True or false?
Cyanide is poisonous because it binds more tightly to the iron in hemoglobin than does O2 and cause suffocation.
The Singapore government gets it right:“Blood Agents: Cyanide-containing compound, affect body functions by poisoning the enzymes, Cytochrome Oxidise. Hence preventing the normal utilization of oxygen by the cells and causing rapid damage to body tissues.”
About Chemical Warfare Agents
False
Let’s do the calculation.
•You have ~ 2g Fe in your body; about 1.5g of Fe is in your blood as hemoglobin.
•1.5 g Fe = 0.03 mol.
•Toxicity of cyanide: The LD50 for ingestion is 50-200 milligrams, or 1-3 milligrams per kilogram of body weight, calculated as hydrogen cyanide. The LC50 for gaseous hydrogen cyanide is 100-300 parts per million. Inhalation of cyanide in this range results in death within 10-60 minutes, with death coming more quickly as the concentration increases. Inhalation of 2,000 parts per million hydrogen cyanide causes death within one minute.
•So, if you weigh 60 kg, between 60-180 mg could be a toxic dose.
•Or, 0.06-0.18g = 0.002-0.007 mol CN- (as HCN) vs 0.03 mol Fe
•That amount of cyanide would block only ~10% of O2 binding sites in hemoglobin, not enough to kill you.
The toxicity of Cyanide is because of its strong binding to Fe in heme in other critical Heme-proteins.
The Many Role of Hemes
• oxygen carrier (hemoglobin)
• electron transfer (cytochromes a,b,c, etc, in respiratory chain)
• cytochrome oxidase (mitochondrial electron transport chain,
oxygen is terminal electron acceptor and is reduced to water)
• detoxification (cytochrome P450, catalase)
• hydroxylation (cytochrome P450 in hormone production)
matrix
cytosol
Cyt c
c1
c
2 x QH2
QH2
2 Q
Q
Complex III
HEME 1
HEME 2
HEME 3
HEME 4
Fe(III) Fe(II)
Complex IV – Cytochrome C Oxidase
Cu(II) Cu(I)
Cu(II)—Cu(I) Cu(I)—Cu(I) HEME 2
HEME 3
HEME 4
HEME 1
Using Iron Porphyrins as Models for Hemoglobin
The system:
Key Features of Hemes• Fe oxidation state• Fe spin state• porphyrin oxidation state• porphyrin hydrophobicity
Low Spin S = 1/2 n = 1
High Spin S = 5/2 n = 5
Intermediate Spin S = 3/2 n = 3
How will the spin state of Fe(porphyrin) complexes change on binding imidazole?
Sample for Evans’ Magnetic Susceptibility Method
Inside capillary: sample in CHCl3, 1) with imidazole 2) without imidazole
Outside capillary: 99.5 %D CDCl3
NMR tube
NMR Spectrum from Evans’ Method
Inside capillary: sample in CHCl3,
produces broad singlet for paramagnetically shifted CHCl3 below 7.3 ppm
Outside capillary: 99.5 %D CDCl3 produces usual sharp singlet for 0.5% CHCl3 at 7.3 ppm
Why is H resonance in CHCl3 shifted downfield and broadened?
• pseudocontact and contact terms• addition of new small magnetic field to local magnetic fields of neighboring nuclei
is used in NMR Shift Reagents to “de-tangle” complicated spectra
How does shift, , relate to a magnetization of paramagnetic sample?
g = 3 0 c
Mass susceptibility (+)
Shift of signal, in Hz
mass susceptibility of solvent-a diamagnetic contribution, a (-) value
Magnetic field(400 MHz, or 400 x 106 Hz)
Concentration of sample,in g/mL
Magnetic field lines of flux
Magnetic field linesaffected by a paramagneticsubstance: attractsSusceptibility, X > 0
Magnetic field linesaffected by a diamagneticsubstance: repelsSusceptibility, X < 0
How does mass susceptibility, g , relate to unpaired electrons in a paramagnetic sample?
g x (Mol. Wt.) = M
corr = M - diamagnetic corrections
where diamagnetic corrections for Fe, porphyrin, Cl, imidazole, a negative number!
eff = 3 R T corr 1/2 = 2.828 (T corr ) 1/2
N 2
eff = (n(n+2))1/2
Mass susceptibility Molar susceptibility
Diamagnetic Corrections (cgs units)
Xo (CHCl3) = - 4.97 x 10-7 cgs
Porphyrin: TPP= -700 x 10-6 cgs TTP= -753 x 10-6 cgs TClPP= -760 x 10-6 cgs
Fe = -13 x 10-6 cgs
Cl = -20 x 10-6 cgs
Imidazole = -38 x 10-6 cgs
Low Spin S = 1/2 n = 1
High Spin S = 5/2 n = 5
Intermediate Spin S = 3/2 n = 3
2. How will the Ered of Fe(porphyrin) complexes change on binding imidazole?
3. Will the Ered potentials also reflect a change in spin state?
1. How will the Ered of Fe(porphyrin) complexes vary with the porphyrin?
Characterization by Cyclic Voltammetry
The Role of Axial Ligation and the Allosteric Effect in Hemoglobin O2 Binding
3d orbitalson Fe
Spin State of Fe affects size of ion
Large, high spin
Fe(2+):
In T state, transmitted by His on protein helix
Small, low spin
Fe(2+):
In R state, transmitted to His
on protein helix