Insights into colour-tuning of chlorophyll optical response in green plants.

Joaquim Jornet-Somoza,Bruce F Milne,James J P Stewart,Miguel A L Marques,Fernando Nogueira,Joseba Alberdi-Rodriguez,Xavier Andrade,Micael J T Oliveira,Angel Rubio

doi:10.1039/c5cp03392f

Abstract

First-principles calculations within the framework of real-space time-dependent density functional theory have been performed for the complete chlorophyll (Chl) network of the light-harvesting complex from green plants, LHC-II. A local-dipole analysis method developed for this work has made possible the studies of the optical response of individual Chl molecules subjected to the influence of the remainder of the chromophore network. The spectra calculated using our real-space TDDFT method agree with previous suggestions that weak interaction with the protein microenvironment should produce only minor changes in the absorption spectrum of Chl chromophores in LHC-II. In addition, relative shifting of Chl absorption energies leads the stromal and lumenal sides of LHC-II to absorb in slightly different parts of the visible spectrum providing greater coverage of the available light frequencies. The site-specific alterations in Chl excitation energies support the existence of intrinsic energy transfer pathways within the LHC-II complex.

Highlights

More than 50% of the light captured by green plants for use in photosynthesis is absorbed by the major light harvesting complex (LHC-II).[9]
Initial calculations focused on the spectra of the isolated Chl chromophores using geometries obtained from the structure optimisation of the full complex (Fig. S3 and S4, Electronic supplementary information (ESI)†)
The experimental Q-band energy obtained from the LHC-II spectrum is 1.86 eV whilst the Soret-band contains two peaks at 2.62 and 2.85 eV.[43]

Summary

Introduction

More than 50% of the light captured by green plants for use in photosynthesis is absorbed by the major light harvesting complex (LHC-II).[9] LHC-II is a trimeric protein assembly with three-fold symmetry and contains over 17 000 atoms. Complexed within the protein framework, each monomer unit contains 14 chlorophyll (Chl) molecules (both types a and b) that are the key functional units in the light-harvesting process in LHC-II (Fig. 1).[4,10] each monomer contains four secondary carotenoid chromophores (lutein (Â2), neoxanthin and violoxanthin) that play important roles in photoprotection and energy transport within LHC-II.[11,12,13] Approximately 7000 of the LHC-II atoms belong to the chromophores (Fig. 1). Bound within cell membranes in vivo, LHC-II possesses a robust structure and maintains structural integrity during purification and crystallisation meaning that the detailed structure has been obtained from crystallographic studies and that it has become possible to perform experimental studies of isolated LHC-II.[14,15,16]

Methods

Results

Conclusion