Roles of pigment arrangement in light-harvesting phycobiliproteins revealed by recombinant techniques combined with two-dimensional electronic spectroscopy

TL;DR

This study combines recombinant protein synthesis and two-dimensional electronic spectroscopy to investigate how pigment arrangement affects light-harvesting dynamics in phycobiliproteins, revealing local pigment-protein interactions as key factors.

Contribution

It demonstrates that pigment spatial arrangement is not the main determinant of excited-state dynamics, emphasizing the importance of local pigment-protein interactions and structural conformation.

Findings

Pigment number and arrangement are not primary factors in energy structure.

Local pigment-protein interactions govern electronic structure and relaxation.

Bent PCB structure reduces conjugation, affecting excited-state properties.

Abstract

We developed methods for protein synthesis and performed two-dimensional electronic spectroscopy (2D-ES) to examine the influence of pigment arrangement on the photoexcitation dynamics of light-harvesting proteins in phycobilisome. We synthesized allophycocyanin (APC), C-phycocyanin (CPC), and mutant CPC lacking the \b{eta}153 phycocyanobilin (PCB) pigment by an Escherichia coli expression system. The number of pigments in the mutant CPC is identical to that in the wild-type APC, and their spatial arrangements are similar. The absorption and fluorescence spectra of the mutant CPC closely resemble those of the wild-type CPC rather than the wild-type APC, indicating that pigment spatial arrangement is not a primary factor in determining the excited-state energy structure. The 2D-ES measurements show that the wild-type CPC retains broad positive signals at 1 ps, signifying incomplete relaxation and persistence of excited vibronic states, unlike APC, which vibrationally relaxes to the bottom of the potential energy surface within the same timeframe. The mutant CPC behaves similarly to the wild-type CPC in the 2D-ES, reinforcing that the pigment number or arrangement is not a dominant factor. Instead, the local pigment-protein interaction governs the electronic structure and relaxation dynamics. Structural analysis reveals that the bent structure of PCB in CPC's {\alpha}-chain versus the planar structure of PCB in APC. The bent PCB in CPC reduces the degree of {\pi}-conjugation, and exhibits excited-state properties distinct from those of the planar structure of PCB in APC. This finding highlights a critical role of the electronic structure governed by the local interaction in ultrafast energy relaxation.