Shuji Takeda

Novel Light-Illumination Scanning Tunneling Microscopy Equipped with Optical Fiber Probe

Single molecular photocurrent generation was investigated using a novel light-illumination scanning tunneling microscope (LI-STM) equipped with an optical fiber probe. For this study, self-assembled monolayers (SAMs) of chimera proteins composed of cytochrome b562 mutant proteins fused with an enhanced green fluorescent protein (EGFP) were prepared. With light illumination, the EGFP was excited, resulting in photocurrent generation due to the energy transfer to the cytochrome part instead of a typical fluorescence. Two different types of measurements were carried out: one employed a conventional LI-STM with a metallic probe and far-field optics, while the metallic probe was replaced with a laboratory-built doubly metal-coated optical fiber probe in another setup, where a local near-field illumination was possible. The comparison of these techniques indicated the superior sensitivity of the new technique to the conventional one, showing the evidence of the intramolecular energy transfer at a single molecular level.

Rational design of a protein-based molecular device consisting of blue fluorescent protein and zinc protoporphyrin IX incorporated into a cytochrome b562 scaffold

To make a single molecular photo-device, it is essential to control the exact orientation of two types of proteins. We made a chimeric protein in which cytochrome b562 was linked to the N-terminus of enhanced green fluorescent protein, cytb562-EGFP. Within cytb562-EGFP, the excitation energy of EGFP was transferred to the cytochrome b562 cofactor fixed proximally to EGFP. Cytb562-EGFP was engineered so that iron protoporphyrin IX was substituted by zinc protoporphyrin IX to make it a suitable cofactor for photo-induced electron transfer. The photosensitizer pigment was optimized and the EGFP was replaced by a blue fluorescent mutant that gave 15% higher energy transfer efficiency.