Physical immobilization of photosystem I (PSI) at self-assembled monolayers on gold: directed adsorption, electron transfer, and biomimetic entrapment

Abstract

This dissertation examines fundamental aspects towards the incorporation of Photosystem I (PSI) into a photoelectrochemical device. Previous research at Vanderbilt University demonstrated that PSI adsorbs to high-energy surfaces as opposed to low-energy surfaces, which allowed for directed adsorption on a micropatterned surface constructed through microcontact printing of self-assembled monolayers (SAMs). The presence of PSI on patterned substrates was verified by scanning electrochemical microscopy (SECM).

The direct electrochemistry of PSI, in the dark, on hydroxyl-terminated SAMs of various chain lengths was examined. Direct electron transfer occurred when the hydroxyl-terminated monolayer was of intermediate chain length (n = 6 to 8 methylene units), but not when the monolayer was short (n = 2, 4) due to monolayer disorder or long (n = 11) due to increased distances for electron tunneling. The electrochemistry of PSI, in the light, demonstrated the transfer of electrons through PSI on a substrate to the solution-phase electron acceptor methyl viologen, providing approximately 3 to 6 nA per cm squared of current.

In order to suppress the amount of background current and entrap PSI on the substrate, mimicking the thylakoid membrane of plants, PSI atop HOC6S/Au was backfilled by exposure to a solution containing a long-chain alkanethiol from both organic and aqueous micellar solvents. The incoming alkanethiol filled the interprotein spaces providing hydrophobic stabilization of the physically adsorbed PSI. The integral membrane protein, PSI, was resistant to denaturation even upon exposure to organic solvents.

Disorganized hydroxyl-terminated SAMs were designed by adsorbing partially fluorinated disulfides to the gold surface and then hydrolytically cleaving away the fluorocarbon tail group. The use of hydroxyl-terminated surfaces with different packing densities did not enhance PSI adsorption; however, varying the packing density of the hydroxyl-terminated monolayer did provide an enhanced rate of backfilling by docosanethiol. The amount of PSI adsorbed onto hydroxyl-terminated surfaces was enhanced as the chain length of the alkanethiol was reduced, reflecting the likely importance of van der Waals interactions between PSI and the gold substrate through the intervening SAM.