Pulse Radiolysis and Spectral Resolutions Free Radical Problems in Biology and Medicine Free Radical Problems in Biology and Medicine Spectral Resolution Technique Spectral Resolution Technique Motivation for Transient Conductivity Motivation for Transient Conductivity

Motivations Oxidative stress Alzheimer’s disease Biological aging Basic issues Neighboring-group effects Details of oxidative scheme

N-Acetyl-L-Methionine Amide N-Acetyl-L-Methionine Amide

STABILIZATION OF SULFUR RADICAL CATIONS VIA INTRAMOLECULAR SULFUR-NITROGEN AND SULFUR-OXYGEN BOND FORMATION STABILIZATION OF SULFUR RADICAL CATIONS VIA INTRAMOLECULAR SULFUR-NITROGEN AND SULFUR-OXYGEN BOND FORMATION Gly-Met Met-Gly

Three-electron bonds 2,1* -> 1,2* origin of 480 nm absorption in SS + radical cations

Advantage of Radiation Chemistry: Can convert all radicals to oxidizing (or reducing) ones G in units of radicals produced for every 100 eV of ionizing radiation absorbed Strongly reducing radical Strongly oxidizing radical Converting reducing to oxidizing

Endo- and Exo-amides Neighboring group participation in compounds 3 but not in compounds 4?

OPTICAL ABSORPTION SPECTRA USED FOR Spectral Resolutions

Endo-amide

Endo-amide at pH 5

Mathematics of Spectral Resolutions A exp ( j ) =  i  i ( j ) c i d  i ( j ) is the extinction coefficient of the i-th species at wavelength j d = optical pathlength Knowns: Unknowns: c i = concentration of i-th species SSD =  i |A exp ( j )   i  i ( j ) c i d| 2 Minimize Sum of Squares of the Deviation by varying the c i ’s the A exp ( j ) for all wavelengths j Would like to solve for the c i ’s But problem is over determined (too much data) and there is also noisy data

Endo-amide pH 5 t = 650 ns SN included no SO

Endo-amide pH 5 t = 650 ns SO included no SN

Time-resolved conductivity N-Acetyl-L-Methionine Amide N-Acetyl-L-Methionine Amide Formation of (SS) + and (SO) + species is paralled by a net decrease in equivalent conductivity

Time-resolved conductivity N-Acetyl-L-Methionine Amide N-Acetyl-L-Methionine Amide +   OH  H + H2OH2O Formation of (SN) species is not paralled by a net change in equivalent conductivity

Concentration profiles Endo-amide at pH 5 Notes: No SO Only SS + is conducting

Alternative Concentration Profiles Endo-amide pH 5 Notes: No SN Both SS + and SO will conduct

Conclusions about Endo-amide experiments The expected transients account for the observed transient absorptions. The expected transients account for the observed transient absorptions. Whether or not SN or SO is present will depend on conductivity experiments yet to be made. Whether or not SN or SO is present will depend on conductivity experiments yet to be made. If SN, only the small amount of SS + will contribute. If SN, only the small amount of SS + will contribute. If SO, we expect a relatively large contribution to the conductivity at small times which decays into a smaller contribution as SO decays. If SO, we expect a relatively large contribution to the conductivity at small times which decays into a smaller contribution as SO decays.

Exo-amide

Exo-amide pH 4 Notes: Same as pH 5 Namely little Neighboring Group Participation

Conductivity Exo-amide pH 4 Notes: Net decrease in conductivity, indicates SO or SS + Since there was little Neighboring Group Participation in absorption, the conductive signal is probably SS +