| 1. |
|
|
| 2. |
- Durbeej, Bo, 1973-
(författare)
-
Quantum Chemical Studies of Protein-Bound Chromophores, UV-Light Induced DNA Damages, and Lignin Formation
- 2004
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)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.
|
|
| 3. |
- Norberg, Daniel, 1977-
(författare)
-
Quantum Chemical Studies of Radical Cation Rearrangement, Radical Carbonylation, and Homolytic Substitution Reactions
- 2007
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Quantum chemical calculations have been performed to investigate radical cation rearrangement, radical carbonylation, and homolytic substitution reactions of organic molecules. The rearrangement of the bicyclopropylidiene radical cation to the tetramethyleneethane radical cation is predicted to proceed with stepwise disrotatory opening of the two rings. Each ring opening is found to be combined with a striking pyramidalization of a carbon atom in the central bond. The isomerization of the norbornadiene radical cation to the cycloheptatriene radical cation (CHT.+), initialized by opening of a bridgehead–methylene bond, is investigated. The most favorable path involves concerted rearrangement to the norcaradiene radical cation followed by ring opening to CHT.+. The barrier of this channel is found to be significantly reduced upon substitution of the methylene group with C(CH3)2. Stepwise mechanisms are predicted to be favored over concerted isomerization for the McLafferty rearrangement of the radical cations of butanal and 3-fluorobutanal. The barrier for the concerted rearrangement is found to be lowered by 17.2 kcal/mol upon substitution, a result which is rationalized by the calculated dipole moments and atomic charges. Recent experiments showed that photoinitiated carbonylation of alkyl iodides with [11C]carbon monoxide may be significantly enhanced by using small amounts of ketones that have nπ* character of their excited triplet state. DFT calculations show the feasibility of an atom transfer type mechanism, proposed to explain these observations. Moreover, the computational results rationalize the observed differences in yield when using various alcohol solvents. Finally, following photolysis of methyliodide, recent electron spin resonance spectroscopy experiments demonstrated that the SH2 reaction •CD3 + SiD3CH3 → CD3SiD3 + •CH3 proceeds with high selectivity over the energetically more favorable D abstraction. The role of geometrical effects, especially the formation of prereactive complexes between methylsilane and methyliodide is studied, and a plausible explanation for the experimentally observed paradox is presented.
|
|
