Measurement of circular dichroism at millisecond time resolution: a stopped-flow circular dichroism system

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Abstract
A new system has been designed for measuring circular dichroism with resolution Delta A approximately 10-5, at approximately 2 ms time constant, and used for stopped-flow circular dichroism spectroscopy. Expressions are derived for signal-to-noise ratio and circular dichroism resolution including contributions from signal current and detector dark current shot noise, and thermal and amplifier noise. An experimental system has been constructed using a mercury arc, piezo-optical birefringence modulator (50 kHz) and silicon photodiode detector. Transient CD signals have been monitored for rapid reactions conducted in a stopped-flow mixing device with dead time less than 1 ms. Phase sensitive detection and analogue division are used for CD signal recovery. The resolution achieved is close to the theoretical prediction. Applications are indicated for the study of ligand-induced conformational changes in biological systems.