A series of oxovanadium(IV) complexes of Schiff bases derived from substituted salicylaldehyde and aminoalcohols has been prepared and characterized. The Schiff bases coordinate through 0, N, and 0 as tridentate bivalent ligands. The primary purpose of the investigation is to describe the structure and bonding in these complexes. The subnormal magnetic properties of the complexes provide much information about both the structure and the bonding in the complexes.

Copper(II) and oxovanadium(IV) ions resemble one another magnetically in having one unpaired electron in their complexes irrespective of their geometrical structures and bond types involved. Copper(II) complexes with antiferromagnetic exchange are well known. On the contrary, antiferromagnetic exchange in oxovanadium(IV) complexes is rather new and not well established. Very few oxovanadium(IV) complexes have been reported to have this anomalous magnetic property. In the investigation of the magnetic properties of oxovanadium(IV) complexes, we have successfully prepared two series of new oxovanadium(IV) complexes with N-salicylideneamino acids.

As cucurbiturilas são moléculas versáteis empregadas em diversos ramos da química, essas gaiolas moleculares formam uma família de homólogos oriundos da condensação da glicolurila e do formaldeído em meio ácido. As aplicações se estendem para campos como catálise, fotoquímica, sínteses orgânica e inorgânica, química de materiais e supramolecular. Particularmente a cucurbit[6]urila, comumente chamada apenas de CB[6], uma molécula bastante simétrica, têm em seus opérculos, sítios propícios à coordenação, especialmente com espécies pequenas e pouco polarizáveis. Aproveitando o caráter dos portais, foi possível sintetizar um complexo inédito, com um sítio catalítico proveniente de íons vanadila, VO2+, ligados às extremidades do cavitando. O acesso ao centro catalítico desta molécula, seria determinado pela captura do substrato no interior do macrociclo, limitado pelo volume do substrato e pelas relações de solvofobicidade, pois a cavidade possui um caráter hidrofóbico. A estrutura desse complexo ainda não foi completamente elucidada, pois não foi possível obter um cristal único e determiná-la através de difração de raios-x. As técnicas de caracterização no estado sólido empregadas indicaram que esse novo complexo teria o seu centro metálico provavelmente numa configuração piramidal de base retangular, isso é evidenciado pelas medidas espectroscópicas que remetem a uma simetria C2V. Foram feitas tentativas de coordenação mono e binuclear, mas algumas técnicas apontam para a formação apenas da espécie mononuclear. Os primeiros testes catalíticos, em condições brandas para a conversão de hidrocarbonetos apresentaram bons resultados frente a substratos lineares e inatividade frente a substratos cíclicos mais volumosos. Os resultados obtidos nos levam a crer que as reações de oxidação de alcanos, com estas espécies, se dão pelo interior da cavidade, ao longo do orbital dz2 do íon (V=O)2+, e não pela parte superior através do átomo de oxigênio ligado ao vanádio. / Cucurbiturils are versatile molecules used in several branches of chemistry. These molecular cages form a family of homologues from the condensation of glycoluril and formaldehyde in acidic medium. Their applications range from catalysis, photochemistry, organic and inorganic synthesis, materials and supramolecular chemistry. Particularly cucurbit[6]uril, or simply CB[6], is a very symmetrical molecule, which posesses coordination points on its portals, especially with hard acids. Due to this characteristic, it was possible to synthesize a new complex, with a vanadyl, VO2+, catalytic center bonded to the extremity of the cavitand. The access to the catalytic center of this molecule should be determined by the entrance of the substrate inside the macrocycle, which is limited by its volume and by solvophobicity, because the cavity is strongly hydrophobic. The structure of this new complex could not be completely solved yet, because it was impossible to obtain a single crystal for X-rays diffraction analysis. The solid state characterization techniques indicated that this new complex has its metallic center in a rectangular-base pyramid configuration. This was shown by the spectroscopic evidences which point to a d1 ion in a C2V symmetry. Attempts were made to synthesize both mono and binuclear species, but all the techniques we have used have demonstrated that only the former one was obtained. The first catalytic tests for hydrocarbons convertion in mild conditions have shown good results towards for linear substrates and no activity at all towards more volumous cyclic molecules. The results we have obtained indicate that the alkane oxidation reactions with these complexes occur inside the cavity along the (V=O)2+ ion dz2 orbital , and not on its superior moiety on the vanadyl oxygen atom.

