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Photochemistry of Eumelanin Precursors Role of Excited State Proton Transfer for UV Photoprotection

Corani, Alice LU (2015)
Abstract (Swedish)
Popular Abstract in Swedish

Melanin är ett pigment som finns i många olika slags vävnader. I hud och hår finns

två slags melanin, pheomelanin som är typiskt för rödhåriga personer och

eumelanin som finns i större mängd i mörkt hår och hud. Konstaterad korrelation

mellan hudcancer och solexponering motiverar forskarna att förstå mer om

melanin och dess växelverkan med UV-ljus. Man vet att rödhåriga personer med

ett överskott på pheomelanin har större sannolikhet att få hudcancer och att

eumelanin förmodligen har en skyddseffekt mot UV ljus. Dock är kunskapen om

bakomliggande ljusinducerade fotokemiska processer dålig. Därför studerar vi

melanin och dess... (More)
Popular Abstract in Swedish

Melanin är ett pigment som finns i många olika slags vävnader. I hud och hår finns

två slags melanin, pheomelanin som är typiskt för rödhåriga personer och

eumelanin som finns i större mängd i mörkt hår och hud. Konstaterad korrelation

mellan hudcancer och solexponering motiverar forskarna att förstå mer om

melanin och dess växelverkan med UV-ljus. Man vet att rödhåriga personer med

ett överskott på pheomelanin har större sannolikhet att få hudcancer och att

eumelanin förmodligen har en skyddseffekt mot UV ljus. Dock är kunskapen om

bakomliggande ljusinducerade fotokemiska processer dålig. Därför studerar vi

melanin och dess molekylära byggstenar under UV-excitation. Melanin är en stor

polymer molekyl med komplicerad fotokemi. Vi börjar därför med att studera

mindre byggstenar av pigmentet, från de allra minsta monomera enheterna till

oligomerer och till slut hela polymeren. Studier av monomerenheter har givit

viktiga insikter om de mekanismer som styr eumelanins funktion. Arbetet på

dimerer visar att redan dessa ganska små enheter har samma fotokemiska

processer som hela eumelaninpigmentet. Vid absorption av UV-ljus initieras en

process där en vätejon, en proton, sparkas ut från pigmentet i samma ögonblick

som UV-ljuset når pigmentmolekylen. Man skulle kunna likna händelseförloppet

vid att melaninet gör sig av med UV-ljusets energi genom att mycket snabbt skjuta

iväg en protonprojektil. Denna projektil gör i sin tur av med energin till

omgivande membranvävnad i form av värme och har därmed omvandlat farlig

UV-energi till ofarlig värme. Den kemiska reaktionen går oerhört snabbt, på

mindre än en tusendel av en miljarddels sekund. Vi har alltså lyckats visa att

protonöverföring är den aktiva mekanismen för eumelanins funktion och att denna

funktion kan härledas till enheter i pigmentet som består av två eller bara ett fåtal

monomera enheter. Melaninet i hudens melanocyter består av tätt packade

melaninpolymerer. För att studera melaninpigmentet i en form som liknar detta har

vi också studerat tunna filmer av eumelanins byggstenar, belagda på ett

kvartssubstrat. Dessa preliminära studier visar att nya processer uppstår när

molekylerna är tätt packade. Framtida arbete får visa vilka dessa processer är. (Less)
Abstract
Melanin is an epidermal pigment commonly known to give darker skin coloration under sun exposure. It is

also present in the hair, eyes, inner ear and brain. The first function of epidermis melanin is believed to be

photoprotective against harmful ultraviolet (UV) light, but the recent increase of skin cancer correlated to an

increase of sun exposure questions the properties of melanin. Its presence in different body parts suggests

that its function is not solely protective against UV-light.

Melanin in epidermis is divided in two categories eumelanin responsible of the dark coloration and

pheomelanin, which does not have great influence on the skin shade, but gives the red coloration of... (More)
Melanin is an epidermal pigment commonly known to give darker skin coloration under sun exposure. It is

also present in the hair, eyes, inner ear and brain. The first function of epidermis melanin is believed to be

photoprotective against harmful ultraviolet (UV) light, but the recent increase of skin cancer correlated to an

increase of sun exposure questions the properties of melanin. Its presence in different body parts suggests

that its function is not solely protective against UV-light.

