In one area, our studies involve DNA containing stereochemically defined, site-specific carcinogen lesions. Such lesions are the result of DNA binding by the metabolically activated forms of polycyclic aromatic hydrocarbons (PAH) that are ubiquitous environmental pollutants. Thus, biological activation of these PAH followed by DNA binding of the ensuing metabolites produce biological events that can ultimately lead to tumor formation and possibly cancer. In mammalian cells PAH undergo metabolic activation to four isomeric diol epoxides (Figure 1).

The metabolically formed electrophilic diol epoxides undergo ring-opening in the presence of DNA and the incipient intermediate is trapped by the exocyclic amino groups of the purine bases, resulting in covalent modification of DNA. Figure 2 shows how adducts are produced by alkylation of DNA by diol epoxides and the subsequent processes by which tumor formation could occur. However, DNA alkylation by the 4 diol epoxides of any PAH is complex, and 8 isomeric 2’-deoxyadenosine adducts and 8 isomeric 2’-deoxyguanosine adducts are formed (Figure 2) by both cis and trans ring opening of the epoxide.

In order to gain a better understanding of steps involved in the tumor formation, it is important to understand the biological responses (such as DNA replication, repair, DNA-dependent RNA synthesis etc.) elicited by each adduct. It therefore critical to develop rational synthetic methodologies for constructing the individual adducts. Once the adducts are synthesized these can be incorporated into specific sites of biologically significant DNA sequences such as ras or tumor suppressor genes. Thus, sets of identical DNA oligomers can be prepared, where the only variation resides in the PAH stereochemistry. Structures of these modified DNA, obtained through X-ray and NMR methods. In conjunction with the structural data, biochemical and biological experimentation with the modified DNA then allows for evaluation of the adduct structure in relation to the biological response.

Our research involves substantial synthetic chemistry, and we collaborate with experts in various fields for structural, biological and biochemical studies. Figure 3 shows four N-ras sequences we have synthesized that contain isomeric adducts of the potent environmental carcinogen benzo[a]pyrene. In our initial experiments we have found that the thermal denaturation properties of the duplex with sequence IV, which contains a highly potent carcinogen, is markedly different from those of the remaining three. This therefore indicates that this lesion is substantially different in its behavior compared to the remaining less tumorigenic isomers.

In another area of chemical carcinogenesis we have been involved with studies aimed at understanding how planarity or lack there of translates to the metabolic profile of a PAH as well as its biological activity. For this purpose we have synthesized 1,4-dimethylbenzo[c]phenanthrene and its metabolites. This PAH is 35 degrees out of plane (Figure 4) compared to the unsubstituted benzo[c]phenanthrene (which is 25 degrees out of plane). Our studies have shown that 1,4-dimethylbenzo[c]phenanthrene and its metabolites exhibit helical properties and undergo slow helical interconversion. The studies also show that 1,4-dimethylbenzo[c]phenanthrene is less readily oxidized to its terminal diol epoxide metabolite by cytochrome P450 1B1 and metabolic activation in the final epoxidation step is substantially influenced by the distortion.

We are actively involved with the development of novel methods in organic synthesis. Among several, one example is our development of palladium-catalyzed approaches for nucleoside modification. In one application we have demonstrated a catalytic method for the introduction of aryl moieties into the exocyclic amino group of 2’-deoxyadenosine (Scheme 1).

We have also shown that nucleoside O-aryl sulfonates are excellent coupling partners with aryl boronic acids in palladium-catalyzed Suzuki-Miyaura reactions (Scheme 2). Efforts towards development of palladium catalysis for other types of nucleoside and DNA modification are also ongoing.

Our research routinely requires the use of HPLC, DNA synthesizer, photochemical equipment, UV-Vis and high-field NMR instrumentation. Our program offers a unique possibility for developing new organic synthesis methods which involve regio and stereochemical control, as well as methods involving organotransition metal chemistry all of which can be brought to bear for studying problems at the chemistry-biology interface.

After completing the Ph.D. from the University of Oklahoma in 1989, Mahesh joined the National Institutes of Health as a Fogarty Fellow in 1990. In 1994 he moved to the private sector where he served as Senior Scientist until 1997. In 1998 he came back to the academic environment (joining the University of North Dakota) and later moving to City College in 2000.

Grants from the NIH and NSF, instrumentation grants from the NSF (500 MHz NMR and Circular Dichroic spectrophotometer), PSC-CUNY awards.

Undergraduates. Undergraduate students have always been part of our research program and have made significant contributions to our studies. Qualifying students are often supported through programs such as the NIH MARC and RISE. Interested students are encouraged to contact Prof. Lakshman.

Graduate students. Students interested in the MA degree can become involved in our research programs through both "Introduction to Research Methodology" and thesis research. Students interested in obtaining the Ph.D. degree are encouraged to visit the graduate center link at http://web.gc.cuny.edu/Chemistry/ Students interested in the MA and the Ph.D. degrees are also encouraged to contact Prof. Lakshman.

Postdoctoral Associates. Our research has been funded by the National Institutes of Health (National Cancer Institute) as well as the National Science Foundation. Therefore, there are times when postdoctoral positions become available. Additionally, individuals who have other sources of stipend funding can also be supported through our programs. Interested individuals are encouraged to contact Prof. Lakshman.