Ryan D. McCulla

Computational investigation of the reaction mechanisms of nitroxyl and thiols.

Nitroxyl, or nitrosyl hydride, (HNO) is a pharmacologically relevant molecule whose physiological responses have been thought to result from modification of intracellular thiols. The reaction of HNO with thiols has been shown to lead to disulfides and sulfinamides. The free energies of reaction (DeltaG) and activation (DeltaG(++)) were determined for the reaction pathways of HNO and five different thiols using computational methods. The methods employed included B3LYP, MP2, and CBS-QB3, as well as IEF-PCM to approximate implicit water solvation. The five examined thiols were hydrogen sulfide, methanethiol, trifluoromethanethiol, thiophenol, and cysteine. A putative N-hydroxysulfenamide intermediate was the initial product for the reaction of HNO with a thiol. Analysis of the Wiberg bond indices indicated that the formation of the S-N bond was concerted with the proton transfers that led to the intermediate. The calculated pK(a) of protonated N-hydroxysulfenamide was approximately 13, and from the protonated N-hydroxysulfenamide intermediate, two irreversible reactions that lead to either the disulfide or sulfinamide were found. The calculated values of DeltaG(++) indicated the preferred reaction pathway would be dependent upon the hydrophobicity of the environment, the availability of a local base, and the identity of the thiol substituent. In a hydrophobic environment, the formation of the disulfide was kinetically favored. Formation of the sulfinamide product was expected to occur upon the protonation of the hydroxy group of the N-hydroxysulfenamide intermediate.

Photodeoxygenation of dibenzothiophene S-oxide derivatives in aqueous media.

The use of atomic oxygen (O((3)P)) as potent oxidant in water has suffered from the lack of a facile, efficient source. The photodeoxygenation of aromatic sulfoxides to the corresponding sulfides in organic solvents has been suggested to produce O((3)P) in low quantum yields. The photolysis of 4,6-dihydroxymethyldibenzothiophene S-oxide and 2,8-dihydroxymethyldibenzothiophene S-oxide in water results in deoxygenation at significantly higher quantum yields than in organic solvents. Depending upon conditions, a variable amount of oxidation of the hydroxymethyl substituent into an aldehyde was observed to accompany deoxygenation. Analysis of the photoproducts indicated the deoxygenation occurred by at least two different pH-sensitive mechanisms. Under basic conditions, photoinduced electron transfer yielding a hydroxysulfuranyl radical that decomposed by heterolytic S-O cleavage was thermodynamically feasible. The thermodynamics of photoinduced electron transfer were expected to become increasingly unfavorable as the pH of the solution decreased. Thus, at neutral and acidic pH, an S-O bond scission mechanism was suspected. The observed increase in the photodeoxygenation quantum yields was consistent with charge separation accompanying S-O bond scission. Oxidative cleavage of alkenes in aerobic conditions suggested O((3)P) was produced during photolysis in these conditions; however, the formation of discrete O(*-)/HO(*) may occur, particularly at low pH.

Ultrafast spectroscopy and computational study of the photochemistry of diphenylphosphoryl azide: direct spectroscopic observation of a singlet phosphorylnitrene.

The photochemistry of diphenylphosphoryl azide was studied by femtosecond transient absorption spectroscopy, by chemical analysis of light-induced reaction products, and by RI-CC2/TZVP and TD-B3LYP/TZVP computational methods. Theoretical methods predicted two possible mechanisms for singlet diphenylphosphorylnitrene formation from the photoexcited phosphoryl azide. (i) Energy transfer from the (π,π*) singlet excited state, localized on a phenyl ring, to the azide moiety, thereby leading to the formation of the singlet excited azide, which subsequently loses molecular nitrogen to form the singlet diphenylphosphorylnitrene. (ii) Direct irradiation of the azide moiety to form an excited singlet state of the azide, which in turn loses molecular nitrogen to form the singlet diphenylphosphorylnitrene. Two transient species were observed upon ultrafast photolysis (260 nm) of diphenylphosphoryl azide. The first transient absorption, centered at 430 nm (lifetime (τ) ∼ 28 ps), was assigned to a (π,π*) singlet S(1) excited state localized on a phenyl ring, and the second transient observed at 525 nm (τ ∼ 480 ps) was assigned to singlet diphenylphosphorylnitrene. Experimental and computational results obtained from the study of diphenyl phosphoramidate, along with the results obtained with diphenylphosphoryl azide, supported the mechanism of energy transfer from the singlet excited phenyl ring to the azide moiety, followed by nitrogen extrusion to form the singlet phosphorylnitrene. Ultrafast time-resolved studies performed on diphenylphosphoryl azide with the singlet nitrene quencher, tris(trimethylsilyl)silane, confirmed the spectroscopic assignment of singlet diphenylphosphorylnitrene to the 525 nm absorption band.

