Dielectrophoresis of chloroplasts

Dielectrophoresis is the migration of neutral particles in a nonuniform electric field (a.c. or d.c.) toward the region of highest field intensity. Dielectrophoresis should be distinguished from electrophoresis which is the migration of charged particles in electric fields. Chloroplasts, isolated from spinach leaves, can be collected on platinum electrodes by dielectrophoresis. Stripped chloroplasts lacking outer envelopes and stroma were prepared from fresh spinach leaves in a 0.5 M sucrose-0.05 M Tris buffer (pH 7.4). The chloroplast preparation was desalted with a mixed anion-cation resin to a resistivity of 3 · 10⁴–5 · 10⁴ ohm · cm. Dielectrophoresis was conducted in a pin-pin type leucite cell 3.2 mm in diameter and 1.5 mm deep. The 0.425-mm diameter electrodes were 0.85 mm apart and 0.05 mm below the surface of the cell. The collection of chloroplasts with ac current is a function of the frequency. 3-(3,4-Dichlorophenyl)-1,1-dimethylurea (DCMU)-stabilized chloroplasts had collection maxima at 300, 1 · 10⁶, and 3 · 10⁷ Hz when run at 50 V. The rate of collection is a function of the square root of the time. Both DCMU and darkness tend to stabilize collections. It is suggested that dielectrophoresis may be a useful tool for the study of chloroplast physiology and perhaps, for the preparation and purification of chloroplasts.