Chapter 6
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Structure-Activity Relationship and AH-B after 40 Years 1
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Terry E. Acree and Michael Lindley 1
Department of Food Science and Technology, Cornell University, 630 West North Street, Geneva, NY 14456 Lindley Consulting, 17, Highway, Crowthorne, Berkshire, United Kingdom 2
For over a century the relationship between chemical structure and sweet taste has interested scientists, not only to explain food perception but also to direct the search for low calorie sweeteners. Notwithstanding, most low calorie sweeteners of commercial value were discovered by serendipity, but the A H Β theory published in 1967 did stimulate research that led directly to the discovery of the sweetness antagonist lactisole and the development of the 'multi-point attachment theory' of Tinti and Nofre used to design 'neotame'. This paper will outline the history of A H - B , its role in the discovery of lactisole and its relevance to the present view of taste perception.
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© 2008 American Chemical Society
Weerasinghe and DuBois; Sweetness and Sweeteners ACS Symposium Series; American Chemical Society: Washington, DC, 2008.
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Introduction In the early I960's sweetness was known to be a property of a number of ligands that some animals evolved to detect and whose detection was described by humans as 'sweet'. Bitterness, however, was known to be stimulated by scores of different ligands distributed throughout the natural environment indicating many different receptor proteins, but not as many as were ultimately discovered for olfaction and certainly not as few as were responsible for ionic tastes: salty and sour. Over the next 20 years several non-carbohydrate sweet ligands were discovered during the search for new low calorie sweeteners of improved technical characteristics and/or to replace those banned by governments. In the absence of tools to study the initial chemistry of sweetness, it was believed that comparing patterns of taste behaviour with patterns of chemical structure could produce some insight (/). Known as Structure Activity Relationships (SAR), these techniques are often used in drug discovery, particularly when computer simulations replace mechanical models. Earlier attempts to use infra-red spectrometry to identify intra-molecular hydrogen bonds in crystalline sugars was abandoned when it was recognized that in solution compounds like glucose and fructose are in equilibrium with several tautomers and that each tautomer is likely in equilibrium with several conformers. The tautomeric composition could be determined directly by forming sugar derivatives in solutions quickly frozen with liquid nitrogen (2) but the conformational composition remained a mystery. A n attempt to use the kinetics and thermodynamics of sugar tautomerization in boric acid solution, measuring changes in both pH and optical rotation, produced theories, but no way of validating them. Reviews of this period include those of Walters (3) and Shallenberger (4).
Early SAR of Sugar Taste Robert Sands Shallenberger first described his thoughts about sweetness in California in April 1963. He had read everything remotely related to the subject and was convinced that sweetness was a direct reflection and an indication of selective bonding between sweeteners and specific proteins that functioned as chemoreceptors. He imagined that this was a reversible bonding reaction and it initiated transduction when the concentration of the bound form reached a threshold; highly potent sweeteners just had higher bonding coefficients. To him, it was simply a matter of relating the structure of sweet tasting chemicals to their activity, as defined by the conscious perception of sweetness. Understanding sweetness might be revealed by S A R analysis of sweet tasting chemicals. When Terry Acree joined his laboratory at the New York State Agricultural Experiment Station as a Cornell University graduate student in July of 1963 the only tools available for structural studies were spectrometers, chromatographs, crude molecular models and rulers. That Fall, Shallenberger
Weerasinghe and DuBois; Sweetness and Sweeteners ACS Symposium Series; American Chemical Society: Washington, DC, 2008.
Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on April 28, 2016 | http://pubs.acs.org Publication Date: March 4, 2008 | doi: 10.1021/bk-2008-0979.ch006
98 began to machine molecular models from steel in the shop in his basement. The models had bonds that rotated and bent and with the addition of foam plastic balls to approximate electron clouds, conformational energy barriers were simulated by the force needed to bend the steel bonds as the models were deformed through all the possible conformations of a single structure without, of course, breaking any covalent bonds. The 1960's was marked by the work of Jacob, Monod and Changeux with their ideas that protein - ligand binding results in conformational changes in protein structure: the allosteric effect. In the case of enzymes this explained non-competitive "activation" and "inhibition" among other inductive phenomena and it provided a model for extra-cellular activation of intercellular receptor protein reactions, i.e., transduction. These binding reactions were presumed not to require the formation of covalent bonds but could be accomplished through the formation and disruption of hydrogen bonds, hydrophobic associations and other London dispersion forces (5). Called the M W C model it proposed that a ligand, L , would bind to a receptor protein RP on the outside of a receptor cell and that remote to the binding site (as it turns out on the inside of the receptor cell) the protein changes structure enough to initiate transduction. This was an allosteric effect. The challenge was then to envision the minimum set of structural features necessary to elicit sweetness, i.e., bind to a receptor. Many sweet compounds were compared to similar non-sweet compounds and two sets were used to formulate the A H - B theory (6)\ the chlorinated alkanes and the cyclic diols. The sweetest chlorinated alkane was chloroform (Figure lb). This molecule had an acidic or electropositive proton capable of forming a hydrogen bond with a negative site on the receptor protein and any of the chlorines could also form a simultaneous hydrogen bond with a suitable proton on the protein i f it was approximately 3Â from the electronegative center. Finally, when a cyclic diol had the "gauche" configuration (Figure la), it was sweet and when it was anti-clinal or eclipsed the compound was not sweet. Most importantly, the distance between an O H proton on any one of the diols and a full p-orbital on an adjacent oxygen could be separated by 3 A (6). However, which of the many diols located on all carbohydrates formed the A H B in sweet sugars and just why most sugars were not sweet; even though they bristled with diols (Figure lc) remained unclear. Although A H - B functional groups could be located on many sweet compounds (Figure 2), it was obvious that A H - B might be a necessary condition for sweetness, but it certainly was not sufficient.
Variations on the Shallenberger/Acree AH-B Model To explain why there were hundreds of compounds with A H - B structures that did not taste sweet, a comparison of the sweet with the non-sweet amino acids yielded part of the answer (7). Glycine and D- and L-alanine are all sweet. Weerasinghe and DuBois; Sweetness and Sweeteners ACS Symposium Series; American Chemical Society: Washington, DC, 2008.
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