Towards building artificial light harvesting complexes: enhanced singlet–singlet energy transfer between donor and acceptor pairs bound to albumins

Abstract

Specific donor and acceptor pairs have been assembled in bovine serum albumin (BSA), at neutral pH and room temperature, and these dye–protein complexes indicated efficient donor to acceptor singlet–singlet energy transfer. For example, pyrene-1-butyric acid served as the donor and Coumarin 540A served as the acceptor. Both the donor and the acceptor bind to BSA with affinity constants in excess of 2 × 10⁵ M⁻¹, as measured in absorption and circular dichroism (CD) spectral titrations. Simultaneous binding of both the donor and the acceptor chromophores was supported by CD spectra and one chromophore did not displace the other from the protein host, even when limited concentrations of the host were used. For example, a 1:1:1 complex between the donor, acceptor and the host can be readily formed, and spectral data clearly show that the binding sites are mutually exclusive. The ternary complexes (two different ligands bound to the same protein molecule) provided opportunities to examine singlet–singlet energy transfer between the protein-bound chromophores. Donor emission was quenched by the addition of the acceptor, in the presence of limited amounts of BSA, while no energy transfer was observed in the absence of the protein host, under the same conditions. The excitation spectra of the donor–acceptor-host complexes clearly show the sensitization of acceptor emission by the donor. Protein denaturation, as induced by the addition of urea or increasing the temperature to 360 K, inhibited energy transfer, which indicate that protein structure plays an important role. Sensitization also proceeded at low temperature (77 K) and diffusion of the donor or the acceptor is not required for energy transfer. Stern–Volmer quenching plots show that the quenching constant is (3.1 ± 0.2) × 10⁴ M⁻¹, at low acceptor concentrations (<35 μM). Other albumins such as human and porcine proteins also served as good hosts for the above experiments. For the first time, non-natural systems have been self-assembled which can capture donor–acceptor pairs and facilitate singlet–singlet energy transfer. Such systems may form a basis for the design and construction of protein-based multi-chromophore self-assemblies for solar light harvesting, conversion and storage.