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According to my textbook, "Too much or too little salt can interfere with the hydrogen bonds that hold an enzyme in its three-dimensional shape". I know that NaCl is held together by an ionic bond, meaning that one atom is strongly positive (in this case sodium) and one atom is strongly negative (in this case chlorine). I would guess the polarity of salt has something to do with its effect on hydrogen bonds, which consist of a hydrogen atom which bonds weakly to a strongly electronegative atom, resulting in a similar behavior to its ionization in glass of water, since water molecules are also polar and form hydrogen bonds with one another.

My question is, how exactly does salinity affect the bonds of an enzyme?

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  • $\begingroup$ I can't find a good reference for this, but I know the DNA double helix is only stable in the presence of cations such as Na+, K+, or Mg2+. The cations help neutralize the negative charges on the phosphate backbone and prevent repulsion. This allows the bases to form hydrogen bonds and hold the structure together. Without the cations the hydrogen bonds wouldn't have enough strength. Something similar may happen in proteins, where ions help to neutralize charges or bridge certain amino acids together and allow the hydrogen bond to form. $\endgroup$
    – user137
    Commented Dec 26, 2014 at 4:57
  • $\begingroup$ @user137 In the presence of water (which is the usual state in living cells) there are no ions bound to DNA. $\endgroup$
    – Chris ♦
    Commented Dec 26, 2014 at 8:54
  • $\begingroup$ @Chris But there has to be some interaction, when using RNA I have to use a 150mM NaCl buffer to get Poly A and Poly U to bind well. $\endgroup$
    – user137
    Commented Dec 27, 2014 at 5:13
  • $\begingroup$ @user137 I am not saying that there is no interaction (which is of course present), but not in the form of "bound" positive ions to the DNA backbone. $\endgroup$
    – Chris ♦
    Commented Dec 27, 2014 at 9:52

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Sodium Chloride is not "polar". It is an ionic compound and dissociates into the respective ions in water (because of the latter's high dielectric constant).

Hydrogen bonds are similar to of dipole-dipole interactions (van der Waals) but much stronger and are therefore classified as a separate type of bond. However excess salt can lead to ion-dipole interactions (which are stronger than H-bond) that may prevent the formation of the hydrogen bond.

Ions such as Cl - can act as chaotropes (cause disorder)[ref]. K+ is also chaotropic while Na+ is kosmotropic (reduces disorder). However both Na+ and Cl - affect the entropy of water very slightly. So the effect of NaCl should be mostly due to ion-dipole interactions.

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