Photosystem I (PS 1), a multi-subunit protein-chlorophyll complex that mediates vectorial, light-induced electron transfer has been self-assembled as an oriented monolayer on a solid metal surface. Unique cysteine mutants are induced for the attachment of PS I through the formation of sulfide bonds. A photopotential of approximately +1 V is observed (see Figure) for single trimer molecules or the monolayer continuum.