Table 3.

Summary of the properties of short H-bonds in the crystal structures.

H-bond	Tyr-OH···N-His	His-NH···−O-QB-OH	Glu-COOH···−O-pCAa
geometry
protein	PSII	PSII	PYP
state	n.d.	QBH−/QBH2	pRCW
donor	D1-Tyr161-OH	D2-His214-NH	Glu46-OH
acceptor	D1-His190-N	−O-QB-OH	−O-pCA
distance (X-ray) (Å)	2.46b	2.47b	2.47c
distance (QM/MM) (Å)	2.47	2.47	2.49
short H-bond formation
trigger	n.d.	electron	photon
mechanism	n.d.	QBH− formation → increase in pKa(Oprox)	photoinduced trans–cis isomerization of pCA → removal of an H-bond from Tyr42
pKa (bulk solvent)	approximately 9···approximately 7	approximately 14···approximately 5d	approximately 4···approximately 9
stability (protein)	stable	transient	transient
associated conformational change	(no significant conformational change)	proton transfer → QB release from the PSII protein	proton transfer → pB state formation
location	inner core	near the bulk surface	near the bulk surface

^apK_a(Glu46) > pK_a(pCA) in the PYP protein environment (in the initial pG state).

^bSee [1].

^cSee [58].

^dAs pK_a(Q_BH^·/Q_B^·−) [79,80].