Shape of Sweet Receptors Studied by Computer Modeling - American

Chapter 13

Shape of Sweet Receptors Studied by Computer Modeling H . Rohse and H.-D. Belitz

Downloaded by IOWA STATE UNIV on April 19, 2017 | http://pubs.acs.org Publication Date: December 31, 1991 | doi: 10.1021/bk-1991-0450.ch013

Institut für Lebensmittelchemie der T U München and Deutsche Forschungsanstalt für Lebensmittelchemie, Lichtenbergstr. 4, D-8046 Garching, Federal Republic of Germany

General models for sweeteners have been developed using a molecule building program by superimposition of the e/n-systems of a large number of sweet and nonsweet compounds from diverse structural classes. The models are based on psychophysical data (sweet thresholds) and clearly show the possible dimensions of the molecules compatible with sweet taste. The sweet potency depends strongly on the location of hydrophobic groups within the given sweet space. The models include amino acids, oxathiazione dioxides, benzisothiazolone dioxides, carboxyalkylbenzamides, naphthoimidazoles and related compounds. Sweet taste i s exhibited by a large number of compounds from diverse s t r u c t u r a l classes. Common structural elements of sweet compounds have been proposed and include the AH/B-system of Shallenberger and Acree (1) and the hydrophobic group X of Kier (2). The b i p o l a r system can be more generally described as an e l e c t r o p h i l i c / nucleop h i l i c (e/n-)system i n conjunction with a more or less extensive contact of the hydrophobic moiety with the corresponding portion of the sweet receptor (3,4). The quality and intensity of a given molecule's taste depends strongly on the e/n-system, the size and shape of the hydrophobic moiety and i t s p o s i t i o n r e l a t i v e l y to the e/n-system. By superimposition of the e/n-systems of sweet and c l o s e l y related nonsweet compounds, i t i s possible to elucidate the dimensions of a molecule compatible with sweet taste, and, thus to develop a basic model for sweet compounds, and to make inferences about the shape of a schematic sweet receptor (5). Since such a model for sweet compounds i s based on psychophysical data (taste thresholds), i t i s integral i n nature and may r e f l e c t a superimposit i o n of several d i f f e r e n t real sweet receptor binding s i t e s . A r e a l receptor binding s i t e , on the other hand, may only be a p a r t i a l r e a l i z a t i o n of the schematic integral receptor, derived by superimp o s i t i o n of d i f f e r e n t sweet compounds.

0097-6156/91/0450-0176$06.00/0 © 1991 American Chemical Society

Walters et al.; Sweeteners ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

13. ROHSE & BELITZ

177

Shape of Sweet Receptors

The c a p a b i l i t y of this superimposition method s h a l l be demon strated by several examples.


Methods The programs CORINA and MARYLIN were used for generation and superimposition of molecules (6). The 3D-coordinates of molecules were calculated by CORINA from the constitution formulae and stereo chemical descriptors. Molecular models were generated and represen ted on a colour graphics screen MARYLIN. The program allowed various manipulations of these models, such as d i f f e r e n t colouring of atoms, t r a n s l a t i o n and rotation of individual models, conformational changes at s p e c i f i c bonds, superimposition of several models and p l o t t i n g the results i n form of stereoscopic views. Conformations were calculated with the programs MM2 (7), MOPAC (8), OPTIPOW (9) or SYBYL (10), depending on the size of molecules and the a v a i l a b i l i t y of parameters. Conformers which represent l o c a l or absolute energy minima were used for superimposition. Taste q u a l i t i e s and sweet taste recognition thresholds were according to a previously established protocol (11). e/n-Systems A series of d i f f e r e n t e/n-systems are known, e.g. OH/OH i n sugars, NH^/COO" i n amino acids and peptides, NH/SO2 i n oxathiazinone dioxides and isothiazolone dioxides, OH/CO i n β-hydroxyketones, C0NH /C00" i n carboxyalkyl benzamides, C0NH /0R i n alkoxyphenyl ureas and NH2/CO i n ureas. In Table I the distances between the e l e c t r o p h i l i c and the nucleophilic groups are summarized for various sweet compounds. The data suggest that the compounds can be c l a s s i f i e d into two groups; the distance being i n the range of 250 pm f o r group 1 and i n the range of 800 pm for group 2. In the following, two d i f f e r e n t basic models were developed for these two groups. 2

2

TABLE I. e/n-Systems of Different Sweet Compounds

No.

