Free Radicals

 

They form in the sunlight,

they float in the smog.

 
They are present in food

and increase when you jog.

 
They form in the liver from all kinds of drugs,

from solvents from nasty

things used to kill bugs-
 
They will damage your heart, even make you grow old,
and they will mutate your genome is what we’re told.
 
Their causes, reactions, and guessed repercussions
are turning up often in journal discussions.
 
The least stable radicals varnish so quick, that publishing on

them is no

minor trick.
 
But those with a half-life that lets them react with

biocomponents will

also attract spectroscopists, spin trappers, and all of the rest--
 
Who’ll show you the things at which others just guess.
Unknown
 

SINGLET OXYGEN

OXIDATION

Molecular Orbital of Singlet
Oxygen

Molecular

Atomic

Atomic

2Pz

2Py

2Px

2Px

2Py

2Pz

*







*

*

2S

2S

1S

1S





*

*

E

1

O

2

and

3

O

2

with Linoleic Acid

0.4

0.8

1.2

1.6

A

b

so

rb

a

n

ce

a

t

2

3

3

n

m

3

O

2

1

O

2

200

400

600

800

1,000

Reaction Time in Minutes

0

MECHANISMS OF

PHOTOSENSITIZED OXIDATION

Photosensitizers

1.

Dye: Methylene Blue, Rose Bengal, Eosin, Crystal Violet,

and Acridine Orange.

2.

Pigment: Chlorophyll, Hematoporphyrines, Riboflavin

Photosensitizer Properties

1. Absorption of the light in the 380 ~ 900

nm range

2. Exhibition of fluorescence and

phosphorescence
reflecting two separate electronically
excited states;
the Singlet state and the Triplet state

3. The singlet state is produced first by the

light

absorbed, but it has a short lifetime,
decaying by fluorescence to the ground
state and by electronic intersystem
crossing to the Triplet state.

4. The triplet state decays to

the ground state at a
slower rate that varies
depending on the
sensitizer.

5.The most effective

photosensitizers have a

high

quantum yield of a long-lived
triplet state.

Photosensitizer Properties

Excitation and Deactivation of
Photosensitizer

Excited state

Ground state

k = 2  10

8

/sec

k = 10 - 10

4

/sec

k = 1- 20  10

8

/sec

k = 1 - 3  10

9

/sec

h

1

Sen*

Sen

3

Sen*

Singlet oxygen formation

ISC

+

3

O

2

Riboflavin

1R F

N

N H

O

N

O H

O H

O H

O H

N

O

H

CH

2

CH

3

H

CH

3

CO

2

CH

3

O

H

H

3

C

H

3

C

N

N

N

N

CH

H

2

C

Mg

R

C H

2

C O

2

C H

3

C H

3

C H

3

C H

3

Chloroph

yll

Excitation and Deactivation of Chlorophyll for

Singlet Oxygen Formation

E

n

e

rg

y

Ground State

hv

Fluorescence

K= 210

8

/sec

1

Sen

(

Chlorophyll

)

1

Sen

*

K=1-
2010

8

/sec

Phosphorescence

K=10-
10

4

/sec

Singlet Oxygen

K=1-
310

9

/sec

3

Sen*

3

O

2

ISC

Formation of Triplet Riboflavin

ISC

1RF

N

NH

O

N

OH

OH

OH

OH

N O

N

NH

O

N

OH

OH

OH

OH

N O

hv

3RF

1

RF*

Singlet Oxygen Trapping

H

3

C

H

3

C

CH

3

CH

3

O

N

CH

3

CH

3

H

TMPD

1

O

2

O

N

H

3

C

H

3

C

O

TAN

•

2

2

+

H

2

O

.
.

+ 2

2

ESR of 2,2,6,6- Tetramethyl- 4 -Piperidone-N-Oxyl

3370 G

3390 G

3410 G

.

.

.

.

Type II

Type I

Excited Triplet Sensitizer

Reactions

Sen

h

1

Sen*

ISC

3

Sen*

+ RH

+ RH

+

3

O

2

1

O

2

+

3

O

2

O

2

-

+ Sen

+

+

3

O

2

ROOH

ROOH

K

2

K

1

R + Sen H

Singlet Oxygen Formation
Mechanisms

Cu

++

+ Ascorbate

+

3O2

H2O2+ 3O2

3O2+ Xanthine +Xanthine Oxidase

Superoxide Dismutase

Spontaneious Dismutation

HOO 

+ 2H

+

+ 2H+

1

O

2

Riboflavin

+ 3O2

Reduced Riboflavin

+ 3O2

+ 2H

+

-



O2

-

O2

+ H+

+ H

2

O

2

Spontaneious Dismutation

(6)

1

O

2

RCOO

RCOO





+

(1)

(12)

ENZYMES

O

2

+

3

SENSITIZER

(2)

H

2

O

2

+

OCl

-

H

2

O

2

+ O

2

-

(3)

+

(4)

OH

O

2

-

(5)

(7)

(8)

(9)

(10)

O

2

-

O

2

-

+

O

2

-

O

2

-

+

+

Y

+

H

2

O

2

HO

2

-

OZONIDES

ENDOPEROXIDES

SENSITIZER

H

2

O

Cl

-

OH

-

+ OH

OH

-

e

-

Y

2H

+

H

2

O

2

RC O

+

RCOH

PRODUCTS

PRODUCTS

H

2

O+ OH

-

(11)

+





.

.

.

.

.

.

_

Production of

1

O

2

by Photochemical,

Chemical, and Biological Systems

SINGLET OXYGEN REACTION

O

O

O
O

Endoperoxide

Allyl

Hydroperoxide

O

O

H

O

O

O

O

O

O

Dioxetane

CH

2

CH

2

+

+

+

1,4-
Cycloaddition:

ENE Reaction :

1,2 –
Cycloaddition:

Singlet Oxygen Oxidation

Reactions of Singlet Oxygen with Double
Bonds

R

R'

H

O

O

I

R

R'

H

O

O

II

Conjugated and Nonconjugated
Hydroperoxides
via the 6-Centered Transition State

R

R'

R

R'

OOH

OOH

R

R'

R

R'

OOH

isomerization

+

h

/sensitizer/O

2

Reversion Flavor in Soybean Oil

Reversion Flavor—The development of a
characterisitc beany and grassy flavor in soybean
oil prior to the development of rancidity

C H3 C H2

C H

C H2

C H

O

2-Pentenylfuran from Linolenic Acid by Singlet

Oxygen

C H3 C H2 C H

C H2

C H

C H

C H

C H2

C H

C H

C H2 ( CH2)6 C O H

O

C H3 C H2 C H

C H2

C H

C H

C H

C H2

C H

C H

C H ( CH2)6 C O H

O

1O

2

O
O

H

C H3 C H2 C H

C H2

C H

C H

C H

C H2

C H

C H

C H ( CH2)6 C O H

O

O

C H3 C H2 C H

C H2

C H

C H

CH2

C H

C H

O

1O

2

C H3 C H2 C H

C H2

C H

C H

C H2

C H

C H

O

O

O

H

C H3 C H2 C H

C H2

C H

C H

C H2

C H

C H

O

O

C H3 C H2 C H

C H2

C H2

C H

C H2

C

C H

O

O

C H3 C H2 C H

C H2

C H

CH

C H

C

C H

O

H

OH

- H2O

C H3 C H2 C H

C H2

C H

O

.

.

.