A1 reduction in intact cyanobacterial photosystem I particles studied by time-resolved step-scan Fourier transform infrared difference spectroscopy and isotope labeling

Abstract

Time-resolved step-scan Fourier transform infrared (FTIR) difference spectroscopy, with 5 mus time resolution, has been used to produce P700(+)A(1)(-)/P700A(1) FTIR difference spectra in intact photosystem I particles from Synechococcus sp. 7002 and Synechocystis sp. 6803 at 77 K. Corresponding spectra were also obtained for fully deuterated photosystem I particles from Synechococcus sp. 7002 as well as fully (15)N- and (13)C-labeled photosystem I particles from Synechocystis sp. 6803. Static P700(+)/P700 FTIR difference spectra at 77 K were also obtained for all of the unlabeled and labeled photosystem I particles. From the time-resolved and static FTIR difference spectra, A(1)(-)/A(1) FTIR difference spectra were constructed. The A(1)(-)/A(1) FTIR difference spectra obtained for unlabeled trimeric photosystem I particles from both cyanobacterial strains are very similar. There are some mode frequency differences in spectra obtained for monomeric and trimeric PS I particles. However, the spectra can be interpreted in an identical manner, with the proposed band assignments being compatible with all of the data obtained for labeled and unlabeled photosystem I particles. In A(1)(-)/A(1) FTIR difference spectra obtained for unlabeled photosystem I particles, negative bands are observed at 1559 and 1549-1546 cm(-)(1). These bands are assigned to amide II protein vibrations, as they downshift approximately 86 cm(-)(1) upon deuteration and approximately 13 cm(-)(1) upon (15)N labeling. Difference band features at 1674-1677(+) and 1666(-) cm(-)(1) display isotope-induced shifts that are consistent with these bands being due to amide I protein vibrations. The observed amide modes suggest alteration of the protein backbone (possibly in the vicinity of A(1)) upon A(1) reduction. A difference band at 1754(+)/1748(-) cm(-)(1) is observed in unlabeled spectra from both strains. The frequency of this difference band, as well as the observed isotope-induced shifts, indicate that this difference band is due to a 13(3) ester carbonyl group of chlorophyll a species, most likely the A(0) chlorophyll a molecule that is in close proximity to A(1). Thus A(1) reduction perturbs A(0), probably via a long-range electrostatic interaction. A negative band is observed at 1693 cm(-)(1). The isotope shifts associated with this band are consistent with this band being due to the 13(1) keto carbonyl group of chlorophyll a, again, most likely the 13(1) keto carbonyl group of the A(0) chlorophyll a that is close to A(1). Semiquinone anion bands are resolved at approximately 1495(+) and approximately 1414(+) cm(-)(1) in the A(1)(-)/A(1) FTIR difference spectra for photosystem I particles from both cyanobacterial strains. The isotope-induced shifts of these bands could suggest that the 1495(+) and 1414(+) cm(-)(1) bands are due to C-O and C-C modes of A(1)(-), respectively.