Monitoring fast reactions of slow cycling systems with time-resolved FTIR spectroscopy

The implementation of a sample changing wheel in combination with a step-scan FTIR spectrometer is described. This device allows collecting and averaging time-resolved data of ten samples, which can be placed successively into the infrared beam. The high time-resolution of the step-scan technique can be retained to monitor the fast kinetics of systems with slow reaction cycles. Averaging the signals of several samples instead of signals of only one sample allows collecting the data with a repetition rate larger than what would be allowed by the time constant of the reaction cycle. Thus, instrumental instabilities due to a very long measuring time are avoided. It is demonstrated that, if at each mirror position the signals of the same samples are averaged, no multiplicative noise is introduced into the spectra. Results on the application of our method to the photocycle of the Asp96 --> Asn mutant of bacteriorhodopsin, which is slowed down by a factor of more than hundred compared to the native protein, are presented. (C) 1999 Elsevier Science B.V. All rights reserved.