the extraction and purification of metals from compounds naturally found in the environment. What do you think? Would alien life be any different?
3.9 Van der Waals Interactions
We have shown how atoms and molecules can be held together by ionic and covalent bonding. There are two other types of bonding that play a crucial role in holding matter together and take prominent roles in life. These two bonding types are generally involved in bonding between molecules. These bonds are generally much weaker than ionic or covalent bonds. They are van der Waals interactions and hydrogen bonding. We first examine van der Waals interactions.
Sometimes also called van der Waals forces, these van der Waals interactions are in the order of a hundred times less strong than covalent bonds. Nevertheless, they are essential in biological systems.
There are three categories of van der Waals forces.
3.9.1 Dipole–Dipole (Keesom) Forces
Molecules have a charge distribution that is never quite even (i.e. it has a directionality or anisotropy), which results from the non-even distribution of the electrons and their negative charge. This uneven electron charge distribution results in a small permanent electric dipole across the length of the molecule.
In Figure 3.11, you can see this schematically illustrated using hydrogen chloride. Chlorine has a higher electronegativity than hydrogen. In other words, it tends to pull electrons toward it. As it tugs hydrogen's electron toward it, it gets a very slight negative charge. This also means that the hydrogen atom develops a slight positive charge (seen from the point of view of the hydrogen atom, the positive charge of its proton is not quite cancelled out by the negative charge of its electron, which is now more associated with the chlorine atom). The result is that the hydrogen atom has a very slight net positive charge. Hence, the molecule now has a polarity or dipole. Now, if two of these molecules meet up, the slightly negatively charged chlorine atom will be attracted to the positively charged hydrogen on the other molecule; they attract one another like small bar magnets (Figure 3.11).
Figure 3.11 The dipoles of two HCl molecules involved in Keesom interactions. They attract each other like tiny bar magnets.
3.9.2 Dipole-Induced Dipole (Debye; Pronounced Deh-beye) Forces
For the second category, van der Waals interactions can be induced in a molecule that has no charge. In this type of interaction, one of the molecules has a charge (HCl), but the other does not, for example as shown in Figure 3.12 with neon. Nevertheless, a charge can be induced in neon by the presence of the charge in HCl, which influences the electron distribution in the neighboring atom or molecule. As a result, the molecules are attracted.
Figure 3.12 An induced dipole in the otherwise uncharged neon is an example of a Debye interaction.
3.9.3 Dispersion Forces
Yet another type of van der Waals interaction is one in which neutral atoms can attract one another. The electron distribution, even in neutral atoms and molecules, is never quite evenly spaced out. A charge anisotropy will exist on account of the electrons in orbit around the atom, which, at a snapshot in time, will always be slightly unevenly distributed around the atom (shown greatly exaggerated in Figure 3.13).
Figure 3.13 Charge imbalance (exaggerated here) around an atom results in a small charge distribution in the atom and creates a dipole. This atom can now engage in van der Waals interactions.
This causes a small dipole moment to be established in the atoms or molecules. These dispersion forces or London forces are important for attraction between inert gases (Ne, Ar, etc.) and covalently bonded molecules (H2, N2, CH4, etc.), but exist between all atoms and molecules to some degree.
3.9.4 Van der Waals Interactions and Life
Both inside and outside cells, van der Waals interactions are involved in the attraction between molecules involved in biological systems. A particularly remarkable example of van der Waals forces in action in biology is the attachment of a gecko to a wall of glass. Geckos have many tens of thousands of setae – tiny hairs – on their feet, each of which attaches to a surface using van der Waals interactions between the surface and the tiny projections on the setae (Figure 3.14). The combined force is large enough to hold up the gecko and accounts for their ability to attach to a smooth glass surface.
Figure 3.14 Looking at van der Waals forces in action. A gecko attached to a window. The hairs on its feet interact with the surface using van der Waals interactions.
This example is particularly remarkable, since it provides a lucid demonstration of the way in which evolution has homed in on forces expressed at the molecular level to give selective advantage (the ability to run up vertical flat walls) at the scale of the whole organism.
Focus: Astrobiologists: Andreas Elsaesser
Affiliation: Experimental Biophysics and Space Science, Freie Universität Berlin, Germany
What was your first degree? I obtained a German “Diplom” (equivalent to MSc) in Physics from the Technical University Munich (Germany) and specialized in astrophysics and nuclear physics, while my master's thesis focused on low temperature plasma physics.
What do you study? The main focus of my research is the interaction of radiation with soft matter, organic molecules, and biological systems. I am interested in biomarker (photo)stability, organic mineral interaction, life detection on other planets and moons, habitability, origin of life, and its limits.
What science questions do you address? With my research, I aim to address questions in relation to the evolution and distribution of life in our Solar System and in other parts of the Universe. More specifically, I would like to understand better: How could we possibly detect life on other planets? What molecular signature should we search for? What are the chances of finding extraterrestrial life, and how common is it?
How did you get involved in astrobiology research? I have always been fascinated by the questions of how life evolved on Earth and whether life could be found elsewhere in the Universe. However, only after my PhD in the field of Nano-Biophysics at the University of Ulster (United Kingdom) did I get closely involved in astrobiology research. It was my first postdoctoral position at Leiden University (The Netherlands), where I studied the photostability of organic molecules and potential biosignatures in the laboratory and in space. From then on, I developed my own line of research with projects in the fields of astrobiology and astrochemistry, and today I lead my own research group for biophysics and space sciences at Freie Universität Berlin (Germany).
Photo: Daniel Kunzfeld for VolkswagenStiftung
3.10 Hydrogen Bonding