1 2 3 4 5 6 7 8 9

Compound

e/n-System

N-Alkyl(aryl) ureas Oxathiazinone dioxides Sulfamic acids Benzisothiazolone dioxides 3-Aminocarboxylic acids 2-Aminocarboxylic acids 2- Aminobenzoic acid 3- Aminobenzoic acid 3 - Carbamoyl -2,4,6- tribromophenylpropionic

Distance' [pm]

H N-C0 NH/SO2 NH-SO3 NH/S0 NH3 /COO UH^ /C00 ΝΗ /(; NH^/COO 2

0

+

+

3

00

240 241 249 256 269 300 304 513 761 770 773 839

10 11 12 Torsion angle of amino and carboxylic groups ± 60°.


178

SWEETENERS:

DISCOVERY, M O L E C U L A R


Basic Model 1 (e/n-distance ~ 250

DESIGN, A N D

CHEMORECEPTION

pm)

In Tables II and III the taste q u a l i t i e s and recognition thresholds of several sweet and nonsweet oxathiazinone dioxides and benzisothiazolone dioxides are summarized (11). The e/n-systems of these two groups of compounds were superimposed (Figure 1). The positions which are compatible and incompatible with sweet taste matched very well, and gave some information on the dimensions of the corresponding schematic sweet receptor. These dimensions are very c l e a r l y shown i n Figure 2, which i l l u s t r a t e s the incompatible, forbidden positions. Data about the taste properties of several sweet and nonsweet amino acids are given i n Table IV. The superimposition of several compounds i s separately shown i n Figure 3, while i n Figure 4 i t i s included i n the schematic receptor of Figure 2, derived from the oxathiazinone and benzisothiazolone dioxides. The amino acids give additional information about positions, which are forbidden for sweet taste. More than one apolar substituent i n the β-position of sweet L-a-aminocarboxylic acids abolished the sweet taste (Lv a l i n e ) , while, on the other hand, polar groups were compatible with sweet taste i n this p o s i t i o n (L-threonine ; the hydroxy group of L-4hydroxyproline was located i n the same area). In consideration of these facts, the sweet 1-aminocycloalkane-l-carboxylic acids f i t into the model only with the amino group i n equatorial p o s i t i o n . From the l i t e r a t u r e (14) i t i s known that the zwitterion of the indicated cyclohexane c r y s t a l l i z e s i n this conformation. Calculations delivered very similar energies for both conformers.

TABLE I I . Taste Q u a l i t i e s and Thresholds of Selected Oxathiazinone Dioxides (11) (cf. formula i n Figure 17)

No

Compound R

1 2 3 4 5 6 7 8 9 10 11 12 13 a

R

H H H H H CH

H 3 2 5 4 9 6 5 CH

3

C

C

C

3 2 2 3 3

H

H

H

H

C

H

C

H

C

H

3

C

H

C

H

C

H

6 5 2 5 3 7 3 4 9 CHo

5 5 7 7

C H

C

Ç 5 H

3

Na Κ H Na H Na Na Κ Κ H Na Na H

C H

C H

C

Quality

1

H

6

b

sw sw/bi sw/bi sw bi sw/bi bi sw/bi sw/bi sw/bi bi bi sw/bi

f

a c

tsw [mmole/1]