El descubrimient o del mayor efecto hipoglicémic o del bis(maltolato)oxovanadio (IV) (BMOV) con respecto a las salesde vanadio, propició el diseño, síntesis y evaluación in vitro e in vivo de una amplia variedad de complejos de oxovanadio(IV) con diversos ligandos orgánicos para su potencial uso como fármacos antidiabéticos que reemplacen , parcial ototalmente , las incómodas inyecciones de insulina en el tratamiento de la diabetes mellitus tipo 1 o los fármacos oraleshipoglicemiante s empleados generalment e en terapias combinadas para la tipo 2.Recientemente , un complejo etil sustituido , análogo al BMOV, ha completado favorablemente la fase clínica I deensayos en humanos. El diseño de nuevos tipos de complejos y el desarrollo de nuevos modos de administración constituyenactualmente un activo campo de investigación / Design of Vanadium Antidiabetic Agents: Development and Recent AdvancesThe discovery of the enhanced hypoglycemic activity of bis(maltolate)oxovanadiu m (IV) (BMOV) compared to simplevanadium salts stimulated the design, synthesis and both in vitro and in vivo evaluation of a large variety of oxovanadium (IV)complexes with different organic ligands for their potential use as antidiabetic drugs that may be able to substitute, either partially or totally, the daily insulin injections used in the treatment of type 1 diabetes mellitus or the orally available hypoglycemic drugs usually used in combined therapy for type 2 diabetes.Recently, a close BMOV analogue, an ethyl substitute, has already successfully completed the phase 1 clinical trial in humans. The design of new types of complexes and the development of alternative delivery methods represent an active research field today.

As cucurbiturilas são moléculas versáteis empregadas em diversos ramos da química, essas gaiolas moleculares formam uma família de homólogos oriundos da condensação da glicolurila e do formaldeído em meio ácido. As aplicações se estendem para campos como catálise, fotoquímica, sínteses orgânica e inorgânica, química de materiais e supramolecular. Particularmente a cucurbit[6]urila, comumente chamada apenas de CB[6], uma molécula bastante simétrica, têm em seus opérculos, sítios propícios à coordenação, especialmente com espécies pequenas e pouco polarizáveis. Aproveitando o caráter dos portais, foi possível sintetizar um complexo inédito, com um sítio catalítico proveniente de íons vanadila, VO2+, ligados às extremidades do cavitando. O acesso ao centro catalítico desta molécula, seria determinado pela captura do substrato no interior do macrociclo, limitado pelo volume do substrato e pelas relações de solvofobicidade, pois a cavidade possui um caráter hidrofóbico. A estrutura desse complexo ainda não foi completamente elucidada, pois não foi possível obter um cristal único e determiná-la através de difração de raios-x. As técnicas de caracterização no estado sólido empregadas indicaram que esse novo complexo teria o seu centro metálico provavelmente numa configuração piramidal de base retangular, isso é evidenciado pelas medidas espectroscópicas que remetem a uma simetria C2V. Foram feitas tentativas de coordenação mono e binuclear, mas algumas técnicas apontam para a formação apenas da espécie mononuclear. Os primeiros testes catalíticos, em condições brandas para a conversão de hidrocarbonetos apresentaram bons resultados frente a substratos lineares e inatividade frente a substratos cíclicos mais volumosos. Os resultados obtidos nos levam a crer que as reações de oxidação de alcanos, com estas espécies, se dão pelo interior da cavidade, ao longo do orbital dz2 do íon (V=O)2+, e não pela parte superior através do átomo de oxigênio ligado ao vanádio. / Cucurbiturils are versatile molecules used in several branches of chemistry. These molecular cages form a family of homologues from the condensation of glycoluril and formaldehyde in acidic medium. Their applications range from catalysis, photochemistry, organic and inorganic synthesis, materials and supramolecular chemistry. Particularly cucurbit[6]uril, or simply CB[6], is a very symmetrical molecule, which posesses coordination points on its portals, especially with hard acids. Due to this characteristic, it was possible to synthesize a new complex, with a vanadyl, VO2+, catalytic center bonded to the extremity of the cavitand. The access to the catalytic center of this molecule should be determined by the entrance of the substrate inside the macrocycle, which is limited by its volume and by solvophobicity, because the cavity is strongly hydrophobic. The structure of this new complex could not be completely solved yet, because it was impossible to obtain a single crystal for X-rays diffraction analysis. The solid state characterization techniques indicated that this new complex has its metallic center in a rectangular-base pyramid configuration. This was shown by the spectroscopic evidences which point to a d1 ion in a C2V symmetry. Attempts were made to synthesize both mono and binuclear species, but all the techniques we have used have demonstrated that only the former one was obtained. The first catalytic tests for hydrocarbons convertion in mild conditions have shown good results towards for linear substrates and no activity at all towards more volumous cyclic molecules. The results we have obtained indicate that the alkane oxidation reactions with these complexes occur inside the cavity along the (V=O)2+ ion dz2 orbital , and not on its superior moiety on the vanadyl oxygen atom.

