L-3, 4-dimethoxy-α-methylphenylalanine (L-DMMD) is an important intermediate for the synthesis of 3-hydroxy-α-methyl-L-tyrosine (L-methyldopa). This paper describes an efficient, accurate, and low-priced method of high-performance liquid chromatography (HPLC) using chiral mobile phase and conventional C18 column to separate L-DMMD from its enantiomers. The effects of ligands, copper salts, organic modifiers, pHs of mobile phase, and temperatures on the retention factors (k') and selectivity (α) were evaluated to achieve optimal separation performance. Then, thermal analysis of the optimal separation conditions was investigated as well. It was confirmed that the optimal mobile phase was composed of 20 % (v/v) methanol, 8 mM L-phenylalanine (L-Phe), and 4 mM cupric sulfate in water of pH 3.2, and the column temperature was set at 20 °C. Baseline separation of two enantiomers could be obtained through the conventional C18 column with a resolution (R) of 3.18 in less than 18 min. Thermodynamic data (∆∆H and ∆∆S) obtained by Van't Hoff plots revealed the chiral separation was an enthalpy-controlled process. To the best of our knowledge, this is the first report regarding the enantioseparation of DMMD by chiral ligand-exchange HPLC.
'Results and discussion' section. Under 'Method utility on the detection of the DMMD' the following sentence should be added at the end of the paragraph: BFig. 9 shows the chromatogram of separation and quantification of impure L-DMMD with slight D-DMMD under the optimized conditions. lso, in the same section under 'Limit of detection and quantification of D-enantiomer', the last sentence BThe chromatogram that the LOQ for unwanted enantiomer was about 0.02 % is shown in Fig. 9.^is incorrect and should be considered as deleted. Fig. 9 The separation and quantification of impure L-DMMD with slight D-DMMD by ligand-exchange HPLC under the optimized conditions. The mobile phase was composed of 20 % methanol, 8 mM L-Phe and 4 mM cupric sulfate in water at pH 3.2, the flow rate was 1.0 mL/min, the detection wavelength was 278 nm. The table inserted in the figure presents the corresponding relation between the peak area and concentration (mM) of enantiomer Dong-Xu Jia and Zheng-Gui Ai contributed equally to this work.The online version of the original article can be found at http://dx
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