Modification of the phylloquinone in the A1 binding site in photosystem I studied using time-resolved FTIR difference spectroscopy and density functional theory.

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/C-:O 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 approximately 6 cm(-1). To test if such upshifts are likely for C=O/C-:O 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/C-:O 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/C-:C 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/C-:C modes of neutral/reduced phylloquinone, respectively.