The photochemistry and photobiology of urocanic acid and indoleacrylic acid
Abstract
The photochemistry and photobiology of E-urocanic acid (E-UA) and a structurally similar analogue, E-indoleacrylic acid (E-IA), have been investigated. The studies involving E-UA have been divided into two separate investigations. First, studies have been initiated probing the effect of UV light on the interaction of UA with mammalian cells. Results show that UA is capable of permeating the membranes of living cells and that UA is photochemically reactive with cellular nucleic acids; a binding level of 3.1 nmol of UA equivalents per mg of DNA is observed. Second, studies probing the photochemical interactions of E-UA with calf thymus DNA have shown that the amount of UA incorporation is dependent on the wavelength(s) of excitation and the secondary structure of DNA. The results of base specificity studies using DNA analogues demonstrate a predilection for UA binding to thymidine; however, all bases show low levels of incorporation as demonstrated by the reactions with the isolated polynucleotides. At least one mechanism for UA incorporation at $\lambda > 270$ nm has been identified as a (2+2) cycloaddition reaction, which is reversible at $\lambda = 254$ nm. The efficiency of this reaction increases with higher frequency light, where DNA absorption increases relative to that of UA. Furthermore, the cycloaddition is quenched by the presence of oxygen, which is attributed to the oxygen catalyzed degradation of UA. A second source for UA incorporation occurs through reaction with deoxyadenosine. Although the mechanism has not been identified, it has been shown to be oxygen facilitated and to occur more efficiently with higher frequency excitation. To probe further the nature of UA binding, the photochemical reactions between E-UA and the two nucleosides, thymidine and deoxyadenosine, have been studied under conditions known to photochemically incorporate UA into DNA. The chemical structures of two diastereomeric cyclobutane structures formed from the reaction of E-UA and thymidine have been elucidated using $\sp1$H NMR, $\sp{13}$C NMR, FAB mass spectroscopy, Correlation Spectroscopy (COLOC), and nuclear Overhauser effects combined with spin-lattice relaxation times (T$\sb1\sp{\rm SE}$). Preliminary studies of the reaction between UA and deoxyadenosine provide strong evidence for products resulting from electron transfer chemistry through a deoxyadenosine excited state. In related studies, the photochemical interactions between DNA and E-indoleacrylic acid (E-IA) have been investigated. This compound shows much higher levels of photochemical binding to DNA compared to UA. There is evidence that thymidine is the primary target of incorporation and, like UA, the mechanism is a reversible (2+2) cycloaddition reaction. The results of these studies provide vital insight for understanding the potential mechanisms by which naturally occurring aromatic acrylic acid compounds interact photochemically with biologically important substituents.
Degree
Ph.D.
Advisors
Morrison, Purdue University.
Subject Area
Organic chemistry
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