A resonance-like effect induced by sinusoidal electric fields in a macromolecular dipole

Abstract

The linear dichroism of DNA fragments with 587 and 859 base pairs has been measured under sinusoidal electric fields over a broad range of frequencies. The sine-amplitude of the dichroism shows a distinct maximum at frequencies around 1 kHz at an effective electric field strength of ≈ 400 V / cm and at a salt concentration around 1 mM; this maximum does not appear at a field strength of ≈ 200 V / cm. At low frequencies, the initial period of the linear dichroism induced by the electric field shows a characteristic dependence on the phase of the enforcing sinusoidal electric field. The essential part of the system response is described by a deconvolution procedure with a quadratic dependence of the optical effect on the field strength and by two relaxation processes with amplitudes of opposite sign. These relaxation processes are virtually identical with those found independently in the relaxation response induced by rectangular field pulses under equivalent experimental conditions; under these conditions the stationary electric dichroism is positive. Additional processes found in the relaxation response induced by sinusoidal field pulses indicate some field-induced conformation change. The ‘resonance’ has been recorded by the electric dichroism, which reflects the physical process of molecular alignment, but it is concluded that this process is accompanied by a resonance effect in the molecular configuration. The resonance effect observed in the present case clearly does not correspond to resonance as defined in physics. Nevertheless, the maximum of a system response at a given frequency due to superposition of relaxation effects - corresponding to e.g. activation and inhibition - may be useful for a selective control of molecular processes.