The Photophysics and Quantum Mechanics of Cryptophyte Photosynthesis

Download or read book The Photophysics and Quantum Mechanics of Cryptophyte Photosynthesis written by Alexander Bastiaan Doust and published by . This book was released on 2006 with total page 464 pages. Available in PDF, EPUB and Kindle. Book excerpt: This thesis is a comprehensive study of the optical properties and the critical role of phycobiliproteins in the absorption of green light for photosynthesis in cryptophyte algae (Rhodomonas CS24 and Chroomonas CCMP270). Investigations of two different isolated proteins, phycoerythrin 545 and phycocyanin 645, are described. In addition, the intact algae are investigated to elucidate the workings of these antennae proteins within the machinery of the organism. The crystal structures of these proteins are known to 0.97 and 1.4 A resolution, respectively. These data confirm that the proteins each bind eight open-chain tetrapyrrole phycobilin chromophores, which, in these proteins, are phycoerythrobilin, dihydrobiliverdin, phycocyanobilin, and mesobiliverdin. These bilins are covalently attached to the protein backbone by cysteine bonds. The extended conformation, conjugation length, protonation state and electronic couplings to other bilins or protein residues all play an important role in ensuring that these pigments function efficiently as photosynthetic light harvesters. Chapter 1 of this thesis outlines the evolution of phycobiliproteins and cryptophyte algae, as well as describing in detail the structures of both phycoerythrin 545 and phycocyanin 645. Chapter 2 provides a background to all the experimental and computational techniques that are employed, as well as the data analysis and global and target modeling of the ultrafast data. Chapter 3 describes the results and discussion of all the work on phycoerythrin 545, with Chapter 4 describing the subsequent experiments that reveal the role of phycoerythrin 545 in the overall photosynthetic energy transfer pathways in intact algae. Chapter 5 is an up-to-date evaluation of phycoerythrin 645, where structural data is combined with steady state and ultrafast heterodyne detected transient grating to generate a non-quantitative model of function. Chapter 6 provides a combined conclusion to all the work that has been performed and presented in this thesis. These phycobilins are the focus of most of the work. Steady-state spectroscopies, including polarization anisotropy and circular dichroism, are combined with ultrafast transient grating and transient absorption spectroscopies and subsequent global analyses to reveal a detailed picture of energy transfer within the proteins. In addition, energy transfer from phycoerythrin 545 to chlorophyll-containing light harvesting complexes and photosystems in intact algae is investigated by steady-state spectroscopy and time-resolved fluorescence. Quantum mechanical computational methods are employed to calculate phycobilin excited states, and generate transition density cubes which are used to accurately determine the electronic coupling between the chromophores in phycoerythrin 545 and phycocyanin 645. Modelling of the phycobilin lineshape function has allowed the precise estimation of the energy transfer times among the bilins in phycoerythrin 545 by way of the generalized Forster theory as well as reproducing the absorption and circular dichroism spectra. Kinetic models for exciton dynamics in both proteins are presented. The roles of the central dimer bilins and the final energy transfer step between the two red-most DBV bilins in phycoerythrin 545 are discussed in detail.