Mechanistic, Biochemical, and Theoretical Studies of Phototoxicity and Photobluing

Author

Lloyd M. Lapoot, The Graduate Center, City University of New YorkFollow

Date of Degree

6-2024

Document Type

Dissertation

Degree Name

Ph.D.

Program

Biochemistry

Advisor

Alexander Greer

Committee Members

Lesley Davenport

Wayne Harding

Sanjai Pathak

Orrette Wauchope

Subject Categories

Biochemistry, Biophysics, and Structural Biology | Chemistry | Computational Chemistry | Organic Chemistry

Keywords

Photochemistry, Photosensitization, Phototoxicity, Adjuvancy

Abstract

This thesis consists of four chapters as detailed below:

Chapter 1 discusses the photoconversion of heptamethine to pentamethine cyanines and of pentamethine to trimethine cyanines. Here, we report mechanistic studies and initial experimental evidence for a previously unexplored 4-carbon truncation reaction that converts the simplest heptamethine cyanine to the corresponding trimethine cyanine. We propose a DFT-supported model describing a singlet oxygen (¹O₂) mediated formation of an allene hydroperoxide intermediate and subsequent 4-carbon loss through a retro-Diels-Alder process. Fluorescence and mass spectrometry measurements provide evidence for this direct conversion process. This 4-carbon truncation reaction adds to growing body of cyanine reactivity and may provide an optical tool leading to a substantial blue-shift of ~200 nm.

Chapter 2 discusses a density functional theoretical (DFT) study implicating a ¹O₂ oxidation process to reach a dihydrobenzofuran from the reaction of the natural homoallylic alcohol, glycocitrine. Our results predict an interconversion between glycocitrine and an iso-hydroperoxide intermediate [R(H)O⁺–O⁻] that provides a key path in the chemistry which then follows. Formations of allylic hydroperoxides are unlikely from a ¹O₂ ‘ene’ reaction. Instead, the dihydrobenzofuran arises by ¹O₂ oxidation facilitated by a 16° curvature of the glycocitrine ring imposed by a pyramidal N-methyl group. This curvature facilitates formation of the iso-hydroperoxide, which is analogous to the iso species CH₂I⁺–I⁻ and CHI₂⁺–I⁻ formed by UV photolysis of CH₂I₂ and CHI₃. The iso-hydroperoxide is also structurally reminiscent of carbonyl oxides (R₂C=O⁺–O⁻) formed in the reaction of carbenes and oxygen. Our DFT results point to intermolecular process, in which the iso-hydroperoxide’s fate relates to O-transfer and H₂O dehydration reactions for new insight to the biosynthesis of dihydrobenzofuran natural products.

Chapter 3 discusses the selectivity of product formation by controlling the philicity of the reaction surface. Although silica surfaces have been used in organic oxidations for the production of peroxides, studies of airborne singlet oxygen at interfaces are limited and have not found widespread advantages. Here, with prenyl phenol coated silica and delivery of singlet oxygen (¹O₂) through the gas phase, we uncover significant selectivity for dihydrofuran formation over allylic hydroperoxide formation. The hydrophobic particle causes prenyl phenol to produce an iso-hydroperoxide intermediate with an internally protonated oxygen atom, which leads to dihydrofuran formation as well as O-atom transfer. In contrast, hydrophilic particles cause prenyl phenol to produce allylic hydroperoxide, due to phenol OH hydrogen bonding with SiOH surface groups. Mechanistic insight is provided by air/nanoparticle interface coated with the prenyl phenol, in which product yield were 6-fold greater on the hydrophobic nanoparticles compared to the hydrophilic nanoparticles and total rate constants (ASI-k_T) of ¹O₂ were 13-fold greater on the hydrophobic vs hydrophilic nanoparticles. A slope intersection method (SIM) method was also developed that uses the airborne ¹O₂ lifetime (τ_airborne) and surface-associated ¹O₂ lifetime (τ_surf) to quantitate ¹O₂ transitioning from volatile to non-volatile and surface boundary (surface···¹O₂). Further mechanistic insight on the selectivity of the reaction of prenyl phenol with ¹O₂ was provided by DFT calculations.

Chapter 4 discusses the photosensitized oxidation of ortho-prenyl phenol leading to byproducts capable of killing ovarian cancer (OVCAR-5) cells in a mechanism separate from singlet oxygen (¹O₂) toxicity. We are interested in cell killing achieved by such ‘priming’ events, where an adjuvant is capable of delivering the second of a ‘one-two’ punch to kill already weakened cells. The byproducts formed in ortho-prenyl phenol photooxidation are capable of priming, but products formed after extended photolysis have neither been tested for toxicity nor characterized. Thus, we undertook in vitro cell and NMR studies to assess compound classes that add toxicity above and beyond the phototoxicity. The photooxidation of prenyl phenol leads to primary products dihydrobenzofuran, hydrogen peroxide, and ‘ene’ allylic hydroperoxides. On extended photolysis, secondary products were detected by 1D and 2D NMR techniques, including dihydrobenzofurans bearing hydroperoxide, alcohol, and epoxide side-groups. The secondary photoproducts enhanced toxicity to ovarian cancer cells ascribed to tandem type II (¹O₂) followed by type I reactions involving oxygen radicals and radical ions to reach the secondary highly toxic products.

Recommended Citation

Lapoot, Lloyd M., "Mechanistic, Biochemical, and Theoretical Studies of Phototoxicity and Photobluing" (2024). CUNY Academic Works.
https://academicworks.cuny.edu/gc_etds/5761