Low-barrier hydrogen bond
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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 [1].
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 [1].
A low-barrier hydrogen bond was found inside a certain aza crown-type compound depicted below [1]:
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.
References
Chemical bonds | |||||
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| "Strong" | |||||
| "Weak" |
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| other | Disulfide bond · Peptide bond · Phosphodiester bond | ||||
| Note: the weakest strong bonds are not necessarily stronger than the strongest weak bonds | |||||
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