Post on 20-Dec-2015
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Pulse Radiolysis andPulse Radiolysis andSpectral ResolutionsSpectral Resolutions
Free Radical Problems in Biology and MedicineFree Radical Problems in Biology and Medicine
Spectral Resolution TechniqueSpectral Resolution Technique
Motivation for Transient ConductivityMotivation for Transient Conductivity
MotivationsMotivations
Oxidative stress• Alzheimer’s disease• Biological aging
Basic issues• Neighboring-group effects• Details of oxidative scheme
N-Acetyl-L-Methionine AmideN-Acetyl-L-Methionine Amide
STABILIZATION OF SULFURSTABILIZATION OF SULFUR RADICAL CATIONS VIA INTRAMOLECULAR SULFUR-NITROGEN AND SULFUR-OXYGEN BOND RADICAL CATIONS VIA INTRAMOLECULAR SULFUR-NITROGEN AND SULFUR-OXYGEN BOND FORMATION FORMATION
Gly-MetMet-Gly
Three-electron bondsThree-electron bonds
Scheme 1
PP
2,1* -> 1,2* origin of 480 nm absorptionin SS+ radical cations
Advantage of Radiation Chemistry:Advantage of Radiation Chemistry:Can convert all radicals to oxidizing Can convert all radicals to oxidizing
(or reducing) ones(or reducing) ones
H2O OH (G = 2.7)
H (G = 0.6)
eaq (G = 2.8)
N2O + eaq OH + OH + N2
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-amidesEndo- and Exo-amides
Neighboring group participation in compounds3 but not in compounds 4?
OPTICAL ABSORPTION SPECTRA USED FOROPTICAL ABSORPTION SPECTRA USED FORSpectral ResolutionsSpectral Resolutions
Endo-amideEndo-amide
Endo-amide at pH 5Endo-amide at pH 5
Mathematics of Spectral ResolutionsMathematics of Spectral Resolutions
Aexp(j) = i i(j) ci d
i(j) is the extinction coefficient of the i-th species at wavelength j
d = optical pathlength
Knowns:
Unknowns: ci = concentration of i-th species
SSD = i|Aexp(j) i i(j) ci d|2
Minimize Sum of Squares of the Deviation by varying the ci’s
the Aexp(j) for all wavelengths j
Would like to solve for the ci’s
But problem is over determined (too much data)and there is also noisy data
Endo-amide pH 5Endo-amide pH 5 t = 650 ns t = 650 ns
SN includedno SO
Endo-amide pH 5Endo-amide pH 5t = 650 nst = 650 ns
SO includedno SN
Time-resolved conductivityTime-resolved conductivity
N-Acetyl-L-Methionine AmideN-Acetyl-L-Methionine Amide
Formation of (SS)+ and (SO)+ speciesis paralled by a net decrease
in equivalent conductivity
Time-resolved conductivityTime-resolved conductivity
N-Acetyl-L-Methionine AmideN-Acetyl-L-Methionine Amide
+OH
H+H2O
Formation of (SN) speciesis not paralled by a net change
in equivalent conductivity
Concentration profilesConcentration profiles Endo-amide at pH 5 Endo-amide at pH 5
Notes:
No SO
Only SS+ isconducting
Alternative Concentration ProfilesAlternative Concentration ProfilesEndo-amide pH 5Endo-amide pH 5
Notes:
No SN
Both SS+ andSO will conduct
ConclusionsConclusionsabout Endo-amide experimentsabout Endo-amide experiments
The expected transients account for the observed The expected transients account for the observed transient absorptions.transient absorptions.
Whether or not SN or SO is present will depend on Whether or not SN or SO is present will depend on conductivity experiments yet to be made.conductivity experiments yet to be made.
If SN, only the small amount of SSIf SN, only the small amount of SS++ will will contribute.contribute.
If SO, we expect a relatively large contribution to If SO, we expect a relatively large contribution to the conductivity at small times which decays into the conductivity at small times which decays into a smaller contribution as SO decays.a smaller contribution as SO decays.
Exo-amideExo-amide
Exo-amide pH 4Exo-amide pH 4Notes:
Same as pH 5
Namely littleNeighboringGroupParticipation
ConductivityConductivityExo-amide pH 4Exo-amide pH 4
Notes:
Net decrease inconductivity,indicates SO orSS+
Since there waslittleNeighboringGroupParticipation inabsorption, theconductive signalis probably SS+