Vibrational Raman optical activity of cyclodextrins

Vibrational Raman optical activity of cyclodextrins

Tetrahedron:A~yme~ttryVol. 1, No. 8. pp. 513-516, 1990 0957-4166/90 $3.00+.00 Pe~gamort Presspie Printed in Great Britain Vibrational Rarnan Optica...

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Tetrahedron:A~yme~ttryVol. 1, No. 8. pp. 513-516, 1990

0957-4166/90 $3.00+.00 Pe~gamort Presspie

Printed in Great Britain

Vibrational Rarnan Optical Activity of Cyclodextrins Laurence D. Barton e, Angelo P.. Gargaro, Zal Q. W e n David D. MacNlcol a n d C o l i n Butters

Chemistry Department, The University, Glasgow, G12 8QQ, U.K.

(Received 5 June 1990)

Abstract: Vibrational Raman optical activity spectra of aqueous solutions of ~-,

[3-

and ~,-D-cyclodextrin in the range 700-1500 cm -1 are reported. As well as showing features characteristic of D-glucose, the ROA spectra all show remarkably intense features between 890 and 960 cm -1 ori9inating in coupled C ( 1 ) - H deformations and glycosidic C-O stretches delocalized around the cyclodextrin rin 9 and which reflect the stereochemistry of the glycosidic links. Vibrational

optical

activity

measurements

stereochemical

information

every part of

the molecule. 1 V i b r a t i o n a l

on

because a vibrational

chiral

molecules

spectrum

can

contains

optical a c t i v i t y in

typical

provide much

bands chiral

associated molecules

new with

in

the

disordered phase was f i r s t observed using the Raman o p t i c a l a c t i v i t y (ROA) technique, which measures

a small

difference

in

the

polarized incident light. 2"3 U n t i l

Raman-scattered

recently,

lack of

intensity

sensitivity

in

right

and

has r e s t r i c t e d

left

circularly

ROA studies

to

favourable samples such as neat liquids; 4'$ b u t a major b r e a k t h r o u g h in ROA i n s t r u m e n t a t i o n based on the use of a backscattering g e o m e t r y 6'7 t o g e t h e r w i t h a cooled CCD d e t e c t o r 8 has now rendered a much wider range of samples accessible to such studies. This communication reports ROA spectra o f c y c l o d e x t r i n s in aqueous s o l u t i o n

which indicate that this technique

has great p o t e n t i a l f o r stereochemical studies o f these and o t h e r polysaccharides. Most carbohydrates (ECD)

measurements,

are

which

not

well

suited

are r e s t r i c t e d

to

to the

conventional

electronic

long-wavelength

bands; 9 nonetheless, some useful i n f o r m a t i o n a b o u t c o n f o r m a t i o n s

tails

circular of

the

dichroism first

ECD

o f c y c l o d e x t r i n s has been

obtained in this way. 10 Vibrational o p t i c a l a c t i v i t y spectra o f simple carbohydrates obtained using ROA's sibling technique o f vibrational circular dichroism (VCD) have been reported, tt b u t no VCD spectra of cyclodextrins have so far been published. The ROA spectra were recorded using the Glasgow multichannel for

backscattering 6'7 and CCD

Aldrich

and

Fluka

and

detection. 8 The c y c l o d e x t r i n

studied

as

near-saturated

samples

solutions,

in

i n s t r u m e n t 12 modified were

purchased

water

for

ct-

from and

T - D - c y c l o d e x t r i n , and in I N NaOH f o r [3-D-cyclodextrin in o r d e r to b o o s t the s o l u b i l i t y . The ROA measurements were made using a focused S00 m W a r g o n - i o n laser beam w i t h a spectral resolution (FWHH) of ~ 8 cm -I. A l l the ROA spectra were acquired f o r 2 hours. The ROA spectra of a-, [3- and ~ , - D - c y c l o d e x t r i n are shown in Figs. 1-3 respectively. The ROA spectra o f D-glucose, D - m a l t o s e and D - m a l t o t r i o s e , to be published elsewhere as part of 513

514

L . D . BARRONet al.

a preliminary survey of ROA spectra o f a range o f carbohydrates, 13 are o f central importance in discussing

the

cyctodextrin

results.

Thus

all

negative-positive c o u p l e t centred at ~ 1340 cm -!

three

cyclodextrin

ROA

spectra

show

a

t h a t also appears in D-glucose (at s l i g h t l y

l o w e r frequency) where i t has been associated w i t h d e f o r m a t i o n s o f the C H 2 O H group; and all three show an ROA "fingerprint" between ~ 980 and 1170 cm -! s i m i l a r to t h a t in glucose and associated w i t h ring C - O and C - C stretching t o g e t h e r with C - O - H

and C - C - H bending which

is characteristic o f the p a t t e r n o f OH ring substituents. The most s t r i k i n g cycloclextrin ROA features are the enormous p o s i t i v e - n e g a t i v e c o u p l e t s between ~ 890 and 960 c m - l : no similar features appear in D-glucose,

but

they appear w i t h

D - m a l t o t r i o s e where they have been associated w i t h

D-maltose

and

modes i n v o l v i n g c o n t r i b u t i o n s f r o m

much

less i n t e n s i t y

in

the

anomeric C ( I ) - H d e f o r m a t i o n s and the glycosidic C - O - C stretch. The general appearance o f the three c y c l o d e x t r i n ROA spectra is i t s e l f quite revealing. Thus the ROA features of

