Single-molecule electron tunneling spectroscopy of the higher plant light-harvesting complex LHC II

Electronic spectroscopy of a single biological molecule is demonstrated wit h similar to 4 Angstrom spatial resolution. The light-harvesting complex II (LHC II), in the ground and photo-excited states, was studied using scanni ng tunneling microscopy and spectroscopy of intact Photosystem II complexes , analysis of the spectra indicates that the main mechanisms of tunneling b etween the STM tip and the surface involve delocalized electronic states of the LHC II and local vibronic states associated with C=C, C=O, C-H, N-H, a nd O-H groups near the LHC II surface. Conduction within the bulk LHC II is then due to ohmic and hopping conduction as well as tunneling between amin o acid residues. Light activation of LHC II occurs via a photoconductive ra ther than a photovoltaic mechanism. There is a dramatic light-induced incre ase in the electronic density of states indicating a light-induced enhancem ent of energy and electron delocalization which is important for the effici ent and rapid transfer of excitation energy from LHC II to the Photosystem II reaction center. (C) 1999 Academic Press.