Nonpolar Side Chains Affect the Photochemical Redox Reactions of Copper(II)-Amino Acid Complexes in Aqueous Solutions

Chen-Jui Lin; Po-Yen Wang; Yi-Liang Lin; Sheng-Te Chang; Chao-Sheng Hsu; Shu-Pao Wu; Chien-Hou Wu

doi:10.1021/acsomega.1c04277

Nonpolar Side Chains Affect the Photochemical Redox Reactions of Copper(II)-Amino Acid Complexes in Aqueous Solutions

ACS Omega. 2021 Oct 13;6(42):28194-28202. doi: 10.1021/acsomega.1c04277. eCollection 2021 Oct 26.

Authors

Chen-Jui Lin¹, Po-Yen Wang², Yi-Liang Lin¹, Sheng-Te Chang¹, Chao-Sheng Hsu¹, Shu-Pao Wu³, Chien-Hou Wu^{1

4}

Affiliations

¹ Department of Biomedical Engineering and Environmental Sciences, National Tsing Hua University, Hsinchu 30013, Taiwan.
² Department of Civil Engineering, Widener University, Chester, Pennsylvania 19013, United States.
³ Department of Applied Chemistry, National Yang Ming Chiao Tung University, Hsinchu 30010, Taiwan.
⁴ Institute of Analytical and Environmental Sciences, National Tsing Hua University, Hsinchu 30013, Taiwan.

Abstract

Photochemical redox reactions of Cu(II) complexes of eight amino acid ligands (L) with nonpolar side chains have been systematically investigated in deaerated aqueous solutions. Under irradiation at 313 nm, the intramolecular carboxylate-to-Cu(II) charge transfer within Cu(II)-amino acid complexes leads to Cu(I) formation and the concomitant decomposition of amino acids. All amino acid systems studied here can produce ammonia and aldehydes except proline. For the 1:1 Cu(II) complex species (CuL), the Cu(I) quantum yields at 313 nm (Φ_Cu(I),CuL) vary by fivefold and in the sequence (0.10 M ionic strength at 25 °C) alanine (0.094) > valine (0.059), leucine (0.059), isoleucine (0.056), phenylalanine (0.057) > glycine (0.052) > methionine (0.032) > proline (0.019). This trend can be rationalized by considering the stability of the carbon-centered radicals and the efficient depopulation of the photoexcited state, both of which are dependent on the side-chain structure. For the 1:2 Cu(II) complex species (CuL₂), the Cu(I) quantum yields exhibit a similar trend and are always less than those for CuL. The photoformation rates of ammonia, Cu(I), and aldehydes are in the ratio of 1:2.0 ± 0.2:0.7 ± 0.2, which supports the proposed mechanism. This study suggests that the direct phototransformation of Cu(II)-amino acid complexes may contribute to the bioavailable nitrogen for aquatic microorganisms and cause biological damage on cell surfaces in sunlit waters.