Coordination-Driven Dimerization of Zinc Chlorophyll Derivatives Possessing a Dialkylamino Group.
Zinc chlorophyll derivatives Zn-1-3 possessing a tertiary amino group at the C31position have been synthesized through reductive amination of methyl pyropheophorbide-d obtained from naturally occurring chlorophyll-a. In a dilute CH2Cl2solution as well as in a dilute 10 %(v/v) CH2Cl2/hexane solution, Zn-1 possessing a dimethylamino group at the C31position showed red-shifted UV/Vis absorption and intensified exciton-coupling circular dichroism (CD) spectra at room temperature owing to its dimer formation via coordination to the central zinc by the 31-N atom of the dimethylamino group. However, Zn-2/3 bearing 31-ethylmethylamino/diethylamino groups did not. The difference was dependent on the steric factor of the substituents in the tertiary amino group, where an increase of the carbon numbers on the N atom reduced the intermolecular N⋅⋅⋅Zn coordination. UV/Vis, CD, and1H NMR spectroscopic analyses including DOSY measurements revealed that Zn-1 formed closed-type dimers via an opened dimer by single-to-double axial coordination with an increase in concentration and a temperature decrease in CH2Cl2, while Zn-2/3 gave open and flexible dimers in a concentrated CH2Cl2solution at low temperature. The supramolecular closed dimer structures of Zn-1 were estimated by molecular modelling calculations, which showed these structures were promising models for the chlorophyll dimer in a photosynthetic reaction center.
Graduate School of Life Sciences, Ritsumeikan University, Kusatsu, Shiga, 525-8577, Japan.
Hiroaki
H
Watanabe
Graduate School of Life Sciences, Ritsumeikan University, Kusatsu, Shiga, 525-8577, Japan.
Yusuke
Y
Kamatani
Graduate School of Life Sciences, Ritsumeikan University, Kusatsu, Shiga, 525-8577, Japan.
Hitoshi
H
Tamiaki
D000588:Amines / Q000737:chemistry*; D002734:Chlorophyll / Q000737:chemistry*; D002942:Circular Dichroism; D019281:Dimerization; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D015394:Molecular Structure; D015032:Zinc / Q000737:chemistry*
D000588:Amines; D002734:Chlorophyll; D015032:Zinc
DOSY; coordination; dimerization; steric hindrance; zinc chlorophyll