Orientation and linear dichroism of chloroplasts and sub-chloroplast fragments oriented in an electric field

Whole or broken spinach chloroplasts, bacterial chromatosphores and CPI chlorophyll · protein complexes in aqueous suspensions at room temperature can be oriented in externally applied electric fields. The orientation is observed by monitoring the electric field induced linear dichroism (LD). With whole chloroplasts a detectable LD signal is observed using voltages as low as 2-3 V (50 Hz alternating voltage) across an 0.3 cm electrode gap, and nearly complete orientation is observed at fields of 30 V · cm^-1. The wavelength dependence of the LD signals using either orienting electric fields (E) alone, or magnetic fields (B) alone, are similar but opposite in sign with E and B pointing in the same direction. The chloroplasts tend to orient in such a way that the membrane planes are parallel to E. The CPI complexes and bacterial chromatophores require much higher electric fields for orientation than whole chloroplasts (for CPI complexes E > 2000 V · cm^-1); rectangular, millisecond duration, voltage pulses are utilized for the observation of electric field induced LD spectra in these cases. Oriented CPI complexes exhibit LD maxima of the same sign at 685 and at 440 nm. The oriented chromatophores exhibit an LD spectrum of either positive or negative sign, depending on the wavelength. The mechanisms of the orientation are discussed.