STUDY OF COVALENT AND NON COVALENT INTERACTIONS OF BIOPOLYMER BY MASS SPECTROMETRY
Tesi di Dottorato
Data di Pubblicazione:
2010
Citazione:
STUDY OF COVALENT AND NON COVALENT INTERACTIONS OF BIOPOLYMER BY MASS SPECTROMETRY / F. Riccardi Sirtori ; coordinatore: Carlo De Micheli ; docente guida: Giancarlo Aldini ; tutor aziendale: Maristella Colombo. Universita' degli Studi di Milano, 2010 Dec 15. 23. ciclo, Anno Accademico 2010.
Abstract:
ESI-MS screening methods directly detect ligand-target non covalent complexes in the gas phase and allow inference of affinity (and specificity) of the ligand-target interaction in solution [1, 2]. The identity of different complexes can be directly assessed as the mass of each molecule works as intrinsic label. Biopolymers can be screened either as a single component or a mixture of different targets; in this way it is possible to determine the selectivity of a new chemical entity for different targets. On the other hand, using ESI-MS it is also possible to identify, within a mixture, components that selectively bind the active site of a certain biopolymer and could be profitably used to screen libraries of known compounds.
The aim of this PhD project was the study by ESI mass spectrometry of different noncovalent complexes formed with biopolymers identified as possible therapeutic targets (Mismatch Repair Mechanism and Hsp90). The noncovalent interaction between biological targets and possible ligands were studied in order to identify potential inhibitors. The binding affinities determined by ESI-MS were compared to existing data obtained by solution phase methods. A novel MS-based method was implemented for testing different biopolymer, of therapeutic interest, against a library of fragments (molecular weight 100-300 Da) constituted of about 2,000 compounds.
Particularly this approach was used for the identification of new molecules able to recognize TG mismatched base pairs in DNA that are responsible for most of the common mutations leading to formation of tumors in humans. TG mismatches are particularly abundant in cells lacking mismatch repair mechanism (MMR). MMR is involved in the correction of DNA polymerase errors that escape proofreading activity. MMR deficiency increases 50-1000-fold spontaneous mutation rates (microsatellite instability MSI) and in addition, MMR deficiency can lead to resistance to several chemotherapeutic agents (DNA damaging agent) [3].
An ESI-MS method was used to study the complexes formed between different DNA duplexes and minor groove binders and intercalator compounds. A hairpin DNA sequence (CTGGsm) bearing a single T:G mismatch and a matched hairpin DNA sequence (CCGG) were prepared and used to set up the method. Two DNA sequences were also synthesized: a self complementary G:C rich DNA sequence (HFM) and a polyAT DNA duplex (A5TG). The association constants (KAs) were directly determined from the MS spectrum and the amount of bound ligand was used to determine the selectivity of a binder among the different DNA sequences. Minor groove binders confirmed their selectivity toward AT rich DNA duplex (A5TG) whereas a tris imidazole lexitropsin derivative proved to be selective for the TG mismatched DNA hairpin (CTGGsm) in agreement with NMR, SPR, and ITC studies.
A medium throughput screening (MTS) method was setup for the screening of the fragment library (about 1,000 cps) and the procedure was validated studying the interactions between the two different DNA sequences (HFM and CTGGsm) and minor groove binders. The screening was performed on the fragment library and hit compounds were afterwards tested against HFM and CCGG in order to identify selective ligands. Among the different hits identified, a slight selectivity (1.3) for the single mismatched sequence was highlighted for the fragment FBA-05-094. Twenty close analogues of FBA-05-094 were subsequently tested against CTGGsm and CCGG DNA with the aim to find more selective and higher affinity ligands. The screening of the fragment library on CTGGsm was repeated using ligand mixtures (5 and 10 components) in order to improve the assay throughput. A good agreement between the results obtained by screening the compounds as single compon
Tipologia IRIS:
Tesi di dottorato
Keywords:
ESI-MS ; non covalent adducts ; DNA ; Hsp90 ; TG mismatch ; Mass Spectrometry
Elenco autori:
F. RICCARDI SIRTORI
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