Atomic force microscopy study of photoelectrical properties of Photosystem I - IsiA supercomplex

Jacek Szczerbiński (Jagiellonian University, Kraków, Poland)
e-mail: szczerbinski.jacek at gmail.com
supervisor: Prof. Robert Szoszkiewicz
e-mail: rs at phys.ksu.edu
Kansas State University Physics REU 2010

This program is funded by the National Science Foundation through grant number PHY-0851599.

My project concerns electron flow in the process of photosynthesis in Synechococcus sp. PCC 7942 cyanobacteria. The general idea of this preliminary study is to treat a photosynthetic supercomplex prevalent in mentioned cyanobacteria as a photodiode and to investigate its properties. The supercomplex should produce spare electrical charge when illuminated with laser light. My aim is to find out the local charge density in the supercomplex by measuring photocurrent flow in different locations on a molecule. Since the constraints of the supercomplex are far below the diffraction limit, atomic force microscope happens to be an ideal tool for this project. The photoelectronic properties of the examined supercomplex have never been investigated with subnanometer resolution. Possible outcomes of the study include development of an efficient biological photodiode with potential applications in solar energetics.

Biological background

The process of photosynthesis consists of absorption of photons by pigments (such as chlorophylls or carotenoids), transport of the excitation through subsequent pigment molecules to a 'trap chlorophyll' P700 and a redox reaction (trap chlorophyll reduces an acceptor first, then it oxidizes a donor). This initial reaction can be interpreted as a transformation of light energy into chemical energy and it eventually leads to creation of ATP and, subsequently, of final products of photosyntesis: O2 and carbohydrates. In this experiment we investigate a photosynthetic complex in absence of acceptors and donors, aiming to discover the location and properties of the processes of electron transfer.

For our investigation we chose a photosyntetic supercomplex PSI-IsiA (A.K.A. PSI-CP43') prevalent in Synechococcus sp. PCC 7942 cyanobacteria in conditions of iron deficiency. It consists of a Photosystem I trimer and a ring of 18 IsiA proteins which act as light harvesting anntenae. The supercomplex has a disc shape of diameter around 50nm.

Aim of the study

Our goal is to acquire simultaneously:

1.) topography of the PSI-IsiA supercomplex molecule,
2.) intensity of current flow between a conductive surface and an AFM tip as a function of location on the molecule (bias voltage is applied and the sample is constantly illuminated).

Knowing both of these for the same PSI-IsiA molecule will let us discover where the processes of acceptance or donation of an electron may take place. Additionally, AFM gives a possibility to apply forces on the molecule and to investigate the relation between indentation into the sample and intensity of current flowing through the tip.

Research strategy

1.) Deposition of PSI-IsiA molecules on a conductive surface. The final result should be separate single molecules attached to the surface.
2.) Acquiring electrical contact between the surface and the AFM tip through a PSI-IsiA molecule.
3.) Simultaneous measurements of topography and current flow on the molecule surface.

Research progress

• Imaging PSI-IsiA supercomplex deposited on gold surface. Measurements in air.
• Imaging empty gold surface and PSI-IsiA deposited on it (sample spin coated). Measurements in water.
• Imaging PSI-IsiA on fresh gold surface in buffer. Modified spin coating program.
• Filtered the buffer to rule out that what we see are buffer impurities. Imaging 3x diluted spin coated PSI-IsiA sample in buffer.
• Imaging empty gold surface more thorougly. Measurements in buffer and in air. Many impurities are present.
• Several methods of cleaning were tested. A cleaning procedure was elaborated. Characterizing the features left on the surface after cleaning in order not to confuse them with PSI-IsiA complex molecules or aggregates.

About me

I'm a sophomore physics major at Jagiellonian University (Kraków, Poland) - the one which Copernicus graduated from! My scientific interests are applications of physical nanoscale tools (especially atomic force microscopy) to research on biological materials. When I'm out of the lab I like travelling around Poland and other places on Earth. My favorite freetime activity is sleeping :)

Links!

Atomic force microscopy & photosynthesis in Wikipedia
Szosz-lab at KSU Department of Physics - the place where this project is held
K-State Physics REU - more information on the Research Experience for Undergraduates program
Department of Physics of Nanostructures and Nanotechnology at Jagiellonian University - my home group website