The photochemistry of urocanic acid with 2'-deoxyadenosine and a study of aryl/ester orbital interactions in the excited state
Abstract
The studies in this work have been divided into two sections. First, the photochemistry of E-urocanic acid (E-UA) and 2$\sp\prime$-deoxyadenosine (dA) has been investigated to determine the origin of the high binding levels observed between poly A and E-UA. These studies have determined that electron transfer chemistry occurs between E-UA and dA, and one resultant photoproduct is 1H-imidazole-4-carboxaldehyde. Mechanistic and $\sp{18}$O labelling studies on this oxidative cleavage reaction are consistent with a mechanism involving a E-UA radical cation and molecular oxygen. The presence of sodium azide appreciably increases aldehyde formation. A UA radical anion has also been implicated in the E-UA/dA photolysis by the observed formation of dihydrourocanic acid (DHU). These studies reconfirm this result and evidence suggests that DHU forms better in argon than in air. Another isolated photoproduct in the E-UA/dA reaction has been determined to be an isomer of deoxyadenosine which, in time, isomerizes back to dA. Due to the large changes in the C-8 and C-2 protons in the $\sp1$H NMR an isomerization involving the purine ring is proposed. The second section involves the study of the aryl/ester interaction in the excited state. The Norrish Type II photochemistry exhibited by 2-ethoxyethylphenyl acetate has been attributed to delocalized excitation of an aryl/ester "superchromophore". This delocalized excitation is a result of extensive orbital mixing of the n$\sb{\rm o(C=O)}$ orbital into the HOMO and the $\pi\sp*\sb{\rm (C=O)}$ orbital into the LUMO, and the degree of this mixing is dependent on the relative orientations of the aryl and ester groups. A MNDO/CI study revealed the contributing transitions to the S$\sb1$ state are also dependent on the orientation of these two groups. Several compounds were studied with varying degrees of orbital mixing, as predicted by MNDO and ab initio calculations, and the results show that the changes in orbital mixing correlate well with the changes in absorption and fluorescence spectra. The insertion of a methoxyl group between the aryl and ester moieties of 2-ethoxylphenylacetate results in large changes in the absorption and fluorescence spectra, a substantial increase in the Norrish Type II photochemistry and changes in the contributing transitions in the S$\sb1$. Although the origin for this large effect is not well understood, electronic factors may be attributing to this effect.
Degree
Ph.D.
Advisors
Morrison, Purdue University.
Subject Area
Organic chemistry
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