The present thesis deals with different aspects of the chemistry of oxovanadium(IV) complexes, their interaction with DNA and protein and photo-induced DNA and protein cleavage activity and photocytotoxicity. Chapter I presents a general introduction on various modes of interactions of organic compounds and transition metal complexes capable of targeting DNA leading to DNA strand scission, emphasizing particularly the photo-induced DNA cleavage activities for their potential application in PDT. The mechanistic pathways associated with the DNA cleavage are discussed. A comparison has been made on the advantages of photoactive metal complexes over organic conjugates. Objective of the present investigation is also dealt in this Chapter. Chapter II of the thesis deals with the synthesis, characterization, DNA binding and photo-induced DNA cleavage activity of ternary oxovanadium(IV) complexes of N-salicylidene-S-methyldithiocarbazate (salmdtc) and phenanthroline bases to explore the photo-induced DNA cleavage activity in UV-A light of 365 nm. Chapter III presents the synthesis, characterization, DNA binding and photo-induced DNA cleavage activity of ternary oxovanadium(IV) complexes containing N-salicylidene-L-methionate (salmet) and N-salicylidene-L-tryptophanate (saltrp) Schiff bases and phenanthroline bases. The objective of this work is to investigate the photo-induced DNA cleavage activity in near-IR light and to see the effect of pendant thiomethyl and indole moieties in the DNA cleavage reactions. Chapter IV deals with the synthesis, characterization, DNA binding, red-light induced DNA cleavage activity and photocytotoxicity of ternary oxovanadium(IV) complexes having N-salicylidene-L-arginine (sal-argH) and N-salicylidene-L-lysine (sal-lysH) Schiff bases and phenanthroline bases. The important results include the visible light-induced DNA cleavage activity and photocytotoxicity of the complexes in human cervical HeLa cancer cells. Chapter V describes the synthesis, characterization, DNA binding and photo-induced DNA and protein cleavage activity and photocytotoxicity of oxovanadium(IV) complexes containing bis(2-benzimidazolylmethyl)amine and phenanthroline bases. The significant results include DNA cleavage activity in near-IR light and photocytotoxicity of the dppz complex in non-small cell lung carcinoma/human lung adenocarcinoma A549 cells in visible light. Further, we have studied the protein cleavage activity of the complexes in UV-A light of 365 nm by using bovine serum albumin (BSA) and lysozyme. Finally, Chapter VI presents the binary oxovanadium(IV) complexes of phenanthroline bases. We have studied their synthesis, characterization, DNA binding and photo-induced DNA and protein cleavage activity and photocytotoxicity. Photocytotoxicity of dppz complex has been studied in human cervical HeLa cancer cells in visible light. Photo-induced protein cleavage activity of the complexes has been studied in UV-A light of 365 nm by using BSA and lysozyme. The references have been compiled at the end of each chapter and indicated as superscript numbers in the text. The complexes presented in this thesis are represented by bold-faced numbers. Crystallographic data of the complexes, characterized structurally by single crystal X-ray crystallography, are given in CIF format in the enclosed CD (Appendix-I). Due acknowledgements have been made wherever the work described is based on the findings of other investigators. Any omission that might have happened due to oversight or mistake is regretted.

