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Abstract
Multiple G‐tracts within the promoter region of the c‐myc oncogene may fold into various G‐quadruplexes with the recruitment of different tracts and guanosine residues for the G‐core assembly. Thermodynamic profiles for the folding of wild‐type and representative truncated as well as mutated sequences were extracted by comprehensive DSC experiments. The unique G‐quadruplex involving consecutive G‐tracts II–V with formation of two one‐nucleotide and one central two‐nucleotide propeller loop, previously proposed to be the biologically most relevant species, was found to be the most stable fold in terms of its Gibbs free energy of formation at ambient temperatures. Its stability derives from its short propeller loops but also from the favorable type of loop residues. Whereas quadruplex folds with long propeller loops are significantly disfavored, a snap‐back loop structure formed by incorporating a 3’‐terminal guanosine into the empty position of a tetrad seems highly competitive based on its thermodynamic stability. However, its destabilization by extending the 3’‐terminus questions the significance of such a species under in vivo conditions.
Electrochemical characterisation of the redox behaviour of quinoide components in membrane models
(2020)
The leading idea of this thesis is to study the effects of (i) membrane composition and (ii) membrane environment (aqueous phases) on the redox properties of membrane-confined redox active compounds. For solutions, it is known since long, how strong solvents affect the redox properties of dissolved redox active species. However, for membranes this question has not yet been addressed, although it can be supposed that such effects may be important to understand the role of membrane-confined redox active compounds in biological systems. To interrogate this problem, a monolayer model was chosen. It consists of a lipid monolayer with embedded menaquinones on mercury electrodes. Since ion transfer across membranes is also a crucial question, in the first part of this project, 2,2-diphenyl-1-picrylhydrazyl (DPPH) was studied as a new redox probe for transferring anions and cation between an organic and an aqueous phase. The important findings of this thesis are: (i) accessing the ion pair equilibrium constant of anions and cations with DPPH redox probe as a model study using the three-phase electrochemistry, (ii) the redox potentials of menaquinone-4, -7, and -9 in 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) monolayers and the acidity constants of menaquinones (MK’s) in membranes monolayer model, and (iii) the effects of membrane composition and the aqueous environment on the thermodynamics and kinetics of MK’s in membrane models.
Abstract
White spot disease (WSD) is one of the most devastating viral infections of crustaceans caused by the white spot syndrome virus (WSSV). A conserved sequence WSSV131 in the DNA genome of WSSV was found to fold into a polymorphic G‐quadruplex structure. Supported by two mutant sequences with single G→T substitutions in the third G4 tract of WSSV131, circular dichroism and NMR spectroscopic analyses demonstrate folding of the wild‐type sequence into a three‐tetrad parallel topology comprising three propeller loops with a major 1 : 3 : 1 and a minor 1 : 2 : 2 loop length arrangement. A thermodynamic analysis of quadruplex formation by differential scanning calorimetry (DSC) indicates a thermodynamically more stable 1 : 3 : 1 loop isomer. DSC also revealed the formation of additional highly stable multimeric species with populations depending on potassium ion concentration.
Abstract
G‐quadruplexes have attracted growing interest in recent years due to their occurrence in vivo and their possible biological functions. In addition to being promising targets for drug design, these four‐stranded nucleic acid structures have also been recognized as versatile tools for various technological applications. Whereas a large number of studies have yielded insight into their remarkable structural diversity, our current knowledge on G‐quadruplex stabilities as a function of sequence and environmental factors only gradually emerges with an expanding collection of thermodynamic data. This minireview provides an overview of general rules that may be used to better evaluate quadruplex thermodynamic stabilities but also discusses present challenges in predicting most stable folds for a given sequence and environment.