Overriding Phthalate Decomposition When Exploring Mycophenolic Acid Intermediates as Selenium-Based ROS Biological Probes

Mahesh B Halle; Tesla Yudhistira; Kyung Jin Lee; Jae Hyuck Choi; Youngsam Kim; Hee-Sung Park; David G Churchill

doi:10.1021/acsomega.8b01571

Overriding Phthalate Decomposition When Exploring Mycophenolic Acid Intermediates as Selenium-Based ROS Biological Probes

ACS Omega. 2018 Oct 31;3(10):13474-13483. doi: 10.1021/acsomega.8b01571. Epub 2018 Oct 18.

Authors

Mahesh B Halle¹, Tesla Yudhistira¹, Kyung Jin Lee¹, Jae Hyuck Choi^{1

2}, Youngsam Kim¹, Hee-Sung Park¹, David G Churchill^{1

2}

Affiliations

¹ Department of Chemistry, Molecular Logic Gate Laboratory, and Department of Chemistry, Molecular Synthetic Biology Laboratory, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Republic of Korea.
² Center for Catalytic Hydrocarbon Functionalization, Institute for Basic Science (IBS), Daejeon 305-701, Republic of Korea.

Abstract

Hypochlorous (OCl^-) acid is the most well-known bacterial oxidant to be produced by neutrophils. Excess amounts of OCl^- can cause various disorders in living systems. Herein, we have designed, synthesized, and characterized two novel organoselenium-based target molecules (Probe-1 and Probe-OCl) based on a synthetic intermediate of mycophenolic acid for the aqueous detection of OCl^-. Probe 1 has been recently reported (Org. Lett. 2018, 20, 3557-3561); both probes show immediate "turn-on" fluorescence (<1 s) upon the addition of OCl^-, display an increase in the fluorescence quantum yield (3.7-fold in Probe-1 and 11.6-fold in Probe-OCl), and are completely soluble in aqueous media without the help of any cosolvent. However, a decrease in the "turn-on" intensity with the oxidized version of Probe-1 in cell assays due to the anhydride/phthalate functionality suggests that probe degradation occurs based on hydrolytic action (a probe degradation half-life of ∼1500 s at 15 μM Probe-1 and 150 μM OCl). Thus, the change of "anhydride" to "methylamide" begets Probe-OCl, which possesses more stability without sacrificing its water solubility properties and responses at short times. Further studies suggest that Probe-OCl is highly stable within physiological pH (pH = 7.4). Surprisingly, in live cell experiments involving U-2 OS cells and HeLa cells, Probe-OCl accumulated and aggregated in lipid droplets and gives a "turn-on" fluorescence response. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays confirmed that Probe-OCl is not toxic. Cuvette aggregation studies were also performed (tetrahydrofuran/H₂O) to demonstrate aggregation-induced fluorescence at longer times. Our current hypothesis is that the "turn-on" fluorescence effect is caused by the aggregation-induced emission mechanism available for Probe-OCl. In this case, in tandem, we reanalyzed the Mes-BOD-SePh derivative to compare and contrast cell localization as imaged by confocal microscopy; fluorescence emission occurs in the absence of, or prior to, Se oxidation.