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 ~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[dbnd]O group of the P _A 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[dbnd]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[dbnd]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[dbnd]O mode vibrations at 1702 and 1754 cm ⁻¹ , respectively.