Chapter 19
Phenoxyalkanoic Acid Sweetness Inhibitors Michael G . Lindley
Downloaded by IOWA STATE UNIV on April 19, 2017 | http://pubs.acs.org Publication Date: December 31, 1991 | doi: 10.1021/bk-1991-0450.ch019
Lintech, P.O. Box 68, Whiteknights, Reading, England
Substituted phenoxyalkanoic acids are potent inhibitors of the sweetness response. The degree of inhibition of sweetness is dependent on the concentration of substituted phenoxyalkanoic acid in a sweetener-inhibitor mixture. In contrast to some other sweetness inhibitors, pre-treatment of the tongue is not necessary and the inhibitory effect is immediate and rapidly reversible. The acidic group, or its salt, is necessary, and the nature, position and degree of substitution on the phenyl moiety influences potency. The results of sensory studies designed to determine the possible mechanism of action are described, and the implications for our understanding of the mechanism whereby sweeteners induce their effects are discussed. The existence of c o m p o u n d s capable of i n h i b i t i n g t h e perception of sweetness have been k n o w n for m a n y years, b u t they have not been the subject of intense study. Nonetheless, the mechanisms w h e r e b y k n o w n i n h i b i t o r s exert t h e i r effects h a v e , t o s o m e degree, been examined a n d a variety of mechanisms s h o w n to be operating. F o r example, N-ethylmaleimide, iodoacetic acid, a n d p - c h l o r o mercuribenzoate (J) p r o b a b l y i n h i b i t s w e e t n e s s p e r c e p t i o n b y c o v a l e n t i n t e r a c t i o n a t t h e t a s t e c e l l m e m b r a n e (2). Gymnemic acid a n d t h e more recently discovered ziziphins are believed to i n h i b i t sweetness t h r o u g h their surface active, detergent like p r o p e r t i e s (3). S i m i l a r l y , s o d i u m d o d e c y l s u l p h a t e i s c a p a b l e o f penetrating and disrupting the phospholipid membranes that are b e l i e v e d t o b e c o m p o n e n t s o f s w e e t n e s s r e c e p t o r s (4). M e t h y l - 4 , 6 dichloro-4,6-dideoxy-a-D-galactopyranoside i n h i b i t s sweet taste N O T E : The work described in this paper was carried out in the Tate and Lyle Research and Development Laboratories, Whiteknights, Reading, England.
0097-6156/91/0450-0251$06.00/0 © 1991 American Chemical Society
Walters et al.; Sweeteners ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
Downloaded by IOWA STATE UNIV on April 19, 2017 | http://pubs.acs.org Publication Date: December 31, 1991 | doi: 10.1021/bk-1991-0450.ch019
252
SWEETENERS:
DISCOVERY, M O L E C U L A R
DESIGN, A N D
CHEMORECEPTION
r e s p o n s e s i n g e r b i l s , a n d there i s s o m e evidence to s u g g e s t t h a t t h e m e c h a n i s m o f i n h i b i t i o n m a y b e c o m p e t i t i v e (5). Finally, the d i u r e t i c a m i l o r i d e is reportedly a n i n h i b i t o r of sweetness, a l t h o u g h there are conflicting results using conventional sensory methodology. Its m e c h a n i s m of a c t i o n h a s b e e n s u g g e s t e d a s t h a t o f a n i n h i b i t o r o f i o n t r a n s p o r t m e c h a n i s m s (6). R e c e n t l y , a n u m b e r of other c l a s s e s of c h e m i c a l s h a v e b e e n d i s c o v e r e d to have s w e e t n e s s i n h i b i t i n g p r o p e r t i e s (7-9). In c o m m o n with other sweetness inhibitors, these substituted phenylalkanoic acids, phenoxyalkanoic acids, and benzoylalkanoic a c i d s i n h i b i t t h e p e r c e p t i o n of s w e e t n e s s of d i f f e r i n g c h e m i c a l s t r u c t u r e s . However, they e x h i b i t n o evidence of d i s r u p t i n g taste c e l l m e m b r a n e s , a n d t h e i r effects are i m m e d i a t e l y reversible. T h e r e f o r e , t h e y are p o t e n t i a l l y v a l u a b l e t o o l s to i n c r e a s e o u r u n d e r s t a n d i n g of the c h e m i c a l s t r i c t u r e s of s w e e t n e s s p e r c e p t i o n . Structure-Activity
