TIFR
Department of Chemical Sciences
School of Natural Sciences

October 14, 2019 at 4.00 pm in AG-69

Title :

The Electron Attachment Induced Radiation Damage to Genetic Materials : The Role of Water

Abstract :

Radiation damage to genetic material is one of the active fields of chemical research with implications in both the cause and cure of cancer.  The origin of the damage for a long time is attributed to ionization and excitation process created by the high energy radiation.  But recent experiments have highlighted the critical role played by the low-energy secondary electron in the radiation damage process[1]. Water has been shown to accelerate the process of radiation damage[2]. We have proposed a mechanism for the water-mediated attachment of electron to nucleobases. The initial electron attached state is localized on the water, and the water bound states act as a doorway for the electron attachment to nucleobases. Subsequently, the electron is transferred  to the nucleobase due to the mixing of the electronic and the nuclear degrees of  freedom in the solvated nucleobase. Our theoretical simulations show that the presence of bulk water increases the rate of the electron transfer, which takes place in the ultrafast time scale. The local structure of water around the nucleobase anion plays a crucial role in the electron attachment process. The computed adiabatic electron affinity of nucleobases and rate of electron transfer from water to the nucleobases shows good agreement with the experimental results validating our proposed mechanism[3].

 

         

References:

[1]B. Boudaïffa, P. Cloutier, D. Hunting, M. A. Huels, L. Sanche, Science (80. )., 2000, DOI:10.1126/science.287.5458.1658.

[2]J. Ma, F. Wang, S. A. Denisov, A. Adhikary, M. Mostafavi, Sci. Adv., 2017, DOI:10.1126/sciadv.1701669.

 [3]      M. Mukherjee, D. Tripathi, A.K. Dutta, 2019 (to be communicated )

 

 

 

September 19, 2019 at 4.00 pm in AG-80

Title :

Cu2ZnSnS4: A potential photovoltaic material for low-cost thin film solar cell

Abstract :

Shifting to renewable energy is the solution for meeting the ever-increasing energy demand as well as preventing the deterioration of the environment due to the use of fossil fuel-based energy sources. The Sun from which abundant energy is received on the Earth is probably the most promising renewable energy source. A solar photovoltaic (PV) device converts the solar radiation directly into electricity. Such as device is comprised of a photo-absorber material that absorbs incident photons and uses their energy for raising the chemical potential of electrons within the material. Other materials interfaced with this photo-absorber help collect these excited electrons at one end of the PV device and thereby help generate a potential difference.

Silicon has been the semiconductor of choice for the role of photo-absorber in a PV device, however, due to lower absorption coefficient (α =103 cm−1; λ < 825 nm) and brittleness, several other materials and related PV technologies have been developed. Among these, the technologies based on CdTe and Cu(In,Ga)Se2 (CIGSe) semiconductors have been commercially successful with a photoconversion efficiency  of more than 20 %. However, the concerns associated with toxic element Cd and less abundant elements In and Ga have propelled the investigation for alternative semiconductors. A quaternary compound Cu2ZnSnS4 (CZTS) is a promising candidate owing to the facts that all the elements used in it are non-toxic and have relatively abundant availability; moreover, it has a high absorption coefficient (α =104 cm−1; λ < 825 nm) and close to an optimum band gap ( = 1.5 eV).

In the present talk, the challenges associated with the synthesis of CZTS thin films via solution chemistry will be discussed and ways to eliminate/control them have been suggested. First, in order to obtain a film with a desired surface morphology, a combined use of the chloride and acetate salts of copper in the precursor solution has been found to be helpful. Further to this, a capping layer of ZnS was necessary on top of the precursor film in order to obtain a CZTS thin film with controlled surface morphology. The CZTS film prepared by this strategy had a stoichiometric composition, the grain size of the order of ~ 200 nm,  = 1.5 eV, and a high hole-carrier density ~ 1019 cm−3.

Further, the tuning of electrical and optical properties of the CZTS via incorporation of silver (Ag) at copper sub-lattice sites in CZTS will be discussed. Thin films of the resulting pentanary alloys (Ag1-xCux)2ZnSnS4 (0 ≤ x ≤ 1) show a remarkable change in their microstructure and electronic properties with increasing Ag content. Going from Cu2ZnSnS4 (x = 0) to Ag2ZnSnS4 (x = 1), the grain size increased from 0.2 to 2 μm which has been correlated to the formation of intermediate phase with relatively lower melting point. 

