LBHB

Low-barrier hydrogen bonds (LBHBs) are a type of hydrogen bond in which the proton is shared almost equally between two electronegative atoms, typically oxygen or nitrogen, creating an unusually short and strong interaction. The concept arises when the donor and acceptor have closely matched pKa values, leading to a shallow or almost single-well potential for proton position and allowing quantum delocalization of the proton across the two centers.

LBHBs are typically characterized by very short donor–acceptor distances, often around 2.5 Å or less, and by spectroscopic and structural features consistent with a highly stabilized, shared proton. Experimental indicators can include downfield shifts of the involved proton in NMR, specific infrared or vibrational signatures, and neutron or X-ray diffraction data that suggest proton sharing. In theory, the strength and barrier height of the bond depend on the relative pKa values and the geometry of the pair.

In biochemistry and enzymology, LBHBs have been proposed as a mechanism to stabilize transition states or facilitate proton transfer during catalysis. They have been suggested in various enzyme active sites, including discussions around catalytic dyads or triads. However, the functional significance of LBHBs remains debated.

Critics argue that many claims can be explained by conventional strong hydrogen bonds with short distances or by conformational effects and electrostatics, rather than by true low-barrier, symmetrically shared protons. Consequently, while LBHBs may occur in some systems, their prevalence and role in catalysis are not universally accepted, and evidence is system-specific.