Step-scan Fourier transform infrared and visible absorption difference spectroscopy, with nanosecond to millisecond time resolution, has been applied to the study of photosystem I particles from photosynthetic oxygen-evolving organisms. In particular, time-resolved infrared (1800-1200 cm<sup>-1</sup>) and visible (680-850 nm) difference spectra associated with flash induced oxidation of the primary electron donor in photosystem I particles from <i>Synechocystis</i> sp. 6803 and Acarychloris marina are presented. These spectra are compared directly to static, photoaccumulated Fourier transform infrared and visible difference spectra from both species. The detailed bonding interactions of P740, the primary donor in photosystem I particles from <i>Acarychloris marina</i>, are very similar to that found for P700, the primary donor in photosystem I particles from <i>Synechocystis</i> sp. 6803. P740 consists of at least two chlorophyll molecules, both of which are probably chlorophyll-d. Combined time-resolved step-scan Fourier transform infrared and visible absorption difference spectroscopy is shown to be a powerful tool for the study of single molecular bond dynamics in proteins as large as 0.4 mega-Daltons.
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