Dear Maria,

Your question has at least two layers. It is very frequently used term with a very fuzzy meaning. It is used much more in casual science conversations then in serious science treatment of water. If it comes to common uses and definitions Google is full of them, so go and read. On the very superficial level it is just a propensity of water molecules to arranged around objects in water. But the casual conversation must stop at this level because a more serious look indicate that this has three separate meanings (therefore the second layer).

1) Many hydrophobic surfaces create clathrates. These are networks of waters that are not forming any productive hydrogen bonds with the underlying surface. Therefore the water molecules are forced to form a strong hydrogen bonded network among themselves. This traps a lot of energy mostly in the form of Entropic arrangement. Every amino acid has at least some hydrophobic surfaces including charged aa. This also contributes to the so called hydrophobic effect.

2) Hydration to any surface contains the mixture of directly coordinated water molecules and nonbonded but not formally forming clathrates. These pseudolayers have an interesting structure that has slightly more and slightly less pseudo layers intermixed depending on the distance from the surface of a regular protein. So indeed in different analyses people call them shells. The Wiki info that they extend to 20Å is just simply rubbish. Hydrodynamic measurmets would give you a different number. Direct diffraction experiments different. Spectroscopy different and there is no consensus what it means.

3) The third meaning is associated with direct screening effects and the first coordination sphere for metal ions or ions in general. Depending on the electronegativity that is loosely associated with a formal charge and ionic radius (that is one of these fuzzy not very well defined characteristics) water molecules form elaborate structures around ions. The most famous are tetrahedral and octahedral which basically means the propensity for four or six water molecules to surround the ion. Every ion can accept (live) in many different coordination spheres so we are only talking about the propensity (probability). So for instance Calcium likes coordination number 7 but that does not mean that 6 or even 5 or 8 would not be found. Larger ions (becasue of the increased ionic radius like on average more. Dehydration of any of these directly coordinated waters cost a lot of energy and this is the basis for selectivity of ion channels.

So as a practical advice go and have a look at Google searches for:

- hydration shell

- ionic radius.

You have to learn how to discriminate info because for instance Wiki page corresponding to the hydration shell is literally worthless.

Dear Maria,

Your question has at least two layers. It is very frequently used term with a very fuzzy meaning. It is used much more in casual science conversations then in serious science treatment of water. If it comes to common uses and definitions Google is full of them, so go and read. On the very superficial level it is just a propensity of water molecules to arranged around objects in water. But the casual conversation must stop at this level because a more serious look indicate that this has three separate meanings (therefore the second layer).

1) Many hydrophobic surfaces create clathrates. These are networks of waters that are not forming any productive hydrogen bonds with the underlying surface. Therefore the water molecules are forced to form a strong hydrogen bonded network among themselves. This traps a lot of energy mostly in the form of Entropic arrangement. Every amino acid has at least some hydrophobic surfaces including charged aa. This also contributes to the so called hydrophobic effect.

2) Hydration to any surface contains the mixture of directly coordinated water molecules and nonbonded but not formally forming clathrates. These pseudolayers have an interesting structure that has slightly more and slightly less pseudo layers intermixed depending on the distance from the surface of a regular protein. So indeed in different analyses people call them shells. The Wiki info that they extend to 20Å is just simply rubbish. Hydrodynamic measurmets would give you a different number. Direct diffraction experiments different. Spectroscopy different and there is no consensus what it means.

3) The third meaning is associated with direct screening effects and the first coordination sphere for metal ions or ions in general. Depending on the electronegativity that is loosely associated with a formal charge and ionic radius (that is one of these fuzzy not very well defined characteristics) water molecules form elaborate structures around ions. The most famous are tetrahedral and octahedral which basically means the propensity for four or six water molecules to surround the ion. Every ion can accept (live) in many different coordination spheres so we are only talking about the propensity (probability). So for instance Calcium likes coordination number 7 but that does not mean that 6 or even 5 or 8 would not be found. Larger ions (becasue of the increased ionic radius like on average more. Dehydration of any of these directly coordinated waters cost a lot of energy and this is the basis for selectivity of ion channels.

So as a practical advice go and have a look at Google searches for:

- hydration shell

- ionic radius.

You have to learn how to discriminate info because for instance Wiki page corresponding to the hydration shell is literally worthless.

Boguslaw, thank you so much for that quick and informative reply!

I would like to estimate what would happen in terms of water molecules if we have an ion exchange system on a membrane that transports, e.g. ion A+ in one direction and B+ in the other direction. If the number of coordinated water molecules would be very different between A and B, the ion exchange would also cause a relative water flux. So I guess this question relates mostly to the ionic radius.

Dear Maria,

Passive versus active transport is a great and very active research area. If I understand you are talking about a passive transport. A single membrane design is an interesting approach but you have to take into the account simple probabilistic transport properties that would intricately depend on the very atomistic details of the membrane. It is very difficult to advise without sufficient details.

Stéphane congratulations on quick and appropriate citations. The only thing I could add would be there is a ton of crystallography literature about water interactions with proteins and individual amino acids. For instance several papers of my collaborator and ex mentor M. Teeter.

Hi Maria, in ion channels and membrane transporters, ions are dehydrated before they are translocated. This means that a difference in the average number of coordinated water molecules between ion A an B will not lead to a net flux of water across the membrane.

Hi Christian, Guus, Stéphane,

thanks for all this information, it was very helpful. I also found this publication that can be interesting:

http://pubs.acs.org/doi/abs/10.1021/cr2000948 (Burakowski & Glinski, Chem. Rev., 2012, 112 (4), pp 2059–2081)