In order to fabricate a molecular rectifier composed of a donor-acceptor molecular pair, we studied two-dimensional (213) crystallization of cytochrome c (cyt c) on a flavin monolayer. Since the fluidity of a supporting monolayer on which proteins are crystallized is an essential factor for self-assembly of proteins, a flavolipid, flavophosphatidylcholine, was designed and synthesized for an clectro-active supporting monolayer with fluidity. First, it was confirmed that a stable flavolipid monolayer with 2D molecular array was formed as a Langmuir monolayer. Then, the adsorption of cyt c molecules onto a flavolipid monolayer was performed at the air/water interface in a Langmuir trough. Finally, it was found that cyt c molecules were self-assembled to form 2D crystals on a flavolipid monolayer. The 2D crystals of cyt c were revealed to grow by a hetero-epitaxial growth process regulated by the 2D molecular arrays of a flavolipid monolayer. It was also found that cyt c molecules were adsorbed onto a flavolipid monolayer with specific molecular orientation facing the heme-crevice side of the cyt c molecule toward a flavolipid monolayer. The orientational adsorption of cyt c onto a flavolipid monolayer made it possible to transfer electrons one-way from flavolipid to cyt c based on the free energy difference as a molecular rectifier. (C) 2004 Elsevier B.V All rights reserved.