Photon Factory Activity Report 2022 #40 (2023)

BL-5A/2020P003, 2022G012

Coordination and photoisomerizaiton of Azobenzene-amino acid
Schiff base copper(II) complexes to lysozyme
Takashiro AKITSU1,*
Department of Chemistry, Faculty of Science,
Tokyo University of Science,
1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan1

We exhibited an amino acid (arginine and threonine) derivative Schiff base copper(II) complexes incorporating
photochromic azobenzene moiety as a photoresponsive site and conjugates it to egg white lysozyme. Spectroscopic andcrystallographic facts showed that the nitrogen atom of the amino acid residue of lysozyme bound to the paramagnetic
copper(II) ion of the complex, and UV light irradiation confirmed photoisomerization of the ligand to cis-form.
1 Introduction
Azobenzene is one of the most famous photocromicdyes due to its high quantum yield, stability, and ease ofsynthesis and derivatization. Since the wavelength ofphotoisomeriza-tion is in the ultraviolet-visible (UV-vis)
region (E → Z: 380 nm, Z → E: > 400 nm). In this study,
by incorporating an azobenzene moiety into the ligand asa photoresponsive site into a Schiff base copper(II)complex containing an amino acid, the photoresponsiveness and switching of a protein (egg whitelysozyme) was investigated with spectroscopic as well as
crystallographic measurements.
2 Experiment
For example, azobenzene-salicylaldehyde [1] (226 mg,1.00 mmol) and L-threonine (or other amio acids) (119 mg,1.00 mmol) were dissolved in methanol (100 mL) andstirred at 313 K for 1.5 hr to give a red solution. Copper(II)acetate-hydrate (199 mg, 1.00 mmol) was added andstirred for 1 hr, and imidazole (68 mg, 1.0 mmol) was
added and stirred for another hour to give a dark green
solution. The reaction solution was allowed to stand at 298
K for 4 days to obtain green needle crystals. Commerciallyavailable chicken egg white lysozyme was used. After
adjusting phosphate buffer solutions of copper(II)complexes and Lysozyme, we confirmed conventional
spectra and grown the binding single crystals [2]. X-ray
crystallography was carried out at KEK-PF BL-5A
3 Results and Discussion
Spectroscopically,
absorption
wavelength
corresponding to the d-d transition of copper in acopper(II) complex shifts from 666 nm to 652 nm as thelysozyme concentration increases, short-wavelength shiftsimilar to a copper(II) complex. This indicates that thecopper(II) ion of a copper(II) complex is bound to theimidazole group of the histidine (or nitrogen atomcontaining) residue of lysozyme. Single crystals weregrown in a conventional way (Fig. 1), though most of themwere binging copper(II) ions only. However,crystallographic verification of binding features are underanalyzing at present (Fig. 2), therefore, experimental
details of the coordination geometry of a copper(II)complex will be presented elsewhere soon.

References[1] K. Kashiwagi and T. Akitsu et al., Symmetry. 12, 808(2020).
[2] T. Akitsu and M. Unno et al., Key Engineering
Materials 888, 105 (2021).

Fig. 1: Pale yellow single crystals of lysozyme containing
a copper(II) complex.

Fig. 2: Electron density of crystals of lysozyme including
a copper(II) complex.
Research Achievements
1. We obtained pale yellow single crystals of lysozyme
containing a copper(II) complex.
2. We could have confirmed that electron densitysuggested as the whole copper(II) complex orcopper(II) ion in lysozyme under a certain condition.
* akitsu2@rs.tus.ac.jp