Table 1.

Comparison between symmetric H-bonds and asymmetric H-bonds.

	single-well H-bond, LBHB	standard H-bond
potential shape	symmetric	asymmetric
pKa difference	close to zero	present
donor–acceptor length	short	long
coupling	large	small
proton transfer	most efficienta	unfavourable
salt-bridge character	low	high
stability (in protein)	less stableb	stable
	in particular when ‘pKa matching’ is satisfied transiently near the protein bulk surface → often leading to conformational changesc

^aSee [5–7].

^bSee [8,9].

^cE.g. [10,11].