Relationships
The initial observation that these phenoxyalkanoic acids c a n i n h i b i t t h e p e r c e p t i o n of s w e e t n e s s w a s m a d e o n t a s t i n g a s o l u t i o n of s u c r o s e a t 5 % (w/v) c o n c e n t r a t i o n w h i c h c o n t a i n e d 0 . 0 5 % (w/v) 4-hydroxy-3-methoxy-phenylpropanoic acid. The mixture was p e r c e i v e d a s b e i n g e q u i - s w e e t t o a 3 % (w/v) s u c r o s e s o l u t i o n , t h u s , the degree of i n h i b i t i o n w a s a p p r o x i m a t e l y 4 0 % . This initial o b s e r v a t i o n l e d to w h a t w a s m e r e l y a s t a n d a r d s t r u c t u r e - a c t i v i t y relationship program. Derivatives were prepared synthetically a n d their inhibitory action assessed u s i n g either the methodology d e s c r i b e d o r o n e i n w h i c h s u c r o s e w a s a t 1 0 % (w/v) a n d t h e i n h i b i t o r u n d e r t e s t a t 0 . 0 0 5 % (w/v). S y n t h e t i c procedures followed general p u b l i s h e d guidelines (10-12). In outline, p h e n y l p r o p a n o i c acid derivatives were p r e p a r e d b y r e d u c t i o n of the c o r r e s p o n d i n g c i n n a m i c a c i d derivative, itself o b t a i n e d b y a P e r k i n c o n d e n s a t i o n of the a p p r o p r i a t e b e n z a l d e h y d e w i t h acetic a n h y d r i d e i n the p r e s e n c e of s o d i u m acetate. P h e n o x y a l k a n o i c acid derivatives were p r e p a r e d b y r e d u c t i o n of the a p p r o p r i a t e p h e n o l w i t h a h a l o a l k a n o i c a c i d or a n ester of a h a l o a l k a n o i c a c i d . It w a s u n f o r t u n a t e t h a t t h e s c o p e o f t h i s p r o g r a m m e w a s l i m i t e d b y its objective. It w a s n o t d e s i g n e d e i t h e r t o u n r a v e l t h e c o m p r e h e n s i v e r e l a t i o n s h i p s p r e s u m e d to e x i s t b e t w e e n s t r u c t u r e a n d effect, o r t o d e v e l o p s t r u c t u r a l c o r r e l a t i o n s b e t w e e n s w e e t n e s s inhibitors a n d k n o w n sweeteners. T h e o b j e c t i v e s w e r e to i d e n t i f y t h e o p t i m u m c o m p o u n d to d e v e l o p c o m m e r c i a l l y . Nonetheless, there were m a n y interesting observations arising from the limited s t r u c t u r e - a c t i v i t y p r o g r a m , s o m e of w h i c h are s e e m i n g l y c o n f l i c t i n g a n d have not yet been rationalised. F o r example, w i t h i n the phenylpropanoic acid series, the p o s i t i o n of a m e t h o x y - s u b s t i t u e n t h a s n o i n f l u e n c e o n the effectiveness of the r e s u l t i n g c o m p o u n d as a n i n h i b i t o r of
Walters et al.; Sweeteners ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
19. LINDLEY
Phenoxyalkanoic Acid Sweetness Inhibitors
253
s w e e t n e s s ( T a b l e I). H o w e v e r , w i t h i n t h e 2 - p h e n o x y p r o p a n o i c a c i d series, positioning of a methoxy-substituent h a s a major i m p a c t o n i n h i b i t o r y p o w e r ( T a b l e II) a n d i t i s n o t i m m e d i a t e l y o b v i o u s w h y t h i s difference exists. T a b l e I.
Effect of S u b s t i t u e n t P o s i t i o n o n I n h i b i t i o n b y Phenylpropanoic acids.
Downloaded by IOWA STATE UNIV on April 19, 2017 | http://pubs.acs.org Publication Date: December 31, 1991 | doi: 10.1021/bk-1991-0450.ch019
Position/Substituent
Inhibition (%) (0.05% Inhibitor i n 5 % Sucrose)
2-Methoxy
40
3-Methoxy
40
4-Methoxy
40
3-Ethoxy
60
4-Ethoxy
60
T a b l e II. E f f e c t o f S u b s t i t u e n t P o s i t i o n o n I n h i b i t i o n b y Phenoxypropanoic Acids Position/Substituent 2- Methoxy
I n h i b i t i o n (%) (0.05% Inhibitor i n 1 0 % Sucrose) 15
3-Methoxy
30
4- M e t h o x y
50
S i m i l a r l y , α-naphthyl p h o s p h a t e does n o t i n h i b i t sweetness, w h e r e a s t h e c o r r e s p o n d i n g s u l p h a t e (α-naphthyl s u l p h a t e ) d o e s . In contrast, α-hydroxy b e n z y l p h o s p h i n i c acid does inhibit s w e e t n e s s , b u t α-hydroxy-a-toluene s u l p h o n i c a c i d d o e s n o t (Figure 1). O t h e r structure-activity relationships developed follow a m o r e consistent pattern. Increasing the lipophilicity of s u b s t i t u e n t s o n the p h e n y l o r p h e n o x y moiety increases the effectiveness u p to a m a x i m u m , when, w i t h further increases i n lipophilicity, there is a d r o p i n i n h i b i t o r y efficiency. A s long ago as 1 9 6 6 , s i m i l a r observations h a d been m a d e o n the relative sweetness of derivatives of 2 - a m i n o - 4 - n i t r o b e n z e n e s . D e u t s c h a n d H a n s c h h a d observed that relative sweetness w a s highly dependent o n the h y d r o p h o b i c b i n d i n g c o n s t a n t (13). Here, the i n h i b i t o r y effectiveness of c o m p o u n d s w a s f o u n d to correlate w i t h t h e i r c a l c u l a t e d l o g Ρ v a l u e s i n a s i m i l a r w a y to t h a t observed b y D e u t s c h a n d H a n s c h for sweeteners. A s the lipophilicity of a s u b s t i t u e n t increases, so does the c o m p o u n d ' s m e a s u r e d "biological response", u p to a m a x i m u m , f o l l o w i n g w h i c h t h e r e i s a s t e a d y f a l l i n e f f e c t i v e n e s s ( T a b l e s III a n d IV). I n t e r e s t i n g l y , e v e n t h i s r e l a t i o n s h i p o n l y h o l d s w i t h i n t h e a l k y l
Walters et al.; Sweeteners ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
254
SWEETENERS:
DISCOVERY, M O L E C U L A R
DESIGN, A N D
CHEMORECEPTION
s u b s t i t u e n t or a l k o x y s u b s t i t u e n t series, not across those s t r u c t u r a l boundaries. T a b l e III. C o r r e l a t i o n o f t h e N a t u r e o f t h e S u b s t i t u e n t w i t h t h e B i o l o g i c a l R e s p o n s e of P h e n o x y p r o p a n o i c A c i d s . R
Log Peak
H
1.60
11
3
2.16
41
C H
2.62
63
3.13
45
3.58
33
CH 2
Downloaded by IOWA STATE UNIV on April 19, 2017 | http://pubs.acs.org Publication Date: December 31, 1991 | doi: 10.1021/bk-1991-0450.ch019
Inhibition (%) (0.005% Inhibitor i n 1 0 %
5
iC3H
7
tC4H
9
Sucrose)
T a b l e IV. C o r r e l a t i o n of the N a t u r e of t h e S u b s t i t u e n t w i t h t h e B i o l o g i c a l R e s p o n s e of P h e n o x y p r o p a n o i c A c i d s . R
Log Peak
Inhibition (%) (0.005% Inhibitor i n 1 0 %
OH
0.93
0
OCH3
1.58
48
OC2H5
1.98
34
OC3H7
2.65
13
OC4H9
3.14
