Differential Model Spectra (DMPs) for the deconvolution of the NIR-signals

The absorption spectra of P700, plastocyanin (PC) and ferredoxin (Fd) are broad and show few features in the near infrared (NIR). A further complication is that the extinction coefficient of P700 is much higher than that of PC, which is higher again than the extinction coefficient of Fd. By choosing specific wavelength pairs it is only possible to get NIR-signals enriched in Fd (785-820 nm), or P700 (820-870 nm) or PC (870-950 nm). To get from there to a clean deconvolution, the software makes use of the differential model plot (DMP) approach, which does not require knowledge of difference spectra and the wavelength dependence of differential extinction coefficients. Christof Klughammer first developed this method for the KLAS-100, a kinetic LED array spectrophotometer for the 510-570 nm wavelength range.

We know the behavior of PC, P700 and Fd under standard conditions quite well and we can, on the basis of that knowledge, design experiments in which, for a short period of time, only redox changes of one of the three components (PC, P700 or Fd) occur. By normalization of the values determined for the 4 difference signals a “spectral fingerprint” for each component is obtained under the given experimental conditions. It should be noted here as well that the FA and FB iron-sulfur centers located on the acceptor side of photosystem I may also contribute to the Fd-signal.