Time-resolved FTIR difference spectroscopy for the study of photosystem I with high potential naphthoquinones incorporated into the A1 binding site 2: Identification of neutral state quinone bands.

Time-resolved step-scan FTIR difference spectroscopy at 77 K has been used to study photosystem I (PSI) from Synechocystis sp. PCC 6803 with four high-potential, 1,4-naphthoquinones (NQs) incorporated into the A₁ binding site. The incorporated quinones are 2-chloro-NQ (2ClNQ), 2-bromo-NQ (2BrNQ), 2,3-dichloro-NQ (Cl₂NQ), and 2,3-dibromo-NQ (Br₂NQ). For completeness 2-methyl-NQ (2MNQ) was also incorporated and studied. Previously, PSI with the same quinones incorporated were studied in the, so-called, anion spectral region between 1550 and 1400 cm^-1 (Agarwala et al. in Biochim Biophys Acta 1864(1):148918, 2023). Here we focus on spectra in the previously unexplored 1400-1200 cm^-1 spectral region. In this region several bands are identified and assigned to the neutral state of the incorporated quinones. This is important as identification of neutral state quinone bands in the regular 1700-1600 cm^-1 region has proven difficult in the past. For neutral PhQ in PSI a broad, intense band appears at ~ 1300 cm^-1. For the symmetric di-substituted NQs (Cl₂NQ/Br₂NQ) a single intense neutral state band is found at ~ 1280/1269 cm^-1, respectively. For both mono-substituted NQs, 2ClNQ and 2BrNQ, however, two neutral state bands are observed at ~ 1280 and ~ 1250 cm^-1, respectively. These observations from time-resolved spectra agree well with conclusions drawn from absorption spectra of the NQs in THF, which are also presented here. Density functional theory based vibrational frequency calculations were undertaken allowing an identification of the normal modes associated with the neutral state quinone bands.