Thesis. (M.Tech. (Chemistry)) -- Vaal University of Technology, 2011. / Bis(acetylacetonato)oxovanadium(IV), Manganese(III) acetylacetonate, Co-Zn and Co-Al were prepared as catalyst precursors for the synthesis of carbon materials in a catalytic chemical vapor deposition (CCVD) reactor. The carbon materials produced were characterized with Raman spectroscopy, Scanning electron microscope (SEM), Energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), High resolution transmission electron microscopy (TEM) and Thermogravimetric analysis, (TGA). Carbon material prepared from bis(acetylacetonato)oxovanadium(IV) catalyst precursor showed the presence of carbon spheres with average diameter of 104μm together with small traces of carbon nanotubes or amorphous carbon. Synthesis of bis(acetylacetonato)oxovanadium(IV) catalyst precursor yielded approximately 92% of carbon material. Carbon material prepared from Manganese(III) acetylacetonate catalyst precursor showed the presence of carbon spheres with diameter of 87.5μm. Synthesis of Manganese(III) acetylacetonate catalyst precursor yielded approximately 97% of carbon material. Carbon material produced from Co-Zn and Co-Al catalyst precursors showed the presence of carbon nanotubes with small amounts of amorphous carbon. The use of Co-Zn catalyst precursor yielded approximately 80% of carbon nanotubes, whereas Co-Al catalyst precursor yielded approximately 98% of carbon nanotubes. / Vaal University of Technology; NRF; SASOL

The present thesis deals with different aspects of the chemistry and photo-biology of various ferrocene-conjugates, their interaction with double helical DNA, DNA photocleavage and photo-enhanced cytotoxicity in visible light, localization and cellular uptake to study the mechanism of cell death. Phenyl analogues of the active complexes have been synthesized and used for comparison in biological assays. Chapter I presents an overview of cancer and its types, various treatments for cancer. A general overview on the Photodynamic Therapy, a new modality of light activated cancer treatment and its various possible mechanism of action, has been made. The promise of photoactivated chemotherapy is discussed with recently developed metal based antitumor agents. Biological applications of few ferrocene conjugates as anticancer and anti-malarial agents are discussed. The objective of the present investigation is also presented in this chapter. Chapter II presents the synthesis, characterization, structure, DNA binding, DNA photocleavage, photocytotoxicity and cellular localization of ferrocene-conjugated dipicolylamine oxovanadium(IV) complexes of curcumin. To explore the role of the ferrocenyl moiety the phenyl analogue of the ferrocenyl complexes is synthesized and used as a control for comparison purpose. Chapter III deals with the photo-induced DNA cleavage and photo-enhanced cytotoxicity of ferrocene-conjugated oxovanadium(IV) complexes of heterocyclic bases. The synthesis, characterization, structural comparisons, DNA binding, DNA photocleavage and photocytotoxic activity in visible light are discussed in detail. Chapter IV describes the synthesis, characterization and structure of ferrocene-conjugated oxovanadium(IV) complexes of acetylacetonate derivatives. The complexes are evaluated for DNA binding, DNA photocleavage and photocytotoxic activity in HeLa, MCF-7, 3T3 cells in visible light. The fluorescent nature of the complexes is used to study the cellular localization of the complexes and the mechanism of cell death induced by the complexes is also discussed. Chapter V presents the photocytotoxic effect of ferrocene-conjugated oxovanadium(IV) complexes of different curcuminoids in HeLa , HepG2 and 3T3 cells. Curcumin based fluorescence has been successfully used to study the cellular uptake and localization behavior of the complexes. The positive role of the ferrocenyl complex is evident from the ~4 fold increase in its photocytotoxicity compared to the phenyl analogue. The apoptotic mode of cell death is evident from nuclear co-staining using Hoechst dye. Chapter VI describes the synthesis, characterization and photochemotherapeutic efficacy of ferrocene conjugates of N-alkyl pyridinium salts. Mitochondria targeting property of ferrocene compound having n-butyltriphenylphosphonium group has been studied by JC-1 assay. FACS analysis showed significant sub G1/G0 phase cell-cycle arrest in cancer cells on visible light