Hydrophobic Interactions

319 Pages·1980·6.242 MB·English

by  Arieh Ben-Naim (auth.)

Hydrophobic
Interactions
Hydrophobic
Interactions
Arieh Ben-Nairn
Department ofP hysical Chemistry
The Hebrew University of Jerusalem
Jerusalem, Israel
Plenum Press· New York and London
Library of Congress Cataloging in Publication Data
Ben-Na'im, Aryeh.
Hydrophobic in teractions.
Includes index.
1. Surface chemistry. 2. Solution (Chemistry) I. Title.
QD506.B45 541'.39 79-510
ISBN-13: 978-1-4684-3547-4 e-ISBN-13: 978-1-4684-3545-0
DOl: 10.1007/978-1-4684-3545-0
© 1980 Plenum Press, New York
Softcover reprint of the hardcover 1st edition 1980
A Division of Plenum Publishing Corporation
227 West 17th Street, New York, N.Y. 10011
All rights reserved
No part of this book may be reproduced, stored in a retrieval system, or transmitted,
in any form or by any means, electronic, mechanical, photocopying, microfilming,
recording, or otherwise, without written permission from the Publisher
To my sons
ODED
RAANAN
and
YUVAL
Preface
My personal involvement with the problem of hydrophobic interactions
(HI) began about ten years ago. At that time I was asked to write a review
article on the properties of aqueous solutions of nonpolar solutes. While
surveying the literature on this subject I found numerous discussions of the
concept of HI. My interest in these interactions increased especially after
reading the now classical review of W. Kauzmann (1959), in which the
importance of the HI to biochemical processes is stressed.
Yet, in spite of having read quite extensively on the various aspects
of the subject, I acquired only a very vague idea of what people actually
had in mind when referring to HI. In fact, it became quite clear that the
term HI was applied by different authors to describe and interpret quite
different phenomena occurring in aqueous solutions. Thus, even the most
fundamental question of the very definition of the concept of HI remained
unanswered. But other questions followed, e.g.: Are HI really a well
established experimental fact? Is there any relation between HI and the
peculiar properties of water? Is the phenomenon really unique to aqueous
solutions? Finally, perhaps the most crucial question I sought to answer
was whether or not there exists hard evidence that HI are really important
-as often claimed-in biological processes.
It is not uncommon to find statements in many textbooks on bio
chemistry referring to HI as "a major driving force in folding of macro
molecules, the binding of substrate to enzymes and most other molecular
interactions in biology" (Stryer, 1975).
Such statements may be correct. However, in spite of my researches
in this field over almost ten years, I cannot confirm that there is at present
either theoretical or experimental evidence that unequivocally demonstrates
vii
viii Preface
the relative importance of HI over other types of interactions in aqueous
solutions. This does not mean that the whole subject is of no interest. On
the contrary, even if the whole field should ultimately prove to be totally
irrelevant to biochemistry, it still deserves careful attention within the
general framework of the study of the properties of aqueous solutions
per se. It is also my belief that HI are, and will prove to be, of central
importance to the understanding of complex biological processes.
The initial aim of this book was to try to answer some of these questions,
but the reader who is looking for definitive answers will surely find this
book disappointing. The reason is very simple: Our present knowledge
and understanding of the HI are only in their very elementary stages.
Much more work must be done on both the experimental and the theoretical
frontiers before we are sufficiently well equipped to attempt to answer
these basic questions. Thus, my aims in writing the book have shifted from
the initial ones to less ambitious ones; namely, I have concentrated on
surveying what is presently known rather than on what is completely
understood in this field.
The style of the book is mostly descriptive, Theory is used either as a
means for processing and interpreting experimental data, or for suggesting
new and relevant experiments. Recently, some new theories have been
devised to predict the strength of the HI. These are too advanced, however,
to be included in this, rather elementary, exposition of the subject.
All chapters in the book deal with aqueous solutions, rather than with
pure liquid water. Our understanding of HI will ultimately depend on a
fuller understanding of the peculiar properties of liquid water, but it is
not possible to establish such a link at present. However, in Chapter 5,
some notions such as the "structure of water" and "structural changes in
the solvent" are discussed in connection with the phenomena of HI.
Chapter I introduces the basic definitions of the concepts that are
used throughout the book. The level of presentation is very elementary.
Although some of the statements are based on statistical mechanical
arguments, the latter are collected in the Appendixes and are not presented
in the main text.
Chapter 2 is devoted to the properties of ideal dilute solutions. These
solutions are useful in the study of the so-called solute-solvent interactions.
These interactions are related, both directly and indirectly, to the main
topic of the book, namely the solute-solute interactions.
Chapters 3 and 4 form the main body of the work. Here, we first
survey the experimental data on the pairwise solute-solute interactions
and then proceed to the more general case, i.e., interactions among many
Preface ix
solute particles. In the last two sections of Chapter 4 we present a very
short review of an immense literature dealing with the role of HI in micellar
and biological systems. The prevalent approach in these two chapters is
to present those experimental methods that provide information on the
nature of the HI. In most of the biochemical literature, the concept of HI is,
by contrast, usually used to explain some peculiar and complex processes.