Photoinduced DNA cleavage by atomic oxygen precursors in aqueous solutions

Reactive oxygen species are known to induce DNA strand cleavage and have been explored as treatments for cancer. The development of aqueous-soluble dibenzothiophene-S-oxide (DBTO) derivatives has made it possible to investigate the mechanism of DNA cleavage by these photoactivatable precursors of atomic oxygen. In addition to the release of atomic oxygen, DBTO can also undergo other processes such as α-cleavage. An objective of this work was to establish whether the extent of strand scission could be attributed to a direct reaction between atomic oxygen and DNA. To accomplish this aim, the extent of strand cleavage upon irradiation of three different DBTO derivatives was measured by the conversion of circular pUC19 plasmid (Form I) to nicked (Form II) as monitored by gel electrophoresis. The interaction of the sulfoxides with DNA was systematically studied by optical melt and fluorescence anisotropy experiments. Thiols are susceptible to rapid oxidation by atomic oxygen, and thus, glutathione was used as a ROS scavenger to determine if DNA cleavage was induced by the release of atomic oxygen. The results from these experiments indicated atomic oxygen was at least partially responsible for the observed strand scission.

Comparison of experimental and computationally predicted sulfoxide bond dissociation enthalpies.

The accurate estimation of S-O bond dissociation enthalpies (BDE) of sulfoxides by computational chemistry methods has been a significant challenge. One of the primary causes for this challenge is the well-established requirement of including high-exponent d functions in the sulfur basis set for accurate energies. Unfortunately, even when high-exponent d functions were included in Pople-style basis sets, the relative strength of experimentally determined S-O BDE was incorrectly predicted. The aug-cc-pV(n+d)Z basis sets developed by Dunning include an additional high-exponent d function on sulfur. Thus, it was expected that the aug-cc-pV(n+d)Z basis sets would improve the prediction of sulfoxide S-O BDE. This study presents the S-O BDE predicted by B3LYP, CCSD, CCSD(T), M05-2X, M06-2X, and MP2 combined with aug-cc-pV(n+d)Z, aug-cc-pVnZ, and Pople-style basis sets. The accuracy of these predictions was determined by comparing the computationally predicted values to the experimentally determined S-O BDE. Values within experimental error were obtained for dialkyl sulfoxides when the S-O BDEs were estimated using an isodesmic oxygen transfer reaction at the M06-2X/aug-cc-pV(T+d)Z level of theory. However, the S-O BDE of divinyl sulfoxide was overestimated by this method.

Oxidation of Plasmalogen, Low‐Density Lipoprotein and RAW 264.7 Cells by Photoactivatable Atomic Oxygen Precursors

The oxidation of lipids by endogenous or environmental reactive oxygen species (ROS) generates a myriad of different lipid oxidation products that have important roles in disease pathology. The lipid oxidation products obtained in these reactions are dependent upon the identity of the reacting ROS. The photoinduced deoxygenation of various aromatic heterocyclic oxides has been suggested to generate ground state atomic oxygen (O[3P]) as an oxidant; however, very little is known about reactions between lipids and O(3P). To identify lipid oxidation products arising from the reaction of lipids with O(3P), photoactivatable precursors of O(3P) were irradiated in the presence of lysoplasmenylcholine, low‐density lipoprotein and RAW 264.7 cells under aerobic and anaerobic conditions. Four different aldehyde products consistent with the oxidation of plasmalogens were observed. The four aldehydes were: tetradecanal, pentadecanal, 2‐hexadecenal and hexadecanal. Depending upon the conditions, either pentadecanal or 2‐hexadecenal was the major product. Increased amounts of the aldehyde products were observed in aerobic conditions.

Thiol Reactivity toward Atomic Oxygen Generated during the Photodeoxygenation of Dibenzothiophene S-Oxide

Aromatic heterocyclic oxides, such as dibenzothiophene S-oxide (DBTO), have been suggested to release ground state atomic oxygen [O(3P)] upon irradiation, and as such, they have been used to create a condensed phase reactivity profile for O(3P). However, thiols, which are highly reactive with O(3P) in the gas phase, were not previously investigated. An earlier study of O(3P) with proteins in solution indicated a preference for thiols. A further investigation of the apparent thiophilicity provided the subject for this study. DBTO was employed as a putative O(3P)-precursor. However, the effective rate of O(3P) formation was found to be dependent on reactant concentrations in certain cases. All reactants were found to increase the rate of deoxygenation to some extent, but in the presence of reactants containing an alcohol linked to a reactive functional group, deoxygenation occurred substantially more rapidly. The rate enhancement was quantified and attributed to the reaction of activated O atom within the solvent cage prior to escape into the bulk solution. Through competition experiments, the relative rate constants of O(3P) with thiols and other functional groups were found. A small preference for primary thiols was observed over other thiols, sulfides, and alkenes. A much larger preference was observed for thiols, sulfides, and alkenes over aromatic groups. In summary, DBTO was successfully used as an O(3P)-precursor, and the thiophilicity of O(3P) was confirmed and quantified.

Asymmetric Dibenzothiophene Sulfones as Fluorescent Nuclear Stains

Asymmetric dibenzothiophene S, S-dioxides (DBTOOs) were synthesized and their photophysical properties examined. Through examination, the molecules fluoresced at wavelengths between 371 and 492 nm with quantum yields of fluorescence nearing 0.59. Three of the sulfonic acid sodium salt analogues were chosen to be introduced to HeLa cells, resulting in illumination of the nucleus by fluorescent microscopy. These compounds function as nuclear stains while also affording the ability to predict the localization of the corresponding sulfoxide precursor to ground-state atomic oxygen.