Low-barrier hydrogen bond

When pKa of the heteroatoms is closely matched, a LBHB becomes possible at a shorter distance (~2.55 Å).

LBHBs also occur on the surfaces of proteins, but are unstable due to their proximity to bulk water, and the conflicting requirements of strong salt-bridges in protein-protein interfaces.

[4] Low-barrier hydrogen bonds have been proposed to be relevant to enzyme catalysis in two types of circumstance.

[7][8] However, in 2012, a low-barrier hydrogen bond has been proposed to be involved in phosphate-arsenate discrimination for a phosphate transport protein.

[9] This finding might indicate the possibility of low-barrier hydrogen bonds playing a catalytic role in ion size selection for some very rare cases.

Energy profiles for different hydrogen bond types between oxygen heteroatoms . Standard hydrogen bonds are asymmetrical, with the hydrogen being associated with one heteroatom. When the pKa between the heteroatoms is equal, a symmetrical hydrogen bond forms with the hydrogen in equilibrium between two locations. At shorter distances, the barrier between the two energy minima is low enough that the hydrogen is equally bound as a low-barrier, or single-well hydrogen bond.
In this aza crown -type, macrocyclic compound, a proton sits between two amide carbonyl oxygens separated by a distance of 2.45 Å. [ 2 ]