Proton transfer mechanism of 1,3,5-tri(2-benzimidazolyl) benzene with a unique triple-stranded hydrogen bond network as studied by DFT-MD simulations

The proton transfer pathway for redox active tyrosine D (TyrD) in photosystem II is a hydrogen-bond network that involves D2-Arg180 and a series of water molecules. Using quantum mechanical/molecular mechanical calculations, the detailed properties of the energetics and structural geometries were investigated. The potential-energy profile of all hydrogen bonds along the proton transfer pathway indicates that the overall proton transfer from TyrD is energetically downhill. D2-Arg180 plays a key role in the proton transfer pathway, providing a driving force for proton transfer, maintaining the hydrogen-bond network structure, stabilising P680•+, and thus deprotonating TyrD-OH to TyrD-O•. A hydrophobic environment near TyrD enhances the electrostatic interactions between TyrD and redox active groups, e.g. P680 and the catalytic Mn4CaO5 cluster: the redox states of those groups are linked with the protonation state of TyrD, i.e. release of the proton from TyrD. Thus, the proton transfer pathway from TyrD may ultimately contribute to the conversion of S0 into S1 in the dark in order to stabilise the Mn4CaO5 cluster when the photocycle is interrupted in S0.

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Hydration and Proton Transfer in 3M™ PEM Ionomers: An Ab Initio Study

MRS Proceedings ◽

10.1557/opl.2012.187 ◽

2012 ◽

Vol 1384 ◽

Cited By ~ 1

Author(s):

Jeffrey K. Clark ◽

Stephen J. Paddison

Keyword(s):

Hydrogen Bond ◽

Proton Transfer ◽

The State ◽

Side Chain ◽

Water Molecules ◽

Side Chains ◽

Hydrogen Bond Network ◽

Group Separation ◽

Proton Dissociation ◽

Bond Network

ABSTRACTElectronic structure calculations were performed to study the effects local hydration, neighboring side chain connectivity, and protogenic group separation have in facilitating proton dissociation and transfer in fragments of 3M ionomers under conditions of low hydration. Two different types of ionomers, each consisting of a poly(tetrafluoroethylene) (PTFE) backbone, were considered: (1) perfluorosulfonic acid (PFSA) ionomeric fragments containing two pendant side chains (–O(CF2)4SO3H) of distinct separation along the PTFE backbone to model different equivalent weight ionomers and (2) single side chain fragments of three bis(sulfonyl imide)- based fragments with multiple and distinct acid groups per side chain having structural and chemical differences mediating protogenic group separation (side chains: –O(CF2)4SO2(NH)- SO2C6H4SO3H) with the sulfonic acid group located in either the meta or the ortho position on the phenyl ring and –O(CF2)4SO2(NH)SO2(CF2)3SO3H). Fully optimized structures of these fragments with and without the addition of water molecules at the B3LYP/6-311G** level revealed that both side chain connectivity and protogenic group separation, along with local hydration, are key contributors to proton dissociation and the energetics of proton transfer in these materials. Specifically, cooperative interaction between protogenic groups through hydrogen bonding and electron withdrawing –CF2– groups are critical for first proton dissociation and the state of the dissociated proton at low levels of hydration. However, the close proximity of protogenic groups in the ortho bis acid precluded second proton dissociation at low hydration as the relatively fixed protogenic group separation promoted interactions between water molecules, while the labile side chains in the PFSA ionomers allowed for greater freedom in the hydrogen bond network formed. Potential energy profiles for proton transfer were determined at the B3LYP/6-31G** level. The energetic penalty associated with proton transfer was found to be strongly dependent on the surrounding hydrogen bond network and the state of the dissociated proton(s), as well as, the separation between protogenic groups.

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