The role of axial ligands for the structure and function of chlorophylls

TY - JOUR

T1 - The role of axial ligands for the structure and function of chlorophylls

AU - Heimdal, Jimmy

AU - Jensen, Kasper

AU - Devarajan, Ajitha

AU - Ryde, Ulf

N1 - The information about affiliations in this record was updated in December 2015. The record was previously connected to the following departments: Theoretical Chemistry (S) (011001039)

PY - 2007

Y1 - 2007

N2 - We have studied the effect of axial ligation of chlorophyll and bacteriochlorophyll using density functional calculations. Eleven different axial ligands have been considered, including models of histidine, aspartate/glutamate, asparagine/glutamine, serine, tyrosine, methionine, water, the protein backbone, and phosphate. The native chlorophylls, as well as their cation and anion radical states and models of the reaction centres P680 and P700, have been studied and we have compared the geometries, binding energies, reduction potentials, and absorption spectra. Our results clearly show that the chlorophylls strongly prefer to be five-coordinate, in accordance with available crystal structures. The axial ligands decrease the reduction potentials, so they cannot explain the high potential of P680. They also redshift the Q band, but not enough to explain the occurrence of red chlorophylls. However, there is some relation between the axial ligands and their location in the various photosynthetic proteins. In particular, the intrinsic reduction potential of the second molecule in the electron transfer path is always lower than that of the third one, a feature that may prevent back-transfer of the electron.

AB - We have studied the effect of axial ligation of chlorophyll and bacteriochlorophyll using density functional calculations. Eleven different axial ligands have been considered, including models of histidine, aspartate/glutamate, asparagine/glutamine, serine, tyrosine, methionine, water, the protein backbone, and phosphate. The native chlorophylls, as well as their cation and anion radical states and models of the reaction centres P680 and P700, have been studied and we have compared the geometries, binding energies, reduction potentials, and absorption spectra. Our results clearly show that the chlorophylls strongly prefer to be five-coordinate, in accordance with available crystal structures. The axial ligands decrease the reduction potentials, so they cannot explain the high potential of P680. They also redshift the Q band, but not enough to explain the occurrence of red chlorophylls. However, there is some relation between the axial ligands and their location in the various photosynthetic proteins. In particular, the intrinsic reduction potential of the second molecule in the electron transfer path is always lower than that of the third one, a feature that may prevent back-transfer of the electron.

KW - density functional theory

KW - axial ligands

KW - chlorophyll

KW - photosynthesis

KW - reduction potential

U2 - 10.1007/s00775-006-0164-z

DO - 10.1007/s00775-006-0164-z

M3 - Article

C2 - 16953415

VL - 12

SP - 49

EP - 61

JO - Journal of Biological Inorganic Chemistry

JF - Journal of Biological Inorganic Chemistry

SN - 1432-1327

IS - 1

ER -