treatment. Finally, the summary of the dissertation and conclusions drawn from the present investigations are presented. The references in the text have been indicated as superscript numbers and compiled at the end of each chapter. The complexes presented in this thesis are represented by bold-faced numbers. Crystallographic data of the structurally characterized complexes are given in CIF format in the enclosed CD (Appendix-I). Due acknowledgements have been made wherever the work described is based on the findings of other investigators. Any unintentional omission that might have happened due to oversight or mistake is regretted. INDEX WORDS: Ferrocene conjugates Crystal structure DNA binding DNA photocleavage Photocytotoxicity Vanadium Cellular Imaging

The present thesis deals with different aspects of the chemistry of oxovanadium(IV) complexes, their interaction with double stranded DNA, photo-induced DNA cleavage, photo-enhanced cytotoxicity in visible light and red light and localisation and cellular uptake to understand the mechanism of cell death. Chapter I presents a general introduction on potential of transition metal complexes as photochemotherapeutic agents. A brief introduction about Photodynamic Therapy (PDT) as a new alternative to chemotherapy for treating cancer has been made. Various modes of interaction of small molecules with duplex DNA are described. Recent reports on metal-based photocytotoxicity, photo-induced DNA cleavage activity and cellular localization are presented in detail. Objective of the present investigation is also dealt in this Chapter. Chapter II of the thesis deals with the synthesis, characterization, DNA binding and photo-induced DNA cleavage activity of ternary oxovanadium(IV) complexes of ONO-donor 2-(2-hydroxybenzylideneamino)phenol (salamp) and phenanthroline bases to explore the photo-induced DNA cleavage activity in UV-A light of 365 nm and photocytotoxicity in visible light. Chapter III deals with the photo-induced DNA cleavage and photocytotoxicity of ternary oxovanadium(IV) complexes containing ONN-donor N-2-pyridylmethylidine-2-hydroxyphenylamine (Hpyamp) Schiff bases and phenanthroline bases. The objective of this work is to investigate the photo-induced DNA cleavage activity in near-IR light. Photocytotoxicity and cell cycle arrest have been studied in HeLa cancer cells. Chapter IV deals serendipitous discovery of planar triazinuim cationic species by vanadyl-assisted novel ring cyclization reaction. The compounds are synthesised, characterized and their DNA binding and anaerobic photoinduced DNA cleavage activity are presented. The importance of the thiazole moiety in the triazinuim species in cellular uptake has been investigated. Photocytotoxicity, localization and cell death mechanism have been studied in HeLa and MCF-7 cells. Chapter V describes the synthesis, characterization, DNA binding, photo-induced DNA cleavage activity and photocytotoxicity of oxovanadium(IV) complexes containing 2-(1H-benzimidazol-2-yl)-N-(pyridin-2-ylmethylene)ethaneamine (Hpy-aebmz) and curcumin as photosensitizer. The effect of conjugating naphthalimide on Hpy-aebmz on photoinduced DNA cleavage and photocytotoxicity has been studied. Cellular uptake, localization and mechanism of cell death induced by complexes have been investigated. Chapter VI presents ternary oxovanadium(IV) complexes having, 2-((1H-benzimidazol-2-yl)methylimino-methyl)phenol (Hsal-ambmz) and phenanthroline bases. The complexes were synthesized, characterized and their DNA binding property studied. Photo-induced DNA cleavage activity and photocytotoxicity in red light has been discussed. Anthracene has been conjugated to a tridentate ligand to investigate cellular uptake, localization and cell death mechanism. Mitochondria targeting property of the complexes having dipeptide has been studied and compared with clinically used drug Photofrin®. The references have been compiled at the end of each chapter and indicated as superscript numbers in the text. The complexes presented in this thesis are represented by bold-faced numbers. Crystallographic data of the complexes, characterized structurally by single crystal X-ray crystallography, are given in CIF format in the enclosed CD (Appendix-I). Due acknowledgements have been made wherever the work described is based on the findings of other investigators. Any unintentional omission that might have happened due to oversight or mistake is regretted.