13-D-cyclodextrin

(as p r e d o m i n a n t l y

monoanion

sharp structure, especially in the c o u p l e t centred at ~ 1340 c m - l : seen

in

the

ROA

conformational T-D-cyclodextrin

spectrum

possibilities are

of

D-maltose, 13

in

{3-cyclodextrin

generally

broader

and

[3-D-cyclodextrin (anion), and are remarkably

and

may

and much

reflect

maltose. less

in base 14) show

the same characteristics are a

similar

The

restriction

of

features

of

those

of

ROA

structured

than

similar to those of D-maltotriose, 13 indicating

that y-cyclodextrin might have a similar increase in conformational possibilities to maltotriose on account of the larger ring.

I

a

O.$

V 8OO

t 1200

tO0O

Figure 1. The backscattered Raman

(I R +

IL)

and ROA

1400

(I R -

era"

IL)

spectra

of

c~-D-cyclodextrin in water. The enormous glycosidic c o u p l e t s seen in all three c y c l o d e x t r i n ROA spectra between ~ 890 and 960 cm -1 have dimensionless ROA intensities A -- ( l R - I L ) / ( I R + I L) of several parts in 102 , which is an o r d e r o f magnitude larger than the largest A-values usually encountered. The fact t h a t the A-values of the similar g l y c o s i d i c ROA feature in m a l t o s e and m a l t o t r i o s e have more typical magnitudes, w i t h those o f m a l t o t r i o s e r o u g h l y double those of maltose, is

Vibrational Rarnan optical activity

515

evidence that these large cyclodextrin ROA couplets originate in coupled vibrational modes of the glycosidic

links delocalized around the cyclodextrin

ring. In fact

Raman bands in this

region have been assigned previously to skeletal ring modes involving the glycosidic bonds: 15'16 but only the bands at 949 cm -1 in a-cyclodextrin and 945 cm -1 in 13-cyclodextrin (and by

analogy

that

at

945 cm -1

in

I~-I~a-Cayclodextrln.

y-cyclodextrin)

~

were

explicitly

associated

with a

~

~" 0 f OIS~IOS h.z, L,.~ "" s 1

I

800

1200 . . I

. . . . .

[

. . . . . .

14.00 I

. . . .

J

Cm 4

Figure 2. The backscattered Raman and R O A spectra of 13-D-cyciodextrin in IN NaOH.

y-I~.-Cyctode~trin

i[o5....

/ OOO

O t OQ

t2OO I

.

1

1400

Cm"

Figure 3. The backscattered Raman and R O A spectra of y-D-cyclodextrin in water. delocalizecl ring mode; 15 yet close inspection of our ROA spectra reveals

that the negative

parts of the glycosidic ROA couplets are not in fact associated with these particular bands

L . D . BARRONet al.

516

b u t are at s l i g h t l y lower f r e q u e n c y . We t h e r e f o r e c o n c l u d e t h a t ,

t h r e e cyclodextrins,

in all

there are a t l e a s t t w o o v e r l a p p i n g b a n d s on t h e l o w - f r e q u e n c y side o f t h e p r e v i o u s l y - a s s i g n e d delocalized ring m o d e t h a t are a l s o delocalized a r o u n d t h e c y c l o d e x t r i n ring. We have a l s o m e a s u r e d ROA s p e c t r a o f a q u e o u s s o l u t i o n s o f ~ - D - c y c l o d e x t r i n c o n t a i n i n g the g u e s t m o l e c u l e s s o d i u m b e n z o a t e and t h e t w o e n a n t i o m e r s o f p h e n y l a l a n i n e . While they are g e n e r a l l y very s i m i l a r to t h e s p e c t r a s h o w n here, t h e r e are s o m e s m a l l b u t

significant

local d i f f e r e n c e s which may r e f l e c t a t r a n s f o r m a t i o n f r o m a "tense' to a m o r e s y m m e t r i c a l "relaxed" c o n f o r m a t i o n c o r r e s p o n d i n g to t h e model o f S a e n g e r e t al. 17 I n s t r u m e n t a l i m p r o v e m e n t s are now in hand which s h o u l d provide an o r d e r of m a g n i t u d e increase in s p e e d features

to

be

of

the

measured

m e a s u r e m e n t s . This when

different

will

guests

enable are

quantitative

incorporated,

differences thereby

in

ROA

providing

a

c o m p l e t e l y new s o u r c e of i n f o r m a t i o n on t h e d e t a i l s of c o n f o r m a t i o n a l c h a n g e s associated with the a c c o m m o d a t i o n o f d i f f e r e n t g u e s t s in c y c l o d e x t r i n s in a q u e o u s s o l u t i o n .

Acknowledgments: We t h a n k t h e S.E.R.C and t h e W o l f s o n F o u n d a t i o n f o r r e s e a r c h g r a n t s , the S.E.R.C. f o r a r e s e a r c h s t u d e n t s h i p for A.R.G., and Prof. P. L. Polavarapu a n d Drs. A. F. Drake and (3. E. T r a n t e r f o r discussions.

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