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
Finally, we come to hydrogen bonding, which plays an enormously important role in biological processes. It is found in molecules containing an OH group (a hydroxyl group) including water, ethanol, methanol, and numerous other molecules. Like van der Waals interactions, hydrogen bonding is weaker than ionic and covalent bonds. Hydrogen bonds are typically 10–100 times weaker than covalent bonds, depending on the molecules involved.
A hydrogen atom, having one electron, can be covalently bonded to only one atom. However, the hydrogen atom can involve itself in an additional electrostatic bond with a second atom of highly electronegative character, such as fluorine or oxygen. This second bond is a hydrogen bond.
How does hydrogen bonding work? Let's go through the basic points using water:
The charge density in a covalent bond such as an OH bond is highly asymmetric, and the center of charge is much closer to the O atom, as we discovered when discussing the dipole in HCl. For the same reasons, the OH bond also has this dipole character.
This leaves the net positively charged H atom behaving more like a lone, positively charged proton (H+).
Other electrons on the O atom distribute themselves so as to minimize repulsion (Figure 3.15, green lobes). These electrons form lobes of electron density on the opposite side of the O atom to the OH bond.
Another H2O molecule orients itself so that its positively charged H is now close to these negatively charged electrons.
Figure 3.15 Hydrogen bonding in water ice. The dotted lines show the hydrogen bonding. The green lobes are the electrons on oxygen that take part in the interaction with the hydrogen atom on other water molecules.
You can see the result of these interactions much more clearly in Figure 3.15, which shows the hydrogen bonding in water ice depicted in two dimensions.
3.10.1 Hydrogen Bonds and Life
Hydrogen bonding is found in many molecular interactions in life. In the next chapter, we discuss in more detail the structure of the information storage molecule of life: DNA. However, here, one feature of DNA is worth exploring: the role of hydrogen bonding in holding the molecule together. DNA is made up of two complementary strands that bind together to form its characteristic double helix shape. These two strands are held together down their middle by hydrogen bonding.
In the diagram in Figure 3.16 you can see the structure of a DNA double helix that has been flattened into a two-dimensional depiction (from its normal three-dimensional helical structure). Along the two edges of the structure, you can see the pentagonal deoxyribose sugars that make up its backbone linked together with phosphate groups. In the middle are the base pairs. The bases are linked in pairs with dotted lines showing the hydrogen bonding between them. The sequence of individual bases along a DNA strand encodes the genetic information. There are four bases: adenine (A), thymine (T), guanine (G), and cytosine (C). Adenine can only bind with thymine, and cytosine can only bind with guanine. There are two hydrogen bonds for the pairing between adenine and thymine bases, and three hydrogen bonds between guanine and cytosine.
Figure 3.16 Hydrogen bonding in the molecule DNA. (a) The dotted lines in the middle of the flattened two-dimensional molecule on the left are the hydrogen bonds that hold the two strands together. On the right (b) is the three-dimensional double helix.
Source: Reproduced with permission of wikicommons, Michael Ströck.
The hydrogen bonding is just strong enough to hold the strands together so that DNA does not fall apart easily. However, the hydrogen bonds are