Melanin in epidermis is divided in two categories eumelanin responsible of the dark coloration and

pheomelanin, which does not have great influence on the skin shade, but gives the red coloration of hair. The

amount of skin cancer has been observed to be greater in patients presenting a fair type of skin. The

mechanism after melanin UV absorption is poorly understood. Two main problems arise in the study of

melanin photochemistry. First the pigment is believed to be an oligomer assembly of different sizes, resulting

in a broad heterogeneity of a studied sample, which makes the distinction of active species difficult. On the

other hand, this is probably a key property of melanin, to ensure a photoprotective barrier against especially

UV-light. The second main difficulty in the study of melanin is the solubility. The larger the pigment the less

soluble in aqueous solution. An additional issue in the study of melanin is the reproducibility of the sample.

The work presented here focuses on eumelanin and its interaction with UV-light. With help of fluorescence

steady state and time-resolved methods we have investigated eumelanin photochemistry. We present here a

model of the energy dissipation mechanism of the pigment after UV absorption. Our method is based first on

the study of synthetic samples, which allows us to have control over the heterogeneity and thus identify the

function of each molecule involved in the whole melanin structure. Secondly, we have performed a bottom

up approach, starting with the study of monomer constituents up to the polymer. Moreover, we have

developed a method to solubilize the polymer, which does not interfere with the photodynamics of the

molecules.

We demonstrate that the main dissipation channel of eumelanin after UV absorption in aqueous solution is

controlled by Excited State Proton Transfer (ESPT). The surrounding solvent is essential to have a rapid and

efficient UV dissipation on the order of hundreds of femtoseconds. We show that the melanin precursor

DHICA, in its polymeric form, is much more efficient than the DHI precursor in the dissipation mechanism.

Our approach brings new insight to the eumelanin photochemistry and shows that one of the eumelanin

components has great photoprotection properties against UV-light, while the other one present longer excited

state lifetimes that leave more time to the molecule to produce radicals and reactive species, possibly

responsible of melanoma formation. We hope to have brought a better understanding to the property of the

black polymer and opened a way to deepen the study of melanin and its interaction with UV-light. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Prof. Huppert, Dan, Tel Aviv University
organization
publishing date
type
Thesis
publication status
published
subject
keywords
Time-resolved, Fluorescence spectroscopy, Excited State Proton Transfer (ESPT), Photochemistry, Eumelanin, Melanin
pages
156 pages
publisher
Division of Chemical Physics, Department of Chemistry, Lund University
defense location
Kemicentrum, Getingevägen 60, Lund
defense date
2015-09-08 10:00
ISBN
978-91-7623-423-5 (pdf)
978-91-7623-422-8 (print)
language
English
LU publication?
yes
id
cfe8859f-0bd2-45d8-8ca8-222a299d42ff (old id 7756509)
date added to LUP
2015-08-24 16:13:06
date last changed
2016-09-19 08:45:09
@phdthesis{cfe8859f-0bd2-45d8-8ca8-222a299d42ff,
  abstract     = {Melanin is an epidermal pigment commonly known to give darker skin coloration under sun exposure. It is<br/><br>
also present in the hair, eyes, inner ear and brain. The first function of epidermis melanin is believed to be<br/><br>
photoprotective against harmful ultraviolet (UV) light, but the recent increase of skin cancer correlated to an<br/><br>
increase of sun exposure questions the properties of melanin. Its presence in different body parts suggests<br/><br>
that its function is not solely protective against UV-light.<br/><br>
Melanin in epidermis is divided in two categories eumelanin responsible of the dark coloration and<br/><br>
pheomelanin, which does not have great influence on the skin shade, but gives the red coloration of hair. The<br/><br>
amount of skin cancer has been observed to be greater in patients presenting a fair type of skin. The<br/><br>
mechanism after melanin UV absorption is poorly understood. Two main problems arise in the study of<br/><br>
melanin photochemistry. First the pigment is believed to be an oligomer assembly of different sizes, resulting<br/><br>
in a broad heterogeneity of a studied sample, which makes the distinction of active species difficult. On the<br/><br>
other hand, this is probably a key property of melanin, to ensure a photoprotective barrier against especially<br/><br>
UV-light. The second main difficulty in the study of melanin is the solubility. The larger the pigment the less<br/><br>
soluble in aqueous solution. An additional issue in the study of melanin is the reproducibility of the sample.<