Direct observation of semiconduction and photovoltaic behaviour in single molecules of the Photosystem II reaction centre

Scanning tunnelling spectroscopy of single intact Photosystem II molecules was used to directly show that the reaction centre behaves as an organic se miconductor and photovoltaic. The main electron conduction mechanisms obser ved were (a) semiconduction associated with the normal step-wise linear for ward electron transfer pathway OEC-Y-z-P680-pheo-Q(A)-Q(B), (b) a photovolt aic effect, (c) tunnelling to surface vibronic and delocalized states, and (d) hopping conduction between polypeptide residues. Parameters for these c onduction processes were obtained for both dark and illuminated conditions. The photovoltaic behaviour of the reaction centre contrasts with the photo conductive behaviour of the associated light-harvesting complex LHC-II. (C) 2000 Elsevier Science B.V. All rights reserved.