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)

Using Iron Porphyrins as Models for Hemoglobin

Fe3+

N

N N

N

Cl

Fe3+

N

N N

N

N

N

NN

R R

RR

FeCl

R = TPP:

R = TTP:

R = TClPP: Cl

Fe(porph)Cl + xs Imid [Fe(porph)(imid)2]+ Cl

N

NHN

NH

N

NH

+ Cl—

The system:

Key Features of Hemes• Fe oxidation state• Fe spin state• porphyrin oxidation state• porphyrin hydrophobicity

Fe3+

N

N N

N

Cl

Fe3+

N

N N

NN

NHN

NH

N

NH

+ Cl—

dxy, dyz, dxz,dxy, dyz, dxz,

dx2-y

2, dz2

dz2

dx2-y

2

dz2

dx2-y

2

increasing axial ligand field strength

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(300 MHz, or 300 x 106 Hz)

Concentration of sample,in g/mL

N S

N S

N S

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

because diamagnetic corrections for Fe, porph, Cl, imid are negative

corr = M - ( diamagnetic corrections, a neg sum)

eff = 3 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

The Role of Axial Ligation and the Allosteric Effect in Hemoglobin O2 Binding

dxy, dyz, dxz,

dx2-y

2, dz2

dz2

dx2-y

2

dxy, dyz, dxz,

Fe2+Fe2+

92 pm 75 pm

3d orbitalson Fe

Spin State of Fe affects size of ion

Fe2+

N

N N

N

N

N

Fe2+

N

N N

N

OO

N

N

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