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Modification of the Phylloquinone in the A1 Binding Site in Photosystem I Studied Using Time-Resolved FTIR Difference Spectroscopy and Density Functional Theory

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posted on 04.04.2006, 00:00 by K. M. Priyangika Bandaranayake, Ruili Wang, Gary Hastings
A phylloquinone molecule (2-methyl-3-phytyl-1,4-naphthoquinone) occupies the A1 binding site in photosystem I. Previously, we have obtained A1-/A1 FTIR difference spectra using labeled and unlabeled photosystem I particles and proposed assignments for many of the bands in the spectra [Sivakumar, V., Wang, R., and Hastings, G. (2005) Biochemistry 44, 1880−1893]. In particular, we suggested that a negative/positive band at 1654/1495 cm-1 in A1-/A1 FTIR DS is due to a CO/CO mode of the neutral/anionic phylloquinone, respectively. To test this hypothesis, we have obtained A1-/A1 FTIR DS for menG mutant PS I particles. In menG mutant PS I, phylloquinone in the A1 binding site is replaced with an analogue in which the methyl group at position 2 of the quinone ring is replaced with a hydrogen atom (2-phytyl-1,4-naphthoquinone). In A1-/A1 FTIR DS obtained using menG mutant PS I particles, we find that the 1654/1495 cm-1 bands are upshifted by ∼6 cm-1. To test if such upshifts are likely for CO/CO modes of neutral/anionic phylloquinone, we have used density functional theory to calculate the “anion minus neutral” infrared difference spectra for both phylloquinone and its methyl-less analogue. We have also undertaken calculations in which the C4O carbonyl group of phylloquinone and its methyl-less analogue are hydrogen bonded (to a water or leucine molecule). We find that, irrespective of the hydrogen bonding state of the C4O group, the CO/CO modes of neutral/reduced phylloquinone are indeed expected to be upshifted by at least 6 cm-1 upon replacement of the methyl group at position 2 with hydrogen. The calculations also suggest that certain CC/CC modes of neutral/reduced phylloquinone do not shift upon replacement of the methyl group. On the basis of these calculated results, we suggest which bands in the A1-/A1 FTIR DS may be associated with CC/CC modes of neutral/reduced phylloquinone, respectively.