In other words, the concept of HI is used as input rather than as output.
I believe that the former approach, though very popular, is unjustified
because it applies a poorly understood concept to explain other, more
complicated phenomena. Thus, to a certain extent, we diverge in Sections
4.8 and 4.9 from our main approach, and present some examples that are
clearly relevant to the subject of the book, although they may not as yet
be used to provide information that will enhance our knowledge of the HI.
Chapter 5 deals with the temperature and pressure dependence of the
HI. Here, the concept of the "structure of water" is used to interpret some
of the peculiarities of the entropy and the enthalpy changes associated with
the process of HI. In order to define and use the concept of the structure
of water we felt it necessary to appeal to some elementary concepts in
statistical mechanics. We also use statistical mechanics in the Appendixes to
provide a better background for some statements made throughout the book.
In a rapidly expanding field of research, it is likely that some results,
either of experimental or of theoretical nature, may become obsolete
during the time lag from the date of submission to actual publication. There
fore I have stressed, in most parts, the general principles rather than
specific results. This approach has particularly influenced my decision to
review only very briefly the recent progress made in the theory of HI.
Tables of data are used for illustration only. It is not claimed that a
particular set of data is the most complete or most accurate of the kind.
As the reader will immediately notice, the data themselves are very scant
and fragmentary. I have also exercised the author's privilege and have
added a few personal comments at the end of some sections. Here I have
tried to express my personal view on the merits and potentialities of the
particular topic discussed in that section and, in some cases, tried to suggest
further experiments that should complement and improve our knowledge
of that field.
Acknowledgments
I am very much indebted to many friends and colleagues who have
read parts of the manuscript and have made useful comments. I am especially
x Preface
grateful to Dr. Carmel Jolicoeur for writing the first draft of Section 3.10
on the application of NMR and ESR to the problem of hydrophobic
interactions. This section has been further revised with the kind help of
Dr. Haim Levanon and Dr. Gerhard Hertz. I am also grateful for helpful
comments from: Tarique Andrea, Rubin Battino, Kulbir Birdi, John
Edsall, Zeev Elkoshi, Waiter Kauzmann, Eddie Morild, David Oakenfull,
Zvi Rappoport, Harold Scheraga, and Jacob Wilf.
The book was typed by Ms. Doris Ganeor and the art work of most
of the figures was done by Ms. Rachel Behrend. Their help and patience
are gratefully acknowledged.
Arieh Ben-Nairn
Jerusalem, Israel
Contents
Chapter 1. Introduction and Fundamental
Equations
1.1. What Are Hydrophobic Interactions? . . ...... . 1
1.2. The Fundamental Expression for the Chemical Potential . 6
1.3. Definition of Hydrophobic Interaction (HI) 15
1.4. Probability Interpretation 19
1.5. Integrals Involving gss(R) . . . . . . . . 22
Chapter 2. Very Dilute Solutions and Hydrophobic
Interaction
2.1. Introduction 25
2.2. Standard Free Energies of Transfer ". 26
2.3. Some Numerical Examples of Ll.ufr between Pure Phases 28
2.4. Some Numerical Examples of Ll.ufr between More Complex Phases 34
2.5. Survey of Theoretical Methods of Estimating Ll.u~(G -+ W) 36
2.6. Some Words of Caution . 42
Chapter 3. Pairwise Hydrophobic Interaction (HI)
3.1. Introduction. . . . . . . . . . . . . . 49
3.2. The Full Pair Correlation Function gss(R) .. 51
3.3. Values of <5GHI at One Specific Distance. . . 57
3.4. Effect of Variation of the Solvent on <5GHI(O'l) 64
3.5. Solute-Solute Affinity at Infinite Dilution . . 73
3.6. Solute-Solute Affinity at Finite Concentrations 78
3.7. Dimerization Equilibrium . . 83
xi
xii Contents
3.8. Ion-Pair Formation by Double Long-Chain Electrolytes . . . . 89
3.9. Kinetics of Aminolysis by Long-Chain Alkylamines of Some
Long-Chain Esters of p-Nitrophenol . . . 97
3.10. Application of NMR and ESR Techniques. . . . . 108
Chapter 4. Hydrophobic Interaction among
Many Solute Particles
4.1. Introduction. . . . . . . . . . . . . . . . . . . . . 117
4.2. A Specific Example for Demonstrating the Nonadditivity Effect. 121
4.3. The Relation between HI and the Standard Free Energy of
Aggregation . . . . . . . . . . . . . . . . . . . . . 127
4.4. Approximate Measure of the HI among m Solute Particles . 131
4.5. An Improved Approximate Measure of the HI . 137
4.6. Application of the Scaled-Particle Theory (SPT) 142
4.7. A Direct Measure of Intramolecular HI . 155
4.8. HI in Aqueous Micellar Solutions 164
4.9. HI in Synthetic and Biological Polymers. 176
Chapter 5. Temperature and Pressure Dependence
of the Hydrophobic Interactions
5.1. Introduction. . . . . . . . . . . . . . . . . . . . . . . .. 185
5.2. Temperature and Pressure Derivatives of the Standard Free Energy
of Transfer . . . . . . . . . . . . . . . . . . . . . . . . . 186
5.3. Temperature and Pressure Derivatives of the HI . . . . . . . . . 198
5.4. Experimental Observations on the Temperature Dependence of the HI 201
5.5. Experimental Information on the Pressure Dependence of the HI . . 209
5.6. Formal Statistical Mechanical Expressions for the Entropy, Enthalpy,
and Volume Changes Associated with the HI . 219
5.7. Definition of the Structure of Water (SOW) . . . . 222
5.8. How Much Structural Change in the Solvent? . . . 228
5.9. Effect of Structural Changes in the Solvent (SCIS) on the
Thermodynamics of Solubility and HI . 234
5.10. A Simple Solvable Model . . . . . . 240
A/iltHltdixes
A.l. The Fundamental Equation for the Chemical Potential in a
Two-Com~onent System. . . . . . . . . . . . . . . 259
A.2. Conditional Standard Free Energy of Transfer . . . . . 265
A.3. Dimerization Free Energy of Molecules of the Type R-Y 271
A.4. Rudiments of t·he Scaled-Particle Theory (SPT). . . . . 276