I have some problems when i use l-dopa as substrate to assay enzyme activity

1.how to prepare 20mM l-dopa solution？i use phosphate buffer (pH=6) to dissolve l-dopa but not able to dissolve it completly

2. the reation mixture turns dark brown after incubation for 30min at 30℃, but sometimes there are many dark particles in the bottom of the centrifuge tube.should i remove or resuspend the particles before measuring A475?

Thanks for your suggestions