Regulated transportation of nanoscale objects with a high degree of spatiotemporal precision is a prerequisite for the development of targeted molecular delivery. In vitro integration of the kinesin-microtubule motor system with synthetic molecules offers opportunities to develop controllable molecular shuttles for lab-on-a-chip applications. We attempted a combination of the kinesin-microtubule motor system with push-pull type azobenzene tethered inhibitory peptides (azo-peptides) through which reversible, spatiotemporal control over the kinesin motor activity was achieved locally by a single, visible wavelength. The fast thermal relaxation of the cis-isomers of azo-peptides offered us quick and complete resetting of the trans-state in the dark, circumventing the requirement of two distinct wavelengths for two-way switching of kinesin-driven microtubule motility. Herein, we report the manipulation of selected, single microtubule movement while keeping other microtubules at complete rest. The photoresponsive inhibitors discussed herein would help in realizing complex bionanodevices.

Artificial control of bio-nanomachines should have a major impact on the development of controllable transport systems for specific cargo transport on chips. Precise spatiotemporal control and local regulation of the bio-motor activity will, however, be necessary if we are to accomplish such a goal. In this study, we exploited the photoswitching properties of azobenzene-based high-energy molecules and inhibitors to control a single kinesin-driven microtubule that has potential to work as a nanocarrier for molecular cargos. In particular, we could influence the local concentration and dispersion of the microtubules at any desired position and time by irradiating a local area of the motility system at one wavelength, while irradiating the entire area at another wavelength, to enrich either cis or trans isomers of photoswitches in the selected region. Furthermore, various regulations (e.g., transporting, bending, breaking) of single microtubules were possible while almost arresting ambient microtubules-all without the need for any surface patterning.

A novel class of photoswitches based on a phenylazothiazole scaffold that undergoes reversible isomerization under visible-light irradiation is reported. The photoswitch, which comprises a thiazole heteroaryl segment directly connected to a phenyl azo chromophore, has very different spectral characteristics, such as a redshifted absorption maximum wavelength and well-separated absorption bands of the trans and cis isomers, than conventional azobenzene and other heteroaryl azo compounds. Substituents at the ortho and para positions of the phenyl ring of the photoswitch resulted in a further shift to longer wavelengths up to 525 nm at the absorption maximum with a small thermal stability compensation. These photoswitches showed excellent photostationary distributions of the trans and cis isomers, thermal half-lives of up to 7.2 h, and excellent reductant stability. The X-ray crystal structure analysis revealed that the trans isomers exhibited a planar geometry and the cis isomers exhibited a T-shaped orthogonal geometry. Detailed ab initio calculations further demonstrated the plausible electronic transitions and isomerization energy barriers, which were consistent with the experimental observations. The fundamental design principles elucidated in this study will aid in the development of a wide variety of visible-light photoswitches for photopharmacological applications.

Recently we demonstrated the photoregulation of the activity of kinesin-1 using an azobenzene-tethered peptide (azo-peptide: Azo-Ile-Pro-Lys-Ala-Ile-Gln-Ala-Ser-His-Gly-Arg-OH). To understand the mechanism behind this photoswitchable inhibition, here we studied the structure-property relationships of a range of azo-peptides through systematic variations in the structures of the peptide and azobenzene units. The vital peptide sequence for kinesin inhibition-mediated through electrostatic, hydrophobic and C-Hπ interactions-was the same as that for the self-inhibition of kinesin. We also identified substituents on the azobenzene capable of enhancing the photoswitchability of inhibition. As a result, we developed a new inhibitor featuring a relatively short peptide unit (-Arg-Ile-Pro-Lys-Ala-Ile-Arg-OH) and an azobenzene unit bearing a para-OMe group. In the trans form of its azobenzene unit, this finely tuned inhibitor stopped the kinesin-driven gliding motility of microtubules completely at a relatively low concentration, yet allowed gliding motility with a relatively high velocity in the cis form obtained after UV irradiation.