Bitter dipeptides

You will have noticed in Table II, that the Q-values are much higher in the case of bitter dipeptides compared with the non-bitter dipeptides. 

Table III shows a series of non-bitter dipeptides. It should be noted here that the Q-values are all below 1300. We can compare this with values of the following Table IV,...

Thurst S, Koksche B (2003) Protease-catalysed peptide synthesis for the site specific incorporation of alpha-fluoroalkyl amino acids into peptides. J Org Chem 68 2290-2296 Trusek-Holownia A (2003) Synthesis of Z-Ala-Phe.OMe, the precursor of bitter dipeptide in the two-phase ethyl acetate-water system catalysed by thermolysin. J Biotechnol 102 153-163 Tuchscherer G, Mutter M (1996) Template assisted protein de novo design. Pure Appl Chem 68(11) 2153-2162... 

Cyclo(Leu-Trp), a bitter compound isolated from the fermentation of milk casein by Bacillus subtilis, opened up the field to flavor and fragrance properties. It was further noted that dipeptides became more bitter when blockage of both the amino and carboxyl groups occurred or the dipeptide was converted into a DKP. This phenomenon opened the field of taste exhibition. ... 

During work on a series of aspartyl dipeptides containing ACC 71 (vide supra, Eq. (28), Sect. 4) at the carboxyl terminus, it was reported that dispartame Asp-ACC-OMe had a distinct sweet taste [302] and that the corresponding n-propyl ester had 250-300 times the sweetness of sucrose [303]. However, replacement of phenylalanine by 2,3-methanophenylalanine gave tasteless analogues of aspartame [293, 304], and some dimethyl-ACC 214 (methanovaline) and tri-methyl-ACC 215 aspartame analogues [Asp-(Me)n-ACC-OMe] have a bitter taste. These taste properties, which depend on the number and position of the methyl substituents, have been explained on the basis of topochemical models thus, a L-shaped conformation of the dipeptide is necessary for sweet taste, Eq. (86) [3051. 

Many bixxer compounds contain both hydrophobic and hydrophilic sites which can alter cell membranes through penetration. There is a correlation between bitter intensity and hydrophobicity-solubility indexes such as fee octanol/water partition coefficient, lo (7). Penetration may directly affect cAMP phosphodiesterase as part of fee transduction process (see below). A bitter receptor protein may be involved wife certain bitters, such as specific structural requirements wife fee bitter tasting dipeptides and denatonium salts (27). The latter is used in some consumer products to avoid accidental ingestion. A receptor mechanism is also supported by fee existence of a genetic "taste blindness" for some bitter materials (see below). 

Bitter taste can be masked by sweeteners, by salt or by dipeptides containing aspartic or glutamic acids (22,25,24,25). The bitter-masking potential of sugars wife quinine was recently assessed, and quinine-equivalent values were derived to predict masking ability of these substances. Attenq)ts to mask bitter taste may be successful only wife certain bitter substances. 

The sweet dipeptide esters of the L-aspartic acid and the L-amino malonic acid (15-21) are interesting exceptions to the bitter taste shared by all other members of the peptide series. Fig. 

The studies on peptides began with a correlation between sweet amino acids and peptides. Since the projection formula of L-Asp-Gly-OMe (4) is similar in size and shape to that of e-Ac-D-Lys (3) which is sweet, we predicted that L-Asp-Gly-OMe would taste sweet in spite of the bitter taste in the literature. Therefore, we synthesized the peptide and tasted it. As expected, it was sweet and its sweetness potency was almost equal to that of e-Ac-D-Lys. Thus, the dipeptide could be correlated to the amino acid. Lengthening (5) or enlargement (6) of the alkyl group of the ester did not affect its sweetness potency (Table 1). 

The f values of the single amino acids given in Table I were determined by Tanford (2) from solubility data and they represent a measure of the hydrophobicity of an amino acid residue. Please note, that the values are relative to the methyl groups of glycine which is taken to be O. In Table II the taste of some "isomericn-dipeptides is described. All the dipeptides are composed of the natural 1-amino acids, as are all the examples, that will follow later. It is interesting to note, that the position of the amino acid has no influence on bitterness ( ). 

The corresponding dipeptide glutamyl-phenylalanine has no phenolic group, and is bitter over the whole pH-range. Q of this compound is 1660 (42). 

Roasting cocoa beans results in the production of volatile and non-volatile compounds which contribute to the total flavor complex. 5-Methyl-2-phenyl-2-hexenal, which exhibited a deep bitter persistant cocoa note, was reported in the volatile fraction (53). It was postulated to be the result of aldol condensation of phenylacetaldehyde and isovaleraldehyde with the subsequent loss of water. The two aldehydes were the principal products of Strecker degradation products of phenylalanine and leucine, respectively. Non-volatiles contained diketopiperazines (dipeptide anhydride) which interact with theobromine and develop the typical bitterness of cocoa (54). Theobromine has a relatively stable metallic bitterness, but cocoa bitterness is rapidly noticed and disappears quickly. 

A cyclic dipeptide, Pro-Leu anhydride, having bitterness was isolated from a traditional Japanese alcoholic drink "sake (48). This peptide Increased the longer sake was stored in sake production. So this peptide seems to contribute to the bitter taste of sake. 

