Ions cooperate in locking water
Ions play a crucial role in a myriad of processes occurring in water, such as the folding and functioning of proteins. However, this role is not well understood and forms a topic of intense debate. AMOLF researchers have used femtosecond (1 fs = 10-15 s) laser techniques to elucidate how ions influence the dynamics of water. The researchers discovered that positively and negatively charged salt ions can have a strong cooperative effect on the structure and dynamics of their surrounding water molecules. Their research findings are published in Science on 21 May 2010.
Both the earth’s surface and the human body consist largely of water, but rarely in its pure form. Generally, water contains solvated salts, such as kitchen salt (NaCl), that split into positively (e.g. Na+) and negatively (e.g. Cl-) charged ions. It is well known that these ions affect the structure and dynamics of the surrounding water molecules, but the strength and extent of this effect have remained elusive. By combining two ultrafast optical experimental techniques – a collaboration between the research groups of Huib Bakker and Mischa Bonn – essential new information on these properties has been obtained.
A water molecule (H2O) consists of a partially negatively charged oxygen atom in between two partially positively charged hydrogen atoms. This charge difference results in a different orientation of water molecules around positively (cation) and negatively (anion) charged ions. Around cations, the oxygen atom points towards the ion (see figure 1); around anions, one of the two OH-groups of the water molecule points towards the ion and forms a hydrogen bond (see figure 2). These interactions between the water molecule and the ion influence the mobility of water, which is of importance for many processes that take place in water, such as (bio-)chemical reactions and conformational changes of proteins.
The researchers have used special femtosecond (1 fs = 10-15 s) laser techniques to measure how fast water molecules reorient in different salt solutions. This reorientation takes place on picosecond time scales (1 ps = 10-12 s). By combining two techniques that are sensitive to reorientation along two different axes of the water molecule – the p (dipole) axis and the µ (OH-group) axis – a complete picture of the effect of ions on water dynamics emerged. Whereas water molecules in bulk water reorient approximately equally fast in all directions, the reorientation of water molecules next to ions is highly anisotropic: the reorientation is much faster around one specific axis than around the other axes (see figures 1 and 2). The water molecules that hydrate an ion are in what can be called a 'semi-rigid hydration shell'. Water molecules surrounding cations reorient in a propeller-like fashion around the p-axis, where the two OH-groups are the rotor blades. Next to an anion a similar effect occurs: now one OH-group keeps pointing towards the ion, whereas the other OH-group is the rotor blade of the propeller. This anisotropic reorientation generally only occurs in the first structural hydration shell around an ion. Depending on the strength of the interaction of the ion with water, the number of water molecules showing this behaviour can reach up to six. Outside of this shell, water behaves as in pure water, meaning that water molecules reorient both around the p-axis and the µ-axis.
A fascinating observation was that the combination of positive and negative ions that both interact strongly with water can lead to a strong rigidity of a large number of water molecules. The ions then cooperate in immobilizing the reorientation around both vectors of the water molecules. This effect reaches much further than just the first structural shell around each separate ion (see figure 3): as a result of this cooperation between the ions up to twenty water molecules can be affected. It is evident that these kind of rigid water structures play an important role for chemical reactions in solutions of ions that interact strongly with water. It is also likely that such cooperativity takes place between ions (such as Ca2+) and oppositely charged functional groups in biological systems (such as proteins or DNA). The effect of ions like Ca2+ on the function of bio-molecular systems may thus have its origin in a cooperative effect on the structure and dynamics of the water molecules between the ions and the bio-molecule(s).
Reference
Cooperativity in Ion Hydration
K.J. Tielrooij, N. Garcia-Araez, M. Bonn and H.J. Bakker,
Science, 21 May 2010.
For more information contact:
Huib Bakker, tel. +31 (0)20 75 47 100
Klaas-Jan Tielrooij, tel. +31 (0)6 14 79 76 37