Fourier transform visible and infrared difference spectroscopy for the study of P700 in photosystem I from Fischerella thermalis PCC 7521 cells grown under white light and far-red light: Evidence that the A-1 cofactor is chlorophyll f

(P700(+)-P700) Fourier transform visible and infrared difference spectra (DS) have been obtained using photosystem I (PSI) complexes isolated from cells of Fischerella thermalis PCC 7521 grown under white light (WL) or far-red light (FRL). PSI from cells grown under FRL (FRL-PSI) contain similar to 8 chlorophyll f (Chl f) molecules (Shen et al., Photosynth Res. Jan. 2019). Both the visible and infrared DS indicate that neither the P-A or P-B pigments of P700 are Chl f molecules, but do support the conclusion that at least one of the A-1 cofactors is a Chl f molecule. The FTIR DS indicate that the hydrogen bond to the 13(1)-keto C=O group of the PA pigment of P700 is weakened in FRL-PSI, as might be expected given that the proteins that bind the P700 pigments are substantially different in FRL-PSI (Gan et al., Science 345, 1312-1317, 2014). The FTIR DS obtained using FRL-PSI display a band at 1664 cm(-1) that is assigned (based on density functional theory calculations) to the 2(1)-formyl C=O group of Chl f, that upshifts 5 cm(-1) upon P700(+) formation. This is much less than expected for a cation-induced upshift, indicating that the Chl f molecule is not one of the pigments of P700. In WL-PSI the A(-1) cofactor is a Chl a molecule with 13(1)-keto and 13(3)-methylester C=O mode vibrations at 1696 and 1750 cm(-1) respectively. In FRL-PSI the A(-1) cofactor is a Chl f molecule with 13(1)-keto and 13(3)-methylester C=O mode vibrations at 1702 and 1754 cm(-1), respectively.