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Bioelectrochemistry of Biomembranes and Biomimetic Membranes



Rolando Guidelli




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To my grandchildren Neri and Petra


This book is largely based on the lecture notes for the course of Bioelectrochemistry, which I have been holding for students of the “biennio di laurea specialistica” (roughly equivalent to the last year of a UK college) at the Department of Chemistry of Florence University, from 2005 to 2010. It is accessible to graduate students and final year undergraduate students in chemistry and biology, as well as researchers in related disciplines including biology, physics, physiology, and pharmacology. It is particularly suitable for students attracted by the fascinating area of biological membranes and their functions and interested in reading a supplemental text that takes practically no argument for granted and leads the reader, step by step, through gradually more fundamental aspects of this area. To this end, the manuscript describes the essential electrochemical basics required to understand why and how electrochemical and electrophysiological tools are fundamental in elucidating the mode of ion transport across biomembranes. In this respect, it is also of interest to many electrochemists attracted by the biological realm (as it happened to me 20 years ago) and wishing to get a better view of the potentialities of the their own background and tools to move more closely into this area. On the other hand, it is of interest to biophysicists and biochemists willing to get an exhaustive overview of the potential of biomimetic membranes for the investigation of the function of membrane peptides and proteins.

The book deals with the bioelectrochemistry of biological membranes and their mimics in a homogeneous and thematically unified way. It is not a collection of selected, separate topics on the bioelectrochemistry of membranes. To understand in depth the structure and function of biological membranes, it is also essential to understand and apply principles of physical chemistry. In particular, the fundamental role played by the transmembrane potential in modulating the function of biomolecules incorporated in the membrane allows us to regard and treat the membrane as an outright electrified interface. Hence, to understand the function of biological membranes and the properties of their experimental models, called biomimetic membranes, a knowledge of some basic principles of electrochemistry and of the most significant electrochemical techniques is required. The purpose of this manuscript is to construct a coherent thermodynamic and electrochemical framework to achieve this goal.

The book is composed of seven chapters. In Chapter 1, some basic concepts of membrane biochemistry and the relative terminology are briefly outlined. Chapter 2 deals with the electrostatics of biomembranes; here, the relation between electric field and electric potential is briefly touched upon, in order to introduce a simplified derivation of the Poisson equation and, subsequently, the derivation of the Poisson–Boltzmann equation and the Gouy–Chapman theory, which are required to determine the profile of the electric potential across a bilayer lipid membrane. In this chapter, thermodynamic concepts are still not adopted, except for Boltzmann's factor, which is used to illustrate the competition between the ordering effect of the electric field and the disordering effect of temperature. Chapter 3 is devoted to thermodynamics; after a brief introduction to some basic concepts of chemical thermodynamics (including the concept of electrochemical potential, which is not clear to many students), the thermodynamics of irreversible processes is explained and its importance in providing the basis for the function of all ion pumps is pointed out. This chapter ends with a number of examples of coupling of chemical reactions to the vectorial transport of molecules across different types of membranes. The first three chapters serve to introduce the reader to the heart of the problem of passive and active transport across membranes, as carried out by embedded proteins. Passive transport by ion channels is dealt with in Chapter 4, whereas primary active transport by ion pumps and secondary active transport by transporters are discussed in Chapter 5. Particular emphasis is placed on the role of the transmembrane potential in modulating the function of ion channels and pumps. In Chapter 4, the channel current is discussed at two levels: at the first level, it is considered to be proportional to the transmembrane potential according to Ohm's law, with the necessary inclusion of the open probability; at the second level, it is obtained by derivation of the Goldman–Hodgkin–Katz equation, and the outcomes of the two approaches are compared. Two levels are also adopted for explaining the transmission of nerve impulses; the second level leads to the derivation of the equation for the action potential. Chapter 5 deals with the energy levels of the enzymatic cycle of ion pumps and describes in detail how this cycle can be investigated under both steady-state and pre-steady-state conditions by means of electrochemical/electrophysiological techniques. Chapter 6 describes the various biomimetic membranes used to incorporate peptides and proteins in order to investigate their functional activity by electrochemical means, starting from the traditional “bilayer lipid membrane” (BLM) and considering the evolution toward more robust and sophisticated experimental models of biomembranes. Before classifying them in detail on the basis of their structural features and before scrutinizing both their advantages and drawbacks, a number of electrochemical techniques used for their investigation is described and illustrated by selected examples. Possible future developments and applications are foreseen. Finally, Chapter 7 describes the salient features of a number of nonelectrochemical, auxiliary techniques that are frequently employed to characterize solid-supported biomimetic membranes, either at a macroscopic or a molecular level; they are complemented by a few illustrative applications.

I am very grateful to those publishers who have allowed me to reproduce figures that appeared in their own publications, and to Wikipedia for the nice images that I downloaded from Wikimedia Commons. The sources of each of these are indicated in the legends. I would also like to acknowledge the constant collaboration of Lucia Becucci, who started working with me on mercury-supported biomimetic membranes as a student in 1992 and has been conducting research in this area until now, demonstrating notable skillfulness in organizing and carrying out research work and in devising stimulating experiments. It is a pleasure to thank my colleague Jacek Lipkowski for his invaluable suggestions and helpful criticism on Chapter 7. Special thanks go to Ms. Anita Lekhwani for her cheerful assistance in the preparation of this book proposal and to Ms. Sumathi Elangovan, the Project Editor, for her helpful advice and excellent editorial suggestions. Recognition should also be accorded to my daughter Ilaria and her husband, Niccolò Amerini, for managing my computer's performance.