Ryoichiro Nakamura

Molecular mechanisms of sweet taste 7: The sweet protein, Thaumatin I

A computer modelling study of the host-guest relationship between the intensely sweet protein named Thaumatin I and our suggested receptor model, an α-helical protein, has revealed a perfect fit within a cleft of the Thaumatin I conformation. The parameters and stereoselective interactions for the initiation of the sweetness sensation are interpreted in accord with our previous stereo-interactive studies on a wide range of low molecular weight sweeteners. Also, the Thaumatin data parallels that of the macromolecular sweet protein Monellin, and their intense sweetness arises from the multiple interactions arising within the docking cleft of the protein receptor. The AHS/BS pair of Thaumatin I has been assigned to Lys97/Asp-98.

Development of an HPLC Method with an ODS Column to Determine Low Levels of Aspartame Diastereomers in Aspartame

α-L-Aspartyl-D-phenylalanine methyl ester (L, D-APM) and α-D-aspartyl-L-phenylalanine methyl ester (D, L-APM) are diastereomers of aspartame (N-L-α-Aspartyl-L-phenylalanine-1-methyl ester, L, L-APM). The Joint FAO/WHO Expert Committee on Food Additives has set 0.04 wt% as the maximum permitted level of the sum of L, D-APM and D, L-APM in commercially available L, L-APM. In this study, we developed and validated a simple high-performance liquid chromatography (HPLC) method using an ODS column to determine L, D-APM and D, L-APM in L, L-APM. The limits of detection and quantification, respectively, of L, D-APM and D, L-APM were found to be 0.0012 wt% and 0.004 wt%. This method gave excellent accuracy, repeatability, and reproducibility in a recovery test performed on five different days. Moreover, the method was successfully applied to the determination of these diastereomers in commercial L, L-APM samples. Thus, the developed method is a simple, useful, and practical tool for determining L, D-APM and D, L-APM levels in L, L-APM.