(624e) Effect of Surface Attachment Characteristics On Photoactivity of Photosystem I Assembly On Thiol-Activated Au Substrates

Cyanobacterial Photosystem I (PSI), a supra-molecular protein complex (MW~300 kDa, disk shaped with diameter ~30 nm & height ~9 nm), is a biological photodiode responsible for driving natural photosynthesis mechanism. Upon exposure to light, PSI undergoes charge separation enabling unidirectional electron flow. This makes PSI a promising candidate for future research in the fabrication of solid-state bio-electronic, hybrid photovoltaic devices, or bio-sensors. But, a first step in the rational design of aforementioned devices and sensors involves in arranging these protein complexes as a uniform homogeneous monolayer on chemically/ physically decorated self-assembled monolayer (SAM) substrates with proper directionality to allow efficient electron capture pathways. Previous studies have suggested that PSI preferentially attaches onto hydroxyl (OH)-terminated alkanethiols with the electron vectors pointing outward. But, our present studies indicate that experimental parameters like solution concentration and deposition protocol can dictate the attachment dynamics of PSI onto alkanethiolate SAM/ Au substrates resulting in complex structural arrangements which can either hinder or facilitate the photo-electrochemistry of such systems.

In this work, morphology and orientations of PSI assembly on OH-terminated alkanethiolate SAM/Au substrates are studied with: 1) gravity-driven and 2) electric field assisted deposition techniques to elucidate the surface attachment dynamics and properties of these protein complexes. Atomic force microscopy (AFM) images and spectroscopic ellipsometry (SE) data indicate the formation of a large number of columnar PSI aggregates on the substrates for gravity-driven deposition from high PSI density aqueous buffer solutions. These larger structures dissipate into a monolayer at lower concentrations indicating formation of Brownian diffusion mediated aggregates in the solution phase. However, in electric-field assisted deposition, electrophoretic mobility of PSI can be tuned based on the field strength to prevent such aggregation thereby allowing a more uniform deposition. Experimental studies of solution phase aggregation dynamics via dynamic light scattering and analytical ultracentrifugation also support these observations, thereby providing an unprecedented level of understanding into the complex dynamics of protein-protein and/or protein-surface interactions. Furthermore, Quartz Crystal Microbalance (QCM) experiments help us study PSI adsorption/ desorption kinetics with different surface treatments for optimal monolayer formation. PSI orientation from QCM based surface arrangements and packing density data, validated by fluorescence polarization spectroscopy to track emission anisotropy from fluorescently tagged PSI molecules are also discussed. Such studies could play significant role in monitoring and tailoring surface attachment of PSI to specific SAM substrates and hence, in the rational design of future bio-electronic devices.