BIOELECTRONICS OF ION-TRANSPORTING MEMBRANE PROTEINS

Bioelectronics has a double meaning in scientific literature. On the one hand, as a branch of basic biophysical sciences, it deals with electric phenomena appearing on any organization level of living systems (A). On the other hand, as a recently developed discipline of information technological science, it explores the potential of biological materials for application in molecular electronics (B). These two areas of research are in close interaction not only with each other, but also with other disciplines of basic applied sciences. Our main goal is to develop novel methods on integrated micro- and nanotechnological platforms for the investigation of light-induced processes in biological membranes, and utilize them in both branches of bioelectronic science. Besides its impact on basic biophysical science, this research is expected to have applications in various branches of molecular electronics.

A) Electric signals associated with membrane transport processes

Electric phenomena, ubiquitous in living systems, carry a lot of information about basic physiological processes that are inaccessible to other techniques (e.g. ECG or EEG). They can all be traced back to the cellular level, namely to membrane-coupled signal- and energy transduction processes. The importance of methodological developments aiming at the detection of the associated electric signals is underpinned, e.g., by the Nobel Prize given for the patch clamp technique (Neher and Sackmann, 1991).

However, application of the most commonly used microelectrode methods to the investigation of transmembrane ionic currents often fails due to technical limitations, whereas alternative optical techniques still suffer from fundamental sensitivity and time resolution problems. Active pump currents, therefore, are still measured on suspensions of cell organelles or cells by macroelectrode methods, in whose elaboration our institute in Szeged played a determining role. The generalization of one of our techniques allowed the detection of intramolecular electric signals in all the three spatial dimensions.

From the application of the technology and its combination with molecular dynamics simulations, we expect fundamental information concerning changes in the electric field distribution inside the molecule during its function. We demonstrated the ability of our technique on the simplest and best-characterized active ion transport protein, the light-driven proton pump, bacteriorhodopsin (bR), where the description of the molecular function is close to atomic-level precision.

Recent developments in nanotechnology also offer the opportunity of a natural extension of our techniques to the investigation of bioelectric phenomena of other objects such as single cells.

B) Protein-based integrated optical switching

Since the start of integrated electronics, the expansion of development has been described by “Moore’s law”: the density (performance) of integrated electronic circuits doubles about every 1.8 years. While this “law” has remained valid for a remarkable period of 30 years, there is a general perception that the evolutionary development has reached a limit. It is agreed that future development needs revolutionary new principles. Presently, all possible candidates are explored in the search for new routes. Molecular electronics combined with optical data processing is regarded as being among the most promising emerging alternative technologies.

Coupling of optical data-processing devices with microelectronics, as well as sensory functions, is one of the biggest challenges in molecular electronics. Suitable nonlinear optical (NLO) materials with high stability and sensitivity are being intensively researched. In addition to organic and inorganic crystals, biological molecules have also been considered for use in optoelectronics, among which bR has generated the most interest.

Our recent results demonstrated the applicability of this protein as an active, programmable nonlinear optical material in all-optical integrated circuits. Based on these findings, a USA patent [Light-driven integrated optical device (US 6,956,984 B2)] has recently been registered. In addition, preliminary results suggest that all-optical switching utilizing the BR to K transition is also possible. Taking into account the fast kinetics of the K intermediate (picosecond rise time), its application in integrated optics or telecommunication could be especially advantageous. Our long-term goal is to elaborate bR-based films supporting applications of optical and optoelectronical components and devices.

Hofmeister effects

Water is the third most abundant molecule in the Universe (after H and CO), and the most abundant on Earth. The major part of living organisms is made up of water (on each level of organization). If water is extracted, proteins do not function. “Water is a matrix providing stability and flexibility of proteins at the same time.” (Philip Ball)

Some unique physical-chemical properties: high electric dipole moment, network of H-bonds, fast proton exchange. According to molecular dynamics modelling, such cluster-formations are more frequent at lower than at higher temperatures:

The reason is the change of H-bond strength versus temperature. What consequences could this have on proteins? Temperature change is the most straightforward tool to change the strength of H-bonds; however, this has an impact on the Brownian motion of protein molecules as well. Addition of salts which do not affect pH, and do not specifically interact with proteins might help this problem. The non-specific effects of neutral salts on protein aggregation and conformation have been known for a long time, and are called Hofmeister effects after their first investigator. According to the investigations, the effects are dominated by anions rather than cations. In 1888, Hofmeister ordered the anions according to their ability of precipitating globular proteins from water:

SO₄^-- > F^- > CH₃COO^- > Cl^- > Br^- > I^- > ClO₄^-, SCN^-      (1)

Cl^- has the least effect (in the middle of the row), while those on the left-hand side of (1) are called kosmotrops (increase in aggregation: “salting out”), and the right-hand-side ones are called chaotrops (increase in solubility: “salting in”). Interestingly, the same row was found for protein conformation and activity, too: Normally, kosmotrops stabilize conformation and increase activity, whereas chaotrops destabilize conformation and decrease activity. Disturbingly, however, the tendency is just the opposite in some cases. Such exceptions make the elaboration of a coherent theory of Hofmeister effects rather difficult. The main goal of our research is to develop and apply a comprehensive theory of Hofmeister effects. The starting point is that both in aggregation and conformational changes, there is a change in water-exposed protein surface area. According to our hypothesis already supported by a growing line of experimental evidence, the salt-dependence of protein-water interfacial tension holds the explanation of Hofmeister effects.

We are going to provide a microscopic interpretation of the effects by the investigation of protein conformational fluctuations. Another goal is to use Hofmeister effects as an experimental tool to pinpoint large conformational changes during protein function.