On the physical basis for the cis-positive rule describing protein orientation in biological membranes

AbstractThe topology of hydrophobic intramembrane proteins is characterized by a statistical asymmetry in the distribution of positively-charged residues on the two sides of the membrane, the ‘inside- or cis-positive rule’. A mechanism is proposed involving only neutral residue transfer. For a tightly bound polypeptide adsorbed on the membrane and not at equilibrium, the pK values of the ionic residues related to dissociation of the proton into the aqueous phase bulk are increased because of interaction with the negative charges at the membrane surface. The pK shift would selectively neutralize aspartate and glutamate residues, favoring their translocation across the membrane, while stabilizing the impermeant positively charged state of lysine and arginine residues

Coupled electron transfers in artificial photosynthesis

Light-induced charge separation in molecular assemblies has been widely investigated in the context of artificial photosynthesis. Important progress has been made in the fundamental understanding of electron and energy transfer and in stabilizing charge separation by multi-step electron transfer. In the Swedish Consortium for Artificial Photosynthesis, we build on principles from the natural enzyme photosystem II and Fe-hydrogenases. An important theme in this biomimetic effort is that of coupled electron-transfer reactions, which have so far received only little attention. (i) Each absorbed photon leads to charge separation on a single-electron level only, while catalytic water splitting and hydrogen production are multi-electron processes; thus there is the need for controlling accumulative electron transfer on molecular components. (ii) Water splitting and proton reduction at the potential catalysts necessarily require the management of proton release and/or uptake. Far from being just a stoichiometric requirement, this controls the electron transfer processes by proton-coupled electron transfer (PCET). (iii) Redox-active links between the photosensitizers and the catalysts are required to rectify the accumulative electron-transfer reactions, and will often be the starting points of PCET