Finally, optimized (Ag1-xCux)2ZnSnS­4 thin films were incorporated in a PV device that resulted in a short circuit current density Jsc of 9.47 mA/cm2, open circuit voltage (Voc) of 600 mV, and a fill factor of 34 % leading to ƞ of 1.92 %. 

September 11, 2019 at 11.30 am in AG-80

Title :

Bispidine coordination chemistry – ligands for medicinal chemistry, bioinorganic modeling and oxidation catalysis

Abstract :

All important properties of coordination compounds – hermodynamics (complex stabilities, metal ion selectivities, redox potentials), kinetics (reaction rates, selectivities and pathways) as well as electronics (spectroscopy and magnetism) – depend on their structure. The structure of metal complexes is the result of metal ion and ligand preferences, and it is shown that ligand preferences prevail, especially with ligands as rigid as the bispidines. The unique geometries of bispidine transition metal and lanthanide complexes will be described and the resulting properties will be discussed. Specific examples and applications that will be presented include the CuII/I couple with applications ranging from oxygen activation, azidirination and positron emission tomography (PET) and high-valent nonheme iron model chemistry (oxidation catalysis).
 
References
P. Comba, M. Kerscher, W. Schiek Progr. Inorg. Chem., 2007, 55, 613.
P. Comba, M. Kerscher, K. Rück, M. Starke Dalton Perspective 2018, 47, 9202

September 13, 2019 at 11.30 am in AG-80

Title :

Targeted Prodrugs to Manipulate Copper Biology of Prostate Cancer

Abstract :

Cancer cells have considerably different metallome than normal cells. Especially prostate cancer has been shown to overexpress several important copper trafficking proteins and recruit high levels of copper, making it more sensitive towards drugs like disulfiram.1 Disulfiram acts by altering the copper biology of prostate cancer. Though disulfiram is a promising anticancer agent, the off-target activities lead to adverse side effects.2 Disulfiram’s off-target effects can be mitigated in the cancer setting by chemical modification of the active pharmacophore, dithiocarbamate, in ways that target it preferentially to prostate cancer cells.3 In this seminar, I will present the design, development, and activity of dithiocarbamate prodrug, a Cu prochelator, that is activated in the prostate cancer microenvironment specifically.

 

References :

 

(1)      Safi, R.; Nelson, E. R.; Chitneni, S. K.; Franz, K. J.; George, D. J.; Zalutsky, M. R.; McDonnell, D. P. Copper signaling axis as a target for prostate cancer therapeutics.Cancer Res 2014, 74, 5819.

(2)      Schweizer, M. T.; Lin, J.; Blackford, A.; Bardia, A.; King, S.; Armstrong, A. J.; Rudek, M. A.; Yegnasubramanian, S.; Carducci, M. A. Pharmacodynamic study of disulfiram in men with non-metastatic recurrent prostate cancer.Prostate Cancer Prostatic Dis 2013, 16, 357.

(3)      Bakthavatsalam, S.; Sleeper, M. L.; Dharani, A.; George, D. J.; Zhang, T.; Franz, K. J. Leveraging γ-Glutamyl Transferase To Direct Cytotoxicity of Copper Dithiocarbamates against Prostate Cancer Cells.Angew. Chem. Int. Ed. 2018, 57, 12780.

August 29, 2019 at 2.30 pm in AG-80

Title :

Engineering Artificial Metalloenzyme for Enantioselective Catalysis

Abstract :

Artificial metalloenzymes (ArMs) offer new opportunities to improve catalytic selectivity and efficiency. They are a class of catalysts that result from incorporation of an organometallic catalyst precursor within a host protein. This combination exploits the reactivity of the transition metal catalysts while taking advantage of the selectivity and adaptability of proteins. Our research has demonstrated formation of ArMs through strain promoted azide-alkyne cycloaddition. It is a ‘click’ bioconjugation approach, where a covalent link is formed between metal complex and protein through cycloaddition of a strained alkyne linker and genetically encoded p-azidophenyl alanine on the protein scaffold. Several scaffold proteins and cofactor components were explored to demonstrate versatility of this method. Extensive biophysical characterization of these Arms was also done by mass spectrophotometry, fluorescence spectroscopy and X-ray crystallography. Scaffold proteins were engineered to get accelerated bioconjugation of metal cofactors without perturbing the structural conformation around the catalytic site. Enantioselective carbene insertion into Si-H bond and olefin has been observed using dirhodium based ArMs. Further improvement in selectivity was explored via structure based single point mutagenesis and directed protein evolution approach.