Quantum Chemical Studies of Protein-Bound Chromophores, UV-Light Induced DNA Damages, and Lignin Formation

Abstract: Quantum chemical methods have been used to provide a better understanding of the photochemistry of astaxanthin and phytochromobilin; the photoenzymic repair of UV-light induced DNA damages; and the formation of lignin. The carotenoid astaxanthin (AXT) is responsible for the colouration of lobster shell. In solution, the electronic absorption spectra of AXT peak in the 470-490 nm region, corresponding to an orange-red colouration. Upon binding to the lobster-shell protein-complex α-crustacyanin, the absorption maximum is shifted to 632 nm, yielding a slate-blue colouration. Herein, the structural origin of this bathochromic shift is investigated on the basis of recent experimental work.The tetrapyrrole phytochromobilin (PΦB) underlies the photoactivation of the plant photoreceptor phytochrome. Upon absorption of 660-nm light, PΦB isomerizes from a C15-Z,syn configuration (in the inactive form of the protein) to C15-E,anti (in the active form). In this work, a reaction mechanism for this isomerization is proposed. DNA photolyases are enzymes that repair DNA damages resulting from far-UV-light induced [2+2] cycloaddition reactions involving pyrimidine nucleobases. The catalytic activity of these enzymes is initiated by near-UV and visible light, and is governed by electron transfer processes between a catalytic cofactor of the enzyme and the DNA lesions. Herein, an explanation for the experimental observation that the repair of cyclobutane pyrimidine dimers (CPD) – the major type of lesion – proceeds by electron transfer from the enzyme to the dimer is presented. Furthermore, the formation of CPD is studied.Lignin is formed by dehydrogenative polymerization of hydroxycinnamyl alcohols. A detailed understanding of the polymerization mechanism and the factors controlling the outcome of the polymerization is, however, largely missing. Quantum chemical calculations on the initial dimerization step have been performed in order to gain some insight into these issues.