The artificial sweetener aspartame is the methyl ester of the dipeptide Asp-Phe. This synthetic peptide is 180 times sweeter (on a gram-for-gram basis) than sucrose (common table sugar). Both of the amino acids in aspartame have the naturally occurring L-configuration. If the D-amino acid is substituted for either Asp or Phe, the resulting compound tastes bitter. 

A number of synthetic peptides are significant commercial products, ranging from the sweet dipeptide aspartame (L-aspartyl-L-phenylalanine methyl ester) to clinically used hormones such as insulin and oxytocin. L-Aspartyl-L-phenylalanine methyl ester (3 Scheme 2) is the methyl ester of the C-terminal dipeptide of gastrin. It was found accidently during the synthesis of gastrin that this synthetic sweetener is about 200 times as sweet as sucrose.f This pleasant sweetness without a bitter aftertaste was the reason that L-aspartyl-L-phenylalanine methyl ester was approved in many countries as a food additive, receiving much attention as a low-calorie sweetener. L-Aspartyl-L-phenylalanine methyl ester can be prepared by various chemical routes and the first enzymatic procedure of commercial interest was described by Isowa et al.h l In the industrial process,L-Asp and DL-Phe were chosen as inexpensive raw materials. L-Asp is available very inexpensively, whereas L-Phe is more expensive than DL-Phe. Z-D-Asp acts as a competitive inhibitor, while D-Phe-OMe... 

The contribution of the d enantiomers to the characteristic taste of foods is currently being evaluated, but it is clear that the d enantiomers generally taste sweeter , or at least less bitter , than do their l isomers. Of course, kitchen preparation can involve many subtle chemical changes that enhance the attractiveness of natural foodstuffs, including racemisation (Man and Bada, 1987) therefore d enantiomers may be introduced in this way. Peptides are taste contributors, for example the bitter-tasting dipeptides Trp—Phe and Trp—Pro and the tripeptide Leu—Pro—Trp that are formed in beer yeast residues (Matsusita and Ozaki, 1993). 

Ramos de Armas, R., Gonzalez Diaz, H., Molina, R., Perez Gonzalez, M. and Uriarte, E. (2004) Stochastic-based descriptors studying peptides biological properties modeling the bitter tasting threshold of dipeptides. Bioorg. Med. Chem., 12, 4815 822. 

The synthetic sweetener aspartame, or NutraSweet (Section 22.21), is the methyl ester of a dipeptide of L-aspartate and L-phenylalanine. Aspartame is about 200 times sweeter than sucrose. The ethyl ester of the same dipeptide is not sweet. If a D-amino acid is substituted for either of the L-amino acids of aspartame, the resulting dipeptide is bitter rather than sweet. 

The dipeptide sweetener a-L-aspartyl-L-phenylalanine methyl ester (a-APM), also know as aspartame or NutraSweet brand sweetener, is currently used in over 5,000 food products. It has a sweetness potency of approximately 200 times sucrose and is unstable under alkaline conditions. This instability is due to the well known, irreversible cyclization that a-APM undergoes to form 3-(S.)-benzyl-6-(S,)-carboxymethyl-2,5-diketopiperazine or DKP, with the release of methanol. This has serious implications when attempting to utilize a-APM in certain food applications where alkaline pH is required. In addition to loosing sweetness, this DKP as well as other DKPs isolated from cocoa are reported to be bitter, and thus could compromise the flavor of certain foods. Therefore, detection and quantitation of DKP is of interest. 

The widely used sugar substitute aspartame, a dipeptide of two (L)-amino acids, is about 200 times sweeter than sucrose. This is certainly surprising, since (L)-amino acids, as mentioned above, have in general a tendency to taste bitter. 

The dipeptide ester L-Asp-L-Phe-OMe (Aspartame) is used in large amounts as a low-calorie sweetener. One of the most economical strategies for its synthesis involves an enzymatic step, which is run at a capacity of 2,000 t ear (Scheme 3.25). Benzyloxycarb(Miyl-(Z)-protected L-aspartic acid is linked with L-Phe-OMe in a thermodynamically cOTitroUed cmidensation reaction catalyzed by the protease thermolysin without formation of the undesired (bitter) (3-isomer. Removal of the product via formation of an insoluble salt was used as driving force to shift the equilibrium of the reaction in the synthetic direction [318, 319]. 

Aspartame is an artificial sweetener sold under the trade name NutraSweet. Aspartame is the methyl ester of a dipeptide formed from L-aspartic acid and L-phenylalanine and can be summarized as Asp-Phe-OCHs. Surprisingly, the analogous ethyl ester (Asp-Phe-OCH2CH3) is not sweet. If either L residue is replaced with its enantiomeric D residue, the resulting compound is bitter instead of sweet. 

Table 1.15. Bitter taste of dipeptide A-B dependence of recognition threshold value (mmol/1) on side chain hydrophobicity (0 sweet or neutral taste)...

Peptides, Kke amino acids, can taste bitter, sweet, salty or indifferent. Most natural and synthetic oligopeptides have a bitter taste (see Section 2.3.3.2). A sweet taste indicates dipeptides derived from L-aspartic acid (2-91) and others derived from its lower homologue L-aminomalonic acid (2-92). is always a hydrogen atom or a methyl group, substituents are alkyls or aryls and substituents are esterified carboxyl groups (usually methyl esters, but some ethyl, propyl, isopropyl and other esters are also sweet). The best... 

---