Cholesterol photooxidation reaction

Cholesterol photooxidation reactions are classified as either type I
(free radicalmechanism)or type II (singlet oxygenmediated) [19,20].
Cholesterol photosensitized oxidation requires singlet oxygen (1O2)
for the initiation phase to occur, being mainly generated when
photosensitizers absorb light, become electronically excited and
interact with triplet oxygen to convert it into reactive singlet oxygen.
The latter is more reactive (32,000 times for monounsaturated
structures and 1600 times for diunsaturated ones) than triplet
oxygen [21], as it has higher redox potential and a lower energy ofactivation. When cholesterol photosensitized oxidation takes place,
1O2 attacks the steroid molecule by an ene addition mechanism on
either side of the double bond [22]. In reactions mediated by 1O2, 5ahydroperoxycholesterol
(5a-HPC), 6a-hydroperoxycholesterol (6a-
HPC) and 6b-hydroperoxycholesterol (6b-HPC) are the only three
hydroperoxides that are generated, 5a-HPC being themost abundant
one [3,23] (Fig. 1). The latter can rearrange and give rise to both 7ahydroperoxycholesterol
(7a-HPC) and 7b-hydroperoxycholesterol
(7b-HPC), which are usually present at different concentration levels
as the a epimer is less favored from a thermodynamics standpoint
[2,3,5]. Hydroperoxides are then rapidly converted into hydroxyl and
keto derivatives by a dismutation reaction, giving rise to 7ahydroxycholesterol
(7a-HC) and 7b-hydroxycholesterol (7b-HC),
together with 7-ketocholesterol (7-KC) (Fig. 1). As observed for
hydroperoxides, 7a-HC is commonly found at lower concentrations
levels than its corresponding epimer. Depending on the lighting
conditions, it might be possible that 7-HCs and 7-KC are interconvertible
[24]; this behavior could be also influenced by an oxidant
environment. While the formation of such oxidation compounds
occurs, hydroperoxides can also follow a bimolecular reaction
pathway, which involves the interaction with a cholesterol molecule
that generates 5a,6a-epoxycholesterol (a-EC) and 5b,6b-epoxycholesterol
(b-EC) (Fig. 1). In presence of water and acidic conditions,
the epoxy derivatives can undergo ring opening and, thus,
produce cholestanetriol (CT, triol). The formation of side-chain COPs
is also possible due the presence of tertiary atoms at C-20 and C-25 in
the side chain of the cholesterol molecule [4]; although the oxidation
mechanisms are similar to those of the ring structure, a lower extent
of side-chain oxidation is usually observed [3]. It is important to
consider that, in any case, the trend of photosensitized oxidation can
be affected by the surrounding environment, which will impact the
type and amount of the oxidation products that are generated.

ผลลัพธ์ (อังกฤษ) 1:

Cholesterol Classified photooxidation reactions are neither either type I
(free radicalmechanism) or type II (singlet oxygenmediated) [19,20].
Cholesterol oxidation Requires photosensitized singlet oxygen (1O2)
for the Initiation phase to occure, Being mainly generated when-
photosensitizers absorbe light, Become electronically excited and
INTERACT with triplet oxygen to convert it Into reactive singlet oxygen.
The Latter is more reactive (32,000 times for monounsaturated
the structure and 1,600 times for diunsaturated Ones) than triplet
oxygen [21], or it has Higher Redox Potential and a Lower ofactivation energy. Cholesterol when- taxes to photosensitized oxidation Place,
1O2 Attacks the steroid molecule by an ene addition Mechanism on
either side of the double bond [22]. In reactions mediated by 1O2, 5ahydroperoxycholesterol
(5a-HPC), 6a-hydroperoxycholesterol (6a-
HPC) and 6b-hydroperoxycholesterol (6b-HPC) are the only Three
hydroperoxides That are generated, 5a-HPC themost abundant Being
one [3.23 ] (Fig. 1). The Latter Can Rearrange and give rise to Both 7ahydroperoxycholesterol
(7a-HPC) and 7b-hydroperoxycholesterol
(7b-HPC), qual Usually are present at Different Concentration levels
nor the epimer is a less favored from a standpoint thermodynamics
[2,3,5 ]. Hydroperoxides are rapidly crack Converted Into hydroxyl and
these derivatives by a dismutation reaction, GIVING rise to 7ahydroxycholesterol
(7a-HC) and 7b-hydroxycholesterol (7b-HC),
Together with 7-ketocholesterol (7-KC) (Fig. 1). Neither observed- for
hydroperoxides, 7a-HC is commonly found at Lower concentrations
levels corresponding epimer than its me. Depending on the Lighting
Conditions, it Possible That Might swear 7-HCS and 7-KC are interconvertible
[24]; Could this behavior swear Also influenced by an oxidant
Environment. WHILE the formation of Sucha COMPOUNDS oxidation
occurs, hydroperoxides Can Also follow a bimolecular reaction
Pathway, qual involves the interaction with a cholesterol molecule
That generates 5a, 6a-epoxycholesterol (a-EC) and 5b, 6b-epoxycholesterol
(b-EC) ( Fig. 1). In presence of water and acidic Conditions,
the epoxy derivatives Can undergoe ring opening and, THUST,
Produce cholestanetriol (CT, triol). The formation of side-chain COPs
is Possible Also due the presence of Tertiary atoms at C-20 and C-25 in
the side chain of the cholesterol molecule [4]; although the oxidation
Mechanisms are Similar to thos of the ring structure, a Lower extent
of side-chain oxidation is observed- Usually [3]. It is Important to
Consider that, in Any case, the trend of photosensitized oxidation Can
swear affected by the surrounding Environment, qual Will impact the
type and Amount of the oxidation Products That are generated.