Low-barrier hydrogen bond

A Low-barrier hydrogen bond or LBHB is a special type of hydrogen bond. This type of bond is especially strong because the distance between acceptor and donor is especially short. In regular hydrogen bonds (for example the O-H...O distance is at least 2.8 Ångström) the hydrogen ion clearly belongs to one of the heteroatoms. When the distance decreases to about 2.55 Å the proton is free to move between the two atoms (hence the low-barrier) and the LBHB forms. When the distance decreases even further (< 2.29 Å) the bond is characterized as a short-strong hydrogen bond or SSHB.

Low-barrier hydrogen bonds are especially relevant to enzyme catalysis because when they form in an transition state they can significantly accelerate otherwise difficult reactions.

A low-barrier hydrogen bond was found inside a certain aza crown-type compound depicted below :


 * [[Image:Encircledproton.png|400px|Encircled proton Day 2007]]

In this compound a proton sits comfortably between two amide carbonyl oxygens separated by a distance of 2.45 Å. It would not be expected there because the macrocyclic ring already has two positively charged quaternary ammonium units.