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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 is 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 .
Additional recommended knowledge
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 :
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.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Low-barrier_hydrogen_bond". A list of authors is available in Wikipedia.|