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43973-GB4
Using a Photolabile Precursor to Study Radical-Mediated Protein Damage

K. Nolan Carter, University of Central Arkansas

����������� Oxidative damage of proteins is believed to involve free radical intermediates.� This damage process can be initiated by reactive oxygen species such as hydroxyl radical.� Protein radicals thus produced can undergo subsequent reactions which are often complex and may result in transfer of damage to other biological molecules such as nucleic acids.� A problem associated with the direct study of this damage process is that hydroxyl radical is highly reactive and can produce multiple radicals from a given amino acid within a polypeptide chain.� Since this is possible at every amino acid within a polypeptide, many protein radicals can result from attack of hydroxyl radical.� This hinders direct study of specific protein radical intermediates involved in this process.� The objective of this research was to develop photochemically active amino acids which generate the same radicals produced by hydroxyl radical, but in a selective fashion.� It was envisioned that these photochemically active amino acids would contain a photolabile group which would be cleaved upon photolysis to generate amino acid radicals.� Radicals such as 1, derived from the reaction of hydroxyl radical with valine can be selectively generated under mild conditions via this approach.� We have successfully produced a photolabile precursor for a primary valine radical, the first such example of a compound of this type.

����������� Since alkyl aryl selenides are known to undergo facile C-Se bond homolysis upon UV irradiation, the phenylselenyl group was chosen as a photolabile group.� Phthaloyl protected diastereomeric valine radical precursors 4 and 5 were prepared from the corresponding diastereomeric bromides (2,3) by treatment with benzeneselenol in the presence of cesium carbonate.� This method is superior to more conventional methods for introduction of the phenylselenyl group such as diphenyl diselenide/sodium borohydride.� Removal of the phthaloyl protecting group with methanolic hydrazine afforded radical precursors 6 and 7 as a 2:1 mixture of diastereomers.� This compound was intended to generate a monomeric valine radical that would serve as a model for the analogous valine radical produced within proteins by hydroxyl radical.�

����������� Upon photolysis at 350 nm, this compound is converted to products consistent with the intermediacy of radicals 8 and 9.� Detailed characterization of photolysates by BSTFA derivatization followed by GC/MS analysis is in progress.

����������� Studies toward the synthesis of tertiary radical precursor 10 have also been conducted.� This compound is intended to produce tertiary valine radical 11.

����������� Due to the scarcity of available methods for the synthesis of tertiary alkyl aryl selenides, synthesis of tertiary radical precursor 10 has yet to be completed.� Having attempted unsuccessfully to prepare this compound from substitution of the corresponding alkyl halide, we are currently attempting to complete the synthesis via 1,4-addition of benzeneselenolate to alkene 12 to produce protected radical precursor 13.

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