MAHESH K. LAKSHMAN


Contact information

Email: lakshman[a]sci.ccny.cuny.edu

Tel: (212) 650-7835, Fax: (212) 650-6107



 
ORGANIC SYNTHESIS AT THE CHEMISTRY BIOLOGY INTERFACE

    Research in our group primarily involves development of novel organic synthesis techniques to address questions of a biological nature.  Current research projects are directed in two major directions: studies on understanding the molecular basis of chemical-induced mutagenesis as well as carcinogenesis, and metal-mediated nucleoside modification methods.

Studies on site-specific DNA modification with metabolites of polycyclic aromatic hydrocarbons

    Polycyclic aromatic hydrocarbons (PAHs) as well as their aza and sulfur analogs are widely prevalent in the environment and many members of this family are metabolized to ultimate carcinogens.  These compounds therefore represent a health risk to humans.  The metabolically formed compounds that cause adverse biological effects are the 4 isomeric bay or fjord region diol epoxides shown in Figure 1.

Figure 1

    All 4 diol epoxides interact with DNA and undergo a ring-scission of the epoxide moiety and the ensuing cationic intermediate is trapped by the exocylic amino groups of the purine bases in cellular DNA.  This covalent modification of DNA is considered to be the first step in the mutagenic and carcinogenic responses eliced by the diol epoxides.  Since the overall mechanism of DNA alkylation is SN1 like, each diol epoxide can produce 4 nucleoside adducts with each of the 2 purine nucleosides.  Thus, metabolism and DNA binding of any single PAH can result in the formation of a total 16 adducts.  The biological effect of each adduct must then be related to the replication and repair mechanisms involved (Figure 2).

Figure 2
  
    In order to understand better the biochemical processes involved in mutagenesis by diol epoxide-DNA adducts, research in our group is directed towards the stereoselective synthesis of individual diol epoxide-nucleoside adducts and incorporation of these into specific sites in DNA.  This entails in most cases development of new synthesis methodology.  Once the site-specifically modified DNA are available, then a variety of physical measurements can be undertaken, as well as determination of their solution structures by NMR.  Experimentation in collaboration with biochemists is aimed at understanding the cellular process that can then be related to the structures of the specific diol epoxide-DNA lesion.  Structures of the various DNA adduct stereoisomers are shown in Figure 3.
Figure 3


Influence of molecular structure on the biological properties of aromatic hydrocarbons

    There has been a proposal that non-planarity of polycyclic aromatic hydrocarbons may influence their DNA adduct forming abilities and the net biological effect of the DNA adducts.  Therefore, our group has been interested in the synthesis of non-planar PAHs as well as their metabolic profiles and biological activities.  In this context we have synthesized 1,4-dimethylbenzo[c]phenanthrene (1,4-DMBcPh) and its metabolites. This PAH is 35 degrees out of plane (Figure 4) compared to the unsubstituted benzo[c]phenanthrene (BcPh, which is 25 degrees out of plane).

Figure 4

    Our studies have shown that 1,4-DMBcPh and its putative metabolites exhibit helical properties and the isomers undergo slow helical interconversion (Figure 5).  In addition, 1,4-DMBcPh is less readily oxidized to its terminal diol epoxide metabolites by cytochrome P450 1B1 and metabolic activation in the final epoxidation step is substantially influenced by the distortion in the molecule.

Figure 5

Metal-catalyzed synthesis of new nucleoside paradigms

    Modified and unnatural nucleosides hold promise as novel pharmacophores in addition to their use in probing biofunction.  For example, nucleoside analogs are used as anti-cancer and anti-viral agents, some are modulators of adenosine receptors and many are products of xenobiotic metabolism and DNA binding.  For these reasons, development of novel methods for nucleoside modification is of considerable importance.  Our research currently focuses on the use of palladium catalyzed methods for accomplishing C-N and C-C bond formation of nucleoside substrates.  Two C-6 halo nucleoside substrates have been used to synthesize N6 aryl 2'-deoxyadenosine analogs and C-6 aryl purine nucleosides (Scheme 1).  The C-2 bromo nucleoside has been used to prepare C-2 aryl 2'-deoxyinosine analogs.

Scheme 1



    We have also demonstrated that C-6 arylsulfonate derivatives of 2'-deoxyguanosine undergo metal insertion readily.  Therefore, these arylsulfonates can be used to prepare N6 aryl 2,6-diaminopurine nucleoside analogs or C-6 aryl 2-aminopurine derivatives (Scheme 2).  However, our studies show that as with the C-6 bromo nucleoside shown in Scheme 1, substantially different catalytic systems are required for C-N and C-C bond formation.

Scheme 2


    Using our fundamental research in this field, we have synthesized xenobiotic metabolism products (examples shown in Figure 6) that can be used for site-specific DNA modification.  We anticipate continued studies of other metal catalyzed processes for nucleoside and DNA modification.


Figure 6
 

Individuals interested in our programs are encouraged to contact Prof. Lakshman

Undergraduates.
Undergraduate students have always been an integral part of our research program and have made significant contributions to our studies.

Graduate students. Students interested in the MA degree can become involved in our research programs through thesis research. Students interested in the Ph.D. degree are encouraged to visit the graduate center link at http://web.gc.cuny.edu/Chemistry/

Postdoctoral Research.  Since our research has been funded by the NSF and the NIH, there are times when postdoctoral positions become available. Additionally, individuals who have other sources of stipend funding can also be supported through our programs.

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