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Kosmotropes and Chaotropes
The terms 'kosmotrope' (order-maker) and 'chaotrope' (disorder-maker) originally denoted solutes that stabilized, or destabilized respectively, proteins and membranes. Later they referred to the apparently correlating property of increasing, or decreasing respectively, the structuring of water. Although useful, the terminology may sometimes be misleading as such properties may vary dependent on the circumstances, method of determination or the solvation shell(s) investigated. For example a solute may not always act in the same way at different concentrations or in the presence of macromolecules or gels [276]. Also some solutes with less well-defined properties (e.g. urea) are sometimes classified as kosmotropes [276] and sometimes as chaotropes [283]. An alternative term used for kosmotrope is 'compensatory solute' as they have been found to compensate for the deleterious effects of high salt contents (which destroy the natural hydrogen bonded network of water) in osmotically stressed cells, but again behavior as a kosmotrope in one system does not mean that a solute may behave as a 'compensatory solute' in another or even that it will stabilize the structuring of water in a third. Both the extent and strength of hydrogen bonding may be changed independently by the solute but either of these may be, and has been, used as measures of order making. It is, however, the effects on the extent of quality hydrogen bonding that is of overriding importance as true kosmotropes shift the ESCS equilibrium to the left and chaotropes shift it to the right.
Ionic kosmotropes should be treated differently from non-ionic kosmotropes, due mainly to the directed and polarized arrangements of the surrounding water molecules. Generally, ionic behavior parallels the Hofmeister series. Large singly charged ions, with low charge density (e.g. SCN-, H2PO4-, HSO4-, HCO3-, I-, Cl-, NO3-, NH4+, Cs+, K+, (NH2)3C+ (guanidinium) and (CH3)4N+ (tetramethylammonium) ions; exhibiting weaker interactions with water than water with itself and thus interfering little in the hydrogen bonding of the surrounding water), are chaotropes whereas small or multiply-charged ions, with high charge density, are kosmotropes (e.g. SO42-, HPO42-, Mg2+, Ca2+, Li+, Na+, H+, OH- and HPO42-, exhibiting stronger interactions with water molecules than water with itself and therefore capable of breaking water-water hydrogen bonds; ionic radii > 1.06Å for cations and > 1.78Å for anions [284]). Thus the hydrogen bonding between water molecules is more broken in the immediate vicinity of ionic kosmotropes than ionic chaotropes. It is not unreasonable that a solute may strengthen some of the hydrogen bonds surrounding it (structure making; e.g. kosmotropic cations will strengthen the hydrogen bonds donated by the inner shell water molecules) whilst at the same time breaking some other hydrogen bonds (structure breaker; e.g. kosmotropic cations will weaken the hydrogen bonds accepted by the inner shell water molecules) [274], so adding to the confusion in nomenclature. Other factors being equal, water molecules are held more strongly by molecules with a net charge than by molecules with no net charge; as shown by the difference between zwitterionic and cationic amino acids [532].
Weakly hydrated ions (chaotropes, K+, Rb+, Cs+, Br-, I-, guanidinium+) may be 'pushed' onto weakly hydrated surfaces by strong water-water interactions with the transition from strong ionic hydration to weak ionic hydration occurring where the strength of the ion-water hydration approximately equals the strength of water-water interactions in bulk solution (with Na+ being borde
The terms 'kosmotrope' (order-maker) and 'chaotrope' (disorder-maker) originally denoted solutes that stabilized, or destabilized respectively, proteins and membranes. Later they referred to the apparently correlating property of increasing, or decreasing respectively, the structuring of water. Although useful, the terminology may sometimes be misleading as such properties may vary dependent on the circumstances, method of determination or the solvation shell(s) investigated. For example a solute may not always act in the same way at different concentrations or in the presence of macromolecules or gels [276]. Also some solutes with less well-defined properties (e.g. urea) are sometimes classified as kosmotropes [276] and sometimes as chaotropes [283]. An alternative term used for kosmotrope is 'compensatory solute' as they have been found to compensate for the deleterious effects of high salt contents (which destroy the natural hydrogen bonded network of water) in osmotically stressed cells, but again behavior as a kosmotrope in one system does not mean that a solute may behave as a 'compensatory solute' in another or even that it will stabilize the structuring of water in a third. Both the extent and strength of hydrogen bonding may be changed independently by the solute but either of these may be, and has been, used as measures of order making. It is, however, the effects on the extent of quality hydrogen bonding that is of overriding importance as true kosmotropes shift the ESCS equilibrium to the left and chaotropes shift it to the right.
Ionic kosmotropes should be treated differently from non-ionic kosmotropes, due mainly to the directed and polarized arrangements of the surrounding water molecules. Generally, ionic behavior parallels the Hofmeister series. Large singly charged ions, with low charge density (e.g. SCN-, H2PO4-, HSO4-, HCO3-, I-, Cl-, NO3-, NH4+, Cs+, K+, (NH2)3C+ (guanidinium) and (CH3)4N+ (tetramethylammonium) ions; exhibiting weaker interactions with water than water with itself and thus interfering little in the hydrogen bonding of the surrounding water), are chaotropes whereas small or multiply-charged ions, with high charge density, are kosmotropes (e.g. SO42-, HPO42-, Mg2+, Ca2+, Li+, Na+, H+, OH- and HPO42-, exhibiting stronger interactions with water molecules than water with itself and therefore capable of breaking water-water hydrogen bonds; ionic radii > 1.06Å for cations and > 1.78Å for anions [284]). Thus the hydrogen bonding between water molecules is more broken in the immediate vicinity of ionic kosmotropes than ionic chaotropes. It is not unreasonable that a solute may strengthen some of the hydrogen bonds surrounding it (structure making; e.g. kosmotropic cations will strengthen the hydrogen bonds donated by the inner shell water molecules) whilst at the same time breaking some other hydrogen bonds (structure breaker; e.g. kosmotropic cations will weaken the hydrogen bonds accepted by the inner shell water molecules) [274], so adding to the confusion in nomenclature. Other factors being equal, water molecules are held more strongly by molecules with a net charge than by molecules with no net charge; as shown by the difference between zwitterionic and cationic amino acids [532].
Weakly hydrated ions (chaotropes, K+, Rb+, Cs+, Br-, I-, guanidinium+) may be 'pushed' onto weakly hydrated surfaces by strong water-water interactions with the transition from strong ionic hydration to weak ionic hydration occurring where the strength of the ion-water hydration approximately equals the strength of water-water interactions in bulk solution (with Na+ being borde