My research was focused on understanding the importance of mammalian RAD51 paralogs in genome maintenance and suppression of tumorigenesis. The investigation carried out during this study has been addressed toward gaining more insights into the involvement of RAD51 paralogs in DNA damage signalling, repair of various types of lesions including double stranded breaks (DSBs), daughter strand gaps (DSGs), interstrand crosslinks (ICLs), and in the protection of stalled replication forks. My study highlights the molecular functions of RAD51 paralogs in Fanconi anemia (FA) pathway of ICL repair, in the ATM and ATR mediated DNA damage responses, in homologous recombination (HR), and in the recovery from replication associated lesions. My research also focused on the development of a novel photoinducible ICL agent for targeted cancer therapy. The thesis has been divided into following sections as follows: Chapter I: General introduction that describes about DNA damage responses and the known functions of RAD51 paralogs across species in DNA repair and checkpoint The genome of every living organism is susceptible to various types of DNA damage and mammalian cells are evolved with various DNA damage surveillance mechanisms in response to DNA damages. In response to DNA damage, activated checkpoints arrest the cell cycle progression transiently and allow the repair of damaged DNA. Upon completion of DNA repair, checkpoints are deactivated to resume the normal cell cycle progression. Defective DNA damage responses may lead to chromosome instability and tumorigenesis. Indeed, genome instability is associated with several genetic disorders, premature ageing and various types of cancer in humans. The major cause of chromosome instability is the formation of DSBs and DSGs. Both DSBs and DSGs are the most dangerous type of DNA lesions that arise endogenously as well as through exogenous sources such as radiations and chemicals. Spontaneous DNA damage is due to generation of reactive oxygen species (ROS) through normal cellular metabolism. Replication across ROS induced modified bases and single strand breaks (SSBs) leads to DSGs and DSBs, respectively. Such DNA lesions need to be accurately repaired to maintain the integrity of the genome. To understand the various cellular responses that are triggered after different types of DNA damage and the possible roles of RAD51 paralogs in these processes, chapter I of the thesis has been distributed in to multiple sections as follows: Briefly, the initial portion of the chapter provides a glimpse of various types of DNA damage responses and repair pathways to deal with the lesions arising from both endogenous as well as exogenous sources. Owing to the vast range of cellular responses and pathways, the following section provides the detailed description and mechanisms of various pathways involved in taking care of wide range of DNA lesions from SSBs to DSBs. Subsequent section of chapter I provides a comprehensive description of maintenance of genome stability at the replication fork and telomeres. Germline mutations in the genes that regulate genome integrity cause various genetic disorders and cancer. Mutations in ATM, ATR, MRE11, NBS1, BLM and FANC (1-16), BRCA1 and BRCA2 that are known to regulate DNA damage signaling, DNA repair and genome integrity lead to chromosome instability disorders such as ataxia-telangiectasia, ATR-Seckel syndrome, AT-like disorder, Nijmegen breakage syndrome, Bloom syndrome, FA, and breast and ovarian cancers respectively. Interestingly, RAD51 paralog mutations are reported in patients with FA-like disorder and various types of cancers including breast and ovarian cancers. Mono-allelic germline mutations in all RAD51 paralogs are reported to cause cancer in addition to the reported cases of FA-like disorder with bi-allelic germline mutations in RAD51C and XRCC2. In accordance, the last section of the chapter has been dedicated to describe the genetics of breast and ovarian cancers and the known functions of tumor suppressors such as BRCA1, BRCA2 and RAD51 paralogs in the protection of genome. Despite the identification of five RAD51 paralogs nearly two decades ago, the molecular mechanism(s) by which RAD51 paralogs regulate HR and genome maintenance remain obscure. To gain insights into the molecular mechanisms of RAD51 paralogs in DNA damage responses and their link with genetic diseases and cancer, the following objectives were laid for my PhD thesis: 1) To understand the functional role of RAD51 paralog RAD51C in FA pathway of ICL repair and DNA damage signalling. 2) To dissect the ATM/ATR mediated targeting of RAD51 paralog XRCC3 in the repair of DSBs and intra S-phase checkpoint. 3) To uncover the replication restart pathway after transient replication pause and the involvement of distinct complexes of RAD51 paralogs in the protection of replication forks. 