Shana Bender

Microalgae-derived biofuels have potential advantages over other renewable, crop-based resources; however, large-scale production is not currently economical due, in part, to challenges in the harvesting step. In this article, we present a novel approach for the dewatering and harvesting of microalgae using flocculants that can be recovered and recycled. Polyampholytes with molecular charges dependent upon pH (ranging from net positively- to net negatively-charged) are used as a model flocculant system and provide reversible electrostatic interactions with the negatively-charged algal cells. These pH-dependent properties allow the polyampholytic flocculants to efficiently desorb from concentrated biomass and, unlike most commercial flocculants that have permanently charged functionalities, be recovered and recycled for further dewatering processes. The behavior of the model polyampholytic flocculants is characterized for the dewatering of Chlorella vulgaris (UTEX 395). The reversible and recyclable flocculants achieve >99% flocculation efficiencies, are recovered at more than 98 wt% yields after biomass dewatering, and can be recycled over five times for flocculation.

Photosystem I (PSI) is one of the two membrane-associated reaction centers involved in oxygenic photosynthesis. In photosynthesis, solar energy is converted to chemical energy in the form of a transmembrane charge separation. PSI oxidizes cytochrome c6 or plastocyanin and reduces ferredoxin. In cyanobacterial PSI, there are 10 tryptophan residues with indole side chains located less than 10 Å from the electron transfer cofactors. In this study, we apply pump-probe difference UV resonance Raman (UVRR) spectroscopy to acquire the spectrum of aromatic amino acids in cyanobacterial PSI. This UVRR technique allows the use of the tryptophan vibrational spectrum as a reporter for structural changes, which are linked to PSI electron transfer reactions. Our results show that photo-oxidation of the chlorophyll a/a′ heterodimer, P700, causes shifts in the vibrational frequencies of two or more tryptophan residues. Similar perturbations of tryptophan are observed when P700 is chemically oxidized. The observed spectral frequencies suggest that the perturbed tryptophan side chains are only weakly or not hydrogen bonded and are located in an environment in which there is steric repulsion. The direction of the spectral shifts is consistent with an oxidation-induced increase in dielectric constant or a change in hydrogen bonding. To explain our results, the perturbation of tryptophan residues must be linked to a PSI conformational change, which is, in turn, driven by P700 oxidation.

... Doyle School of Chemistry and Biochemistry Georgia Institute of Technology Dr. Wendy Kelly School of Chemistry and Biochemistry Georgia Institute of Technology Dr. Ingeborg Schmidt-Krey School of Biology Georgia Institute of Technology Dr. Nael McCarty Department ...

Photosystem I (PSI) is a multisubunit protein complex which carries out light-induced, transmembrane charge separation in oxygenic photosynthesis. In PSI, the electron-transfer pathway consists of chlorophyll and phylloquinone molecules, as well as iron-sulfur clusters. There are two phylloquinone molecules, which are located in structurally symmetric positions in the reaction center. It has been proposed that both phylloquinone molecules are active as the A1 secondary electron acceptor in bidirectional electron-transfer reactions. The PSI A1 acceptors are of interest because they have the lowest reduction potential of any quinone found in nature. In this work using light-induced FT-IR spectroscopy, isotope-edited spectra are presented, which attribute vibrational bands to the carbonyl stretching vibrations of A1 and A1- and the quinoid ring stretching vibration of A1. Bands are assigned by comparison with hybrid Hartee-Fock density functional calculations, which predict vibrational frequencies, amplitudes, and isotope shifts for the phylloquinone singlet and radical anion states. The results are consistent with an environmental interaction increasing the frequency of the singlet CO vibration and decreasing the frequency of the anion radical CO vibration, relative to model compounds. This environmental interaction may be the asymmetric hydrogen bond to A1/A1-, electrostatic interactions with charged amino acid side chains, or a pi-pi interaction with the indole ring of a nearby tryptophan. Such differential effects on the structure of A1 and A1- may be associated with a destabilization of the anion radical. These studies give novel information concerning the effect of the protein matrix on the PSI electron-transfer cofactor.