Does biology actively use non-trivial quantum phenomena? Previously coherent oscillations (in 2D electronic spectra) for the soluble light harvesting phycobiliprotein (PBP) antenna from cryptophyte algae were observed, suggesting these algae may use quantum effects in the light harvesting process.
Our laboratory has determined the structures of multiple PBPs from several organisms and discovered two distinctly different quaternary structural arrangements, termed ‘open’ and ‘closed’.
Each PBP is comprised of a conserved structure of two αβ dimers. Open and closed conformations of PBP differ in the quaternary conformation. A single amino acid insertion in the ‘open’ form causes a large ~73˚ rotation between the two αβ heterodimers. This structural change between the open and closed form is significant for light harvesting function because it disrupts strong excitonic coupling between two central chromophores. Only the ‘closed’ form shows quantum coherence.
Transcriptomics suggest some cryptophyte species can express both open and closed forms of PBP, indicating that the type of protein expressed (open or closed) is under the control of the organism, leading to the proposal that some cryptophyte algae may switch between proteins that do and do not support quantum phenomena. Does the organism use different types of these proteins under different environmental conditions? When would the organism use the close quantum coherent form?
We present the protein crystal structures of HA560 and HA645, two co-expressed proteins isolated from the cryptophyte Hemiselmis andesenii. HA560 shows an open conformation, while HA645 shows a closed conformation, demonstrating that H. andersenii makes both open and closed light harvesting proteins at the same time. Analysis of the UV-visible/fluorescence spectra of these proteins differs dramatically, suggesting that they may work together to form a light-funnel, and implying that the closed ‘quantum coherent’ form is intimately linked, possibly as the terminal acceptor in the light harvesting process.