4) To design photoinducible ICL agent that can be activated by visible light for targeted cancer therapy. Chapter II: Distinct roles of FANCO/RAD51C protein in DNA damage signaling and repair: Implications for Fanconi anemia and breast cancer susceptibility RAD51C, a RAD51 paralog has been implicated in HR. However, the underlying mechanism by which RAD51C regulates HR mediated DNA repair is elusive. In 2010, a study identified biallelic mutation in RAD51C leading to FA-like disorder, whereas a second study reported monoallelic mutations in RAD51C associated with increased risk of breast and ovarian cancers. However, the role of RAD51C in the FA pathway of DNA cross-link repair and as a tumor suppressor remained obscure. To understand the role of RAD51C in FA pathway of ICL repair and DNA damage response, we employed genetic, biochemical and cell biological approaches to dissect out the functions of RAD51C in genome maintenance. In our study, we observed that RAD51C deficiency leads to ICL sensitivity, chromatid-type errors, and G2/M accumulation, which are hallmarks of the FA phenotype. We found that RAD51C is dispensable for ICL unhooking and FANCD2 monoubiquitination but is essential for HR, confirming the downstream role of RAD51C in ICL repair. Furthermore, we demonstrated that RAD51C plays a vital role in the HR-mediated repair of DSBs associated with replication. Finally, we showed that RAD51C participates in ICL and DSB induced DNA damage signaling and controls intra-S-phase checkpoint through CHK2 activation. Our analyses with pathological mutants of RAD51C displayed that RAD51C regulates HR and DNA damage signaling distinctly. Together, these results unravel the critical role of RAD51C in the FA pathway of ICL repair and as a tumor suppressor. Chapter III: ATM-and ATR-mediated phosphorylation of XRCC3 regulates DNA double-strand break-induced checkpoint activation and repair The RAD51 paralogs XRCC3 and RAD51C have been implicated in HR and DNA damage responses, but the molecular mechanism of their participation in these pathways remained obscured. In our study, we showed that an SQ motif serine 225 in XRCC3 is phosphorylated by ATR kinase in an ATM signaling pathway. We found that RAD51C in CX3 complex but not in BCDX2 complex is essential for XRCC3 phosphorylation, and this modification follows end resection and is specific to S and G2 phases. XRCC3 phosphorylation was found to be required for chromatin loading and stabilization of RAD51 and HR-mediated repair of DSBs. Notably, in response to DSBs, XRCC3 participates in the intra-S-phase checkpoint following its phosphorylation and in the G2/M checkpoint independently of its phosphorylation. Strikingly, we found that XRCC3 distinctly regulates recovery of stalled and collapsed replication forks such that phosphorylation was required for the HR-mediated recovery of collapsed replication forks but is dispensable for the recovery of stalled replication forks. Together, our findings suggest that XRCC3 is a new player in the ATM/ATR-induced DNA damage responses to control checkpoint and HR-mediated repair. Chapter IV: RAD51 paralogs protect stalled forks and mediate replication restart in an FA-BRCA independent manner Mammalian RAD51 paralogs RAD51 B, C, D, XRCC2 and XRCC3 are critical for genome maintenance. To understand the crucial roles of RAD51 paralogs during spontaneously arising DNA damage, we have studied the RAD51 paralogs assembly during replication and examined the replication fork stability and its restart. We found that RAD51 paralogs are enriched onto the S-phase chromatin spontaneously. Interestingly, the number of 53BP1 nuclear bodies in G1-phase and micro-nucleation which serve as markers for under replicated lesions increases after genetic ablation of RAD51C, XRCC2 and XRCC3. Furthermore, we showed that RAD51 paralogs are specifically enriched at two major fragile sites FRA3B and FRA16D after replication fork stalling. We found that all five RAD51 paralogs bind to nascent DNA strands after replication fork stalling and protect the fork. Nascent replication tracts created before fork stalling with hydroxyurea degrade in the absence of RAD51 paralogs but remain stable in wild-type cells. This function was dependent on ATP binding at the walker A motif of RAD51 paralogs. Our results also suggested that RAD51 paralogs assemble into BCDX2 complex to prevent generation of DSBs at stalled replication forks, thereby safeguarding the pre-assembled replisome from the action of nucleases. Strikingly, we showed that RAD51C and XRCC3 in complex with FANCM promote the restart of stalled replication forks in an ATP hydrolysis dependent manner. Moreover, RAD51C R258H mutation that was identified in FA-like disorder abrogates the interaction of RAD51C with FANCM and XRCC3, and prevents fork restart. Thus, assembly of RAD51 paralogs in different complexes prevents nucleolytic degradation of stalled replication forks and promotes restart to maintain genomic integrity. Chapter V: Trans-dichlorooxovandium(IV) complex as a potent photoinducible DNA interstrand crosslinker for targeted cancer therapy Although DNA ICL agents such as MMC, cisplatin and psoralen are known to serve as anticancer drugs, these agents affect normal cells as well. Moreover, tumor resistance to these agents has been reported. We have designed and synthesized a novel photoinducible DNA crosslinking agent (ICL-2) which is a derivative of oxovanadiumterpyridine complex with two chlorides in trans position. We found that ICL-2 can be activated by UV-A and visible light to enable DNA ICLs. ICL-2 efficiently activated FA pathway of ICL repair. Strikingly, photoinduction of ICL-2 induces prolonged activation of cell cycle checkpoint and high degree of cell death in FA pathway defective cells. Moreover, we showed that ICL-2 specifically targets cells that express pathological RAD51C mutants. Our findings suggest that ICL-2 can be potentially used for targeted cancer therapy in patients with gene mutations in FA and HR pathway.