Analysis of the absorption spectrum of the Water-Soluble Chlorophyll-Protein Complex from Lepidium virginicum by ZINDO

Abstract

To study structure and spectroscopy correlation, Chlorophyll (Chl) a/b dimers, in a sandwich configuration, were generated from the X-ray crystallographic data for a water-soluble Chl-protein complex (WSCP) from Lepidium virginicum. The dimers were optimized with the molecular mechanics method using the MM2 parameters and the energy and oscillator strength of their electronic transitions were calculated with ZINDO using the INDO/1 parameterization. The transitions for all Chl dimer were split into high- and low-energy components with respective large and small oscillator strengths. These calculated results were explained in terms of excitonic coupling in a sandwiched dimer and the coupling strength was estimated to be 190cm-1 for Chl a - Chl a, 150cm-1 for Chl b - Chl b, 80cm-1 for Chl a - Chl b. On the basis of the results, the spectral shape of the Qy band for the WSCP was reproduced by Gaussian fitting in the constraint of a Chl a/b molar ratio of 1.8 determined from exp.. The fitting also provided the composition of the dimers : Chl a - Chl a 55% , and Chl b - Chl b 30% and Chl a - Chl b 15%.