Lecture 10 Chemistry Flashcards
Threshold E to produce e transitions in water
7.4 ev
Depending on the amount of E transferred to the e, the molecule can undergo
ionization (E threshold in water 13eV, excitation (7.4 ev), thermal transfer (vibrational rotation)
Average E loss and most probably E loss
E straggling
E needed to break C=C
4.9 ev
cell composition
70-85% H2O, 10-20% protein, 10% carbs, 2-3% lipids
Initial physical event
transfer of 7-100 ev to cause ionizations or excitations in H2O molecule
E of subexcitation e in H2O
<7.4 ev
The time scale for physical stage
10^(-16)s
Species produced at the physical stage of the interaction of particle in water
H2O* (excitation), H2O+ (ionization), e (E=7.4 ev)
Prechemical stage
initial reactants produce radicals as they diffuse in medium.
what is a radical
atom that contains an unpaired e and thus is very reactive (can be neutral or charged)
what is a e aq
e captured by water through dipolar interactions, becoming solvated
E needed to break O-H bond
5 ev
Name radicals produced in water due to irradiation
H2O8=HO. (oxidizer)+ H., H2O+ = H+ + OH., H2O-= H. +OH-, ,and e aq from H2O
what is oxidation
the loss of e. e transferred to an oxidizing agent which becomes reduced
Time scale for prechemical stage
10^-15-10^-12 s
Chemical stage time scale
10^-12s
Characterize the chemical stage
the radicals and ions react with each other HO. +HO. = H2O2, HO. +e aq = OH-, HO. +H. = H2O , H+ + e aq=H. and etc. Most of the reactions remove the chemically reactive species
Time when most of the radicals and ions diffuse that far that the further reaction is unlikely
10^-6. The chemical stage is over by this time. G value will not change after
the formula for radical diffusion
D = distance^2/6 time
Simulations of the track of charged particle is possible via
Monte Carlo method : generation of e, passage model. generations of reactive species, diffusion of species, recombination events.
G value
Radiochemical yield: # of species produced/100 ev of E loss by charged particle and its secondaries when it stops in water.
How different the species produced by a, b, e radiation in 10^-15s in track regions
they are the same H2O*, H2O+, e. The difference in the spatial E deposition patterns (track density)
Limitation of Fricke dosimeter
stong LET dependence
Fricke dosimeter characteristics
a most useful method to measure # of reactive species in solution;
1mM FeSO4 in 0.8M H2SO4: when irradiated Fe2+ oxidized to Fe3+ which has blue color (spectrophotometer measurement),
D response linear up to 400 Gy and up to 700Gy
How many Fe3+ are produced in Fricke dosimeter
G= 2G(H2O2)+3G(H.)+G(HO.) if O2 is present, otherwise:
G(H)+G(OH)+2G(H2O2)
What is the use of a Fricke dosimeter?
to test scavengers and the effect of LET on E deposition as an effect on scavengerable species
Direct effect of radiation
E deposited into a molecule (DNA): ionization, excitation leading to radicals formation inside the biomolecule. The dose-response relationship is linear
Indirect action of radiation
ionization of the solvent molecules. Indirect actions are diffusion-limited and the dose-response rate can be complex.
Primary species that are responsible for rad damage in DNA
HO.
For high LET radiations
direct action is a predominant mechanism of DNA damage
Repair processes of DNA damage
Recombination, restoration, repair
Recombination of DNA
recombination: reaction of nearby radical with the DNA radical to regenerate the original DNA (<10^-11s);
Restoration
restitution: chem restoration of DNA (no enzyme involved). Some intracellular agents can react with radicals
The most important intracellular agent that reacts with radicals
gsh glutathione. It donates one H. to the DNA and restores it. <10^-3s
Repair stage of DNA
cellular enzymes recognize damage and repair DNA within min and hours
Scavengers
Chemicals that can react with reactive species like HO. can block indirect effects of radiation: can be helpful in estimating the relative contributions of direct and indirect effects; define chemistry, and they showed that 60-70% of all damages from low LET are caused by HO.
name several scavenger reactants
N2O, O2, Acid