c

sac/g d

10 130 ' 150 (Na) 30 d

0.08-0.12 0.06-0.09

e

d

d

f

-(10) 0.09-0.11 -(10) 0.1 -0.2 0.07-0.11 0.3 -0.5 -(10)*; -(10) 0.08-0.15

j 130

d

f

d

70 (Na) 30 (Na) d

£

c

sw: sweet, b i : b i t t e r Sweet recognition threshold f c(saccharose)/c(sweetener); the factor corresponds to a sweetener solution, which i s isosweet to 4 % saccharose s o l u t i o n According to r e f . 19 Relative sweetness potencies up to -180 are reported (20) No sweet taste up to 10 mmol/1 8 R: -CH=CH-CH=CHg a c

e

f


13. ROHSE & BELITZ

179


TABLE I I I . Taste Qualities of Selected Benzisothiazolone (cf. formula i n Figure 17).


No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 a

Substitution Pattern 2 4 5 X

6

Quality Sweet Not Sweet

7

a

+ +

N0 NHCOCHo CI 2

+ CI Br NH

+ + + +

2

+ + +

F Cl Br I N0 OH b

L—R L_ b_J R

2

+ + + + + +

Dioxides

Ref. 21 21-23 21,22 24 23,24 23 21,23 16,22,24 16,22-24 16,22-24 16,22,24 21,23 23 24,25 24,25 24,26

A l l substituants without functional groups abolish the sweet taste R: -CH=CH-CH=CHb

Figure 1. Superposition of Selected Sweet and Nonsweet Oxathiazinone Dioxides (Table II, No. 5,12) and Benzisothiazolone Dioxides (Table I I , No.14-16). (o/o Positions allowed/forbidden for sweet taste, e,n: e/n-system, stereoscopic drawing)



180

SWEETENERS:


DESIGN, A N D

CHEMORECEPTION

Figure 2. Superposition of Selected Sweet and Nonsweet Oxathiazinone Dioxides and Benzisothiazolone Dioxides. (Same compounds as i n Figure 1; only the forbidden positions are shown)

The superimposition of the e/n-system of 2-aminobenzoic acids (Table IV, No.33-36) allows i d e n t i f i c a t i o n of some further positions incompatible with sweet taste (Figure 6). From Figure 5 i t can be seen that the sweet 1-aminocyclooctane1-carboxylic a c i d i s compatible with the schematic receptor, but the adamantane derivative i s not, because i t occupies a forbidden p o s i t i o n for an apolar substituent, as previously discussed. Several benzoates have been found to be sweet, and the taste q u a l i t y and thresholds are described i n Table V. In the case of these compounds, the cations and the carboxylate anions are assumed to act as e/n-systems. Other examples of sweet t a s t i n g substances, i n which the e/n-system i s provided by two separate components, are known from the l i t e r a t u r e (13) . The benzoates matched very well with the schematic sweet receptor, with the exception of the nonsweet 3,5-dichloro derivative. Figure 7 shows that one of i t s chlorine atoms projects into the area forbidden for sweet taste. A probable conformation of 6S,1'S-Hernandulcin, a sweet sesquiterpene from L i p p i a dulcis Trev. (14), i s shown i n Figure 8. The sweet isomer f i t t e d very well into the schematic sweet receptor, with the isobutenyl group p a r a l l e l to the receptor's backside (Figure 9). The terpene's area of hydrophobic contact with the schematic receptor corresponds to that of saccharin. Since Hernandulcin i s sweet, D-tert-butylglycine, formerly reported as tasteless (4), should be sweet too, or Hernandulcin would occupy a forbidden p o s i t i o n within the schematic receptor, as i s demonstrated i n Figure 9. Reinvestigation of butylglycine at higher concentrations indeed revealed the sweet taste of t h i s compound with a threshold of 60-80 mmol/1. In Figure 10 i t i s shown that additional D-amino acids can be included i n the schematic sweet receptor without overlapping any of the previously i d e n t i f i e d forbidden positions. The same i s true f o r


13.

ROHSE & BELITZ


181


TABLE IV. Taste Quality and Threshold of Selected Amino Acids

Quality

Compound

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

sw L-Alanine sw/bi L-2-Aminobutyric acid bi L-2-Aminovaleric acid bi L-Valine bi L-Isoleucine sw D-1-Butylglyc ine sw/bi L-Proline ne D-Proline sw L-4-Hydroxyprο1ine L-allo-4-Hydroxyproline ne sw L-Serine sw L-Threonine sw D-Phenylalanine L-Phenylalanine bi sw/bi 1-Aminoeyelohexane-1-carboxylic acid 1-Aminocycloheptane-l-carboxylic acid sw/bi sw/bi 1-Aminocyclooctane-l-carboxylic acid bi 1-Aminocyclononane-1-carboxylic acid sw D,L-3,5-Dibromophenylalanine sw D-Tryptophan sw 1-Me thy1-D-1ryp tophan 6-Methyl-D-tryptophan 5-Me thoxy-D-1ryp tophan 4-Me thy1-D-1ryp tophan D, L- 2-Amino- 3 -(1-naphthyl)propionic sw acid D, L- 2 - Amino - 3 - ( 2 - naphthyl ) prop ionic sw acid D,L-2-Amino-3-ethylpentane carboxylic sw acid 1-Amino-4-me thylcyc1ohexane-1sw/bi carboxylic acid 1-Amino-4-e thyIcyclohexane-1bi carboxylic acid 1-Amino-4-t-butylcyclohexane-Ι ne ο arboxy l i e acid D,L-9-Aminobicyclo(3 . 3 . l)nonane-9bi carboxylic acid D,L-2-Aminoadamantane- 2 -carboxylic ne acid ne 2-Amino-3-methylbenzoic acid sw 2-Amino-4-methylbenzoic acid sw 2-Amino-5-methylbenzoic acid ne 2-Amino-6-methylbenzoic acid

26 27 28 29 30 31 32 33 34 35 36

sw: sweet, b i : b i t t e r , ne: neutral concentration

Sweet

Ref.

3

No.

'tsw. 12 12

-18 -16

60 25

-80 -40

27 27 27 27 27

27 27 27 5 - 7 27 -35 27 25 -45 27 35 - 3 27 1 27 1 - 3 28 2 - 4 28 2 - 4 28 28 16 0.,2 - 0 .4 28 0.,2 - 0 .4 18 0.,02- 0 .03 18 0.,2 - 0 .4 18 0..04- 0 .06 18 0.,1 - 0 .2 0.,6 - 1 .0 10

-20

8

-10

28 28 28 28

5 1

-(50) 7 3 -(50)

28 29 29 29 29

recognition threshold


SWEETENERS:


DESIGN, A N D C H E M O R E C E P T I O N


182

Figure 4. Superposition of the Sweet Receptor Model 1 (Figure 2) with Selected Sweet and Nonsweet Amino Acids



13.

R O H S E & BELITZ


Figure 5. Superposition of the Sweet Receptor Model 1 (Figure 4 including an additional forbidden position, derived from Dproline (Table IV, No.8)) with 1-Amino-cyclooctane-1-carboxylic acid (Table IV, No.17)

η

Figure 6. Superposition of the Sweet Receptor Model 1 (Figure 5) with 2-Aminobenzoic Acids (Table IV, No.33-36)


184

SWEETENERS:

DISCOVERY, M O L E C U L A R DESIGN, A N D C H E M O R E C E P T I O N

TABLE V. Taste Qualities and Thresholds of Selected Benzoates


No. Anion 1 2 3 4 5 6 7 8 9 10 11 12 13

Cation

Benzoate Benzoate Benzoate Benzoate Benzoate Benzoate Benzoate 2-Methylbenzoate 3-Methylbenzoate 4-Methylbenzoate 3 -Chlorobenzoate 4-Chlorobenzoate 3,5-Dichlorobenzoate

+

a

L i (; 68) Na i: 97) K (:133) NH4 ([143) Mg2+

Shape of Sweet Receptors Studied by Computer Modeling - American

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