoxifen ( F i g u r e 1 ) , which is s y n t h e s i z e d i n b o t h t h e c i s and t r a n s f o r m s . These two compounds can be s e p a r a t e d using B-cyclodextrin bonded phase c h r o m a t o g r a p h y , w i t h t h e c i s - t a m o x i f e n e l u t i n g p r i o r t o the t r a n s - t a m o x i f e n ( 6 , 7 ) . Using the i n d i v i d u a l x - r a y c r y s t a l structures for these agents (8,9), the respective inclusion complexes i n B - c y c l o d e x t r i n were modeled u s i n g computer i m a g i n g , as i l l u s t r a t e d i n F i g u r e 4, a ( t r a n s - t a m o x i f e n ) and b ( c i s - t a m o x i f e n ) . It is very apparent that these two agents interact quite d i f f e r e n t l y with t h e B - c y c l o d e x t r i n . The t r a n s - t a m o x i f e n i s a b l e t o form a b e t t e r i n c l u s i o n complex than can the c i s - t a m o x i f e n , w i t h i t s phenyl s i d e group p e n e t r a t i n g 6.3Â (measured from the 3 - h y d r o x y l o f the B - c y c l o d e x t r i n to the lowest atom o f the r e s p e c t i v e m o l e c u l e of tamoxifen) compared to the 5.7Â penetration of the c i s - t a m o x i f e n . In a d d i t i o n to the g r e a t e r l e v e l o f c o m p l e x a t i o n , i t appears that the trans-tamoxifen may have some additional i n t e r a c t i o n between i t s a l i p h a t i c s i d e - c h a i n and the mouth o f t h e B-cyclodextrin. Summary The u s e f u l n e s s o f c y c l o d e x t r i n as a s e p a r a t i o n medium f o r the r e s o l u t i o n o f s t e r o i s o m e r s , whether t h e y be s t r u c t u r a l isomers, d i a s t e r e o m e r s o r e n a n t i o m e r s , has become r e a d i l y a p p a r e n t . The u n d e r s t a n d i n g o f how and why a p a r t i c u l a r s e p a r a t i o n o c c u r s is n i c e l y enhanced with t h e use o f computer m o d e l i n g o f t h e x - r a y crystal structures of the agents. For example, t h e computer m o d e l i n g of the compounds i n t h i s r e v i e w i l l u s t r a t e d the importance o f the 2- and 3 - h y d r o x y l groups f o r the r e s o l u t i o n o f e n a n t i o m e r s , whereas d i f f e r e n t i a l i n c l u s i o n c o m p l e x a t i o n was demonstrated to m e d i a t e the r e s o l u t i o n o f s t r u c t u r a l isomers such as o r t h o , meta and para n i t r o p h e n o l . In p a r t i c u l a r , a l t h o u g h n o t shown i n the above r e s u l t s , t h e use o f the computer imaging may g r e a t l y improve e f f o r t s t o r a t i o n a l l y d e r i v a t i z e c y c l o d e x t r i n in o r d e r to o p t i m i z e a particular separation. It has been demonstrated i n o t h e r r e p o r t s t h a t c y c l o d e x t r i n i s l i m i t e d i n i t s c a p a b i l i t y t o s e r v e as an e f f e c t i v e medium f o r the r e s o l u t i o n o f e n a n t i o m e r i c compounds, and c h i r a l compounds t h a t form v e r y good i n c l u s i o n complexes a r e not



16.

ARMSTRONG

Figure

2.

Computer Imaging of Cyclodextrin Inclusion Complexes

Computer imaging o f c r y s t a l s t r u c t u r e s o f the i n c l u s i o n complexes of para (A), meta (Β) and ortho (C) n i t r o p h e n o l with α - c y c l o d e x t r i n . The complex i s shown with van d e r Waals' r a d i i , and the f r o n t s e c t i o n o f t h e complex cut away i n o r d e r to expose the nitrophenol molecule.


277


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ORDERED MEDIA IN C H E M I C A L SEPARATIONS

F i g u r e 3.

Computer imaging o f the i n c l u s i o n complexes o f d - ( A ) and 1-(B) propranolol with β-cyclodextrin. The c h e m i c a l s t r u c t u r e s a r e i l l u s t r a t e d with van d e r Waal s' radii shown f o r o n l y t h e secondary amine o f p r o p r a n o l o l and the 2- and 3- h y d r o x y l groups o f the β - c y c l o d e x t r i n .

Figure

Computer imaging o f the i n c l u s i o n complexes o f t r a n s (A) and c i s (B) t a m o x i f e n with β - c y c l o d e x t r i n . The complex i s shown with van d e r Waals' r a d i i , and the f r o n t s e c t i o n o f the complex cut away i n o r d e r to expose the n i t r o p h e n o l m o l e c u l e .

4.


16. ARMSTRONG Computer Imaging of Cyclodextrin Inclusion Complexes 279 always n e c e s s a r i l y r e s o l v e d from each o t h e r ( 6 ) . In some c a s e s however, a s m a l l d e r i v a t i z a t i o n m o d i f i c a t i o n o f t h e c y c l o d e s t r i n can a l l o w f o r the needed s e p a r a t i o n (10). With improved and e a s i e r methods o f computer m o d e l i n g and energy m i n i m i z a t i o n c a l c u l a t i o n s , combined with the l o w e r i n g c o s t o f o b t a i n i n g such a s y s t e m , t h e use o f computer imaging s h o u l d c o n t i n u e to be a most v a l u a b l e r e s o u r c e i n the s t u d y o f c y c l o d e x t r i n s and t h e i r v a r i e d f u n c t i o n s .

Acknowledgments


T h i s work was s u p p o r t e d by g r a n t CH 329 from the American Cancer S o c i e t y . The computer imaging was completed a t the Computer G r a p h i c s L a b o r a t o r y ( D r . R. L a n g r i d g e , d i r e c t o r ; s u p p o r t e d by NIH g r a n t RR 1081) with the a s s i s t a n c e o f N. P a t t a b i r a m a n .

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Hinze, W. Separation 10(2):159-237.

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Harata, Κ., Hisahi, U. and Tanaka, J. Japan, 1978. 51:1627-1634.

3.

Harata, K.

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Iwqsaki, F. and Kawano, Y. Acta. Cryst., 1978. 34:1286-1290.

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Armstrong, D. W., DeMond, W., Alak, Α., Hinze, W. L., Riehl, T. E. and Bui, K. H. Analy. Chem. 1985. 57(1):234-237.

6.

Armstrong, D. W., Ward, T. J . , Armstrong, R. D. and Beesley, T. E. Science. 1986, 232:1132-1135.

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Armstrong, R. D., Ward, T. J . , Pattabiraman, N., Benz, C. and Armstrong, D. W. J. Chromatog 1987. 414:192-196.

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Kilbourn, Β. T. and Owston, P. 736:1-5.

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Precigoux, P. G., Courseille, C., Geoffre, S. and Hospital, M. Int. Union Cryst. 1979. 12:3070-3072.

Bull.

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Bull. Chem. Soc. Japan, 1977. 51;1416-1424.

G. J. Am. Chem. Soc. 1970.

10. Armstrong, D. W., Ward, T. J . , Czech, Α., Czech, R. A. and Bartsch, R. A. J. Org. Chem. 1985. 50:5556-5559. RECEIVED

February 17, 1987


Ordered Media in Chemical Separations - American Chemical Society

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