A SPLIT BEAM METHOD FOR MEASURING TIME-RESOLVED CIRCULAR-DICHROISM

An improvement to the Lewis-Kliger method for measuring transient circ ular dichroism on the nanosecond time scale is described. The method u ses a single-probe beam that is split into two different beams of plan e polarized light entering the sample and a retarder hum opposite dire ctions in different succession. Rochon polarizers are used as high-qua lity polarizing beam splitters to select the slow axis component of th e emerging elliptical polarized light beams. The intensities of the Li ght beams are determined by an imaging spectrograph coupled to an inte nsified charge coupled device detector. The split beam method reduces the need for very precise calibration of the central strain plate acti ng as a retarder and controlling the ellipticity of the probe light. T he necessary calculations are simple and can be shown to be equivalent to the formulas derived by Lewis and Kliger. The static CD spectrum o f vitamin B-12 is presented and compared to a spectrum obtained with a commercial instrument and standard technique. The time resolution of the instrument is demonstrated by observation of photobleaching of car bon monoxy myoglobin from horse heart muscle. (C) 1997 American Instit ute of Physics.