TABLE 1.

Geometry of the H-bond network at Y_Z/D after energy optimization of Y_Z/D and D1-His¹⁹⁰/D2-His¹⁸⁹ side chains in the presence of negatively charged tyrosine

	Distance [Å]
Atom pair	Original*	Optimized†
YZ
NɛHis‡-OηYZ§	2.7	2.7
NδHis‡-OδAsn¶	3.3	4.1
Obicarbonate-OηYZ§	3.4	2.9
OH2O-OηYZ§	(2.7)‖	(2.6)‖
OH2O-NɛHis‡	(3.4)‖	(3.0)‖
YD
NɛHis‡-OηYD§	3.0	2.6
NδHis‡-NɛArg**	2.4	3.2
NδHis‡-NηArg**	3.2	3.5

^*Based on the atomic coordinates of the PSII crystal structure (1).

^†Based on energy optimized atomic coordinates of PSII with CHARMM (see text).

^‡D1-His¹⁹⁰ for Y_Z and D2-His¹⁸⁹ for Y_D.

^§Hydroxyl oxygen atom of Y_Z/D.

^¶Side-chain oxygen atom of D1-Asn²⁹⁸.

^‖The H₂O molecule was placed in the PSII crystal structure between the Mn-cluster Ca²⁺ and Y_Z. It was first energetically optimized alone and subsequently together with Y_Z and the D1-His¹⁹⁰ side chains.

^**Side-chain nitrogen atom of D2-Arg²⁹⁴.