New Methodologies for the Construction of Amino Acid Isosteres
The areas of research listed below are synthetic organic and physical organic subsets to the impressive projects found within Dr. Hondal’s group here at the University of Vermont. My research in synthetic organic chemistry has two main parts. The first involves the elucidation of new methodologies for the construction of amino acid isosteres with particular emphasis on oxidized vicinal cysteinyl cysteine dyads and oxidized vicinal cysteinyl selenocysteine dyads. These oxidized vicinal dyads form an eight-membered disulfide ring with multiple conformations and are rare substructural element in proteins. They are often found in specific γ-turn types and this unit is of functional importance to those few that possess its structure such as with the thioredoxin reductases (TRs). During construction of these dyads elements of flexibility and rigidity are installed so that we can come to an understanding of how substitution affects conformation of the vicinal disulfide ring and how it affects activity within TRs. Unrelated to TR, this research has elucidated a number of possible γ-turn mimics that have been hitherto unknown.
The second involves the construction of sulfur- or selenium-containing amino acid isosteres and other small molecules to reach a chemical understanding of why an enzyme would utilize selenium as opposed to sulfur. While these chalcogens are similar, it is much harder for an enzyme to incorporate selenium, when compared to sulfur, so there must be a chemical explanation as to why an enzyme would use selenium as opposed to sulfur. Through studies involving Thioredoxin Reductase we have come to understand that a potential reason for biology to use selenium, is its ability to be reduced even when it is in the over-oxidized selenenic (RSeO2H) form by exogenous thiol. For sulfur this over-oxidized form (sulfinic; RSO2H) cannot be reduced. For the enzyme this means that use of selenium confers protection from oxidative stress. If the enzyme is oxidized at a selenocysteine (Sec) residue and made inactive, exogenous thiol can convert it from the inactive Sec-SeO2H form into the active Sec-SeH form of the enzyme. This cannot be said for the sulfur-homolog since if it is oxidized at a cysteine (Cys) residue and made inactive, exogenous thiol cannot convert it from the inactive Cys-SO2H form into the active Cys-SH form of the enzyme
My research in physical organic chemistry deals with determining how conformation of the vicinal disulfide or selenosulfide ring affects its redox potential. Thiol-disulfide and thiol-selenosulfide exchange reactions are monitored via HPLC or NMR, which allow these potentials to be determined. The ability to relate conformation of the ring to redox potential provides a guide as to what types of ring structure prefer to be in the reduced or oxidized state. This can then provide insight as to what type of conformations are needed for redox cycling to occur in the TRs. Similarly, rates of reduction of oxidized and over-oxidized forms of selenium (RSeOH; RSeO2H; RSeO3H) and sulfur (RSOH; RSO2H; RSO3H) are measured by HPLC and NMR to determine how fast or slow these processes occur. This provides a chemical rationale as to why selenium would be utilized over sulfur, since selenium confers stability by not becoming inactive under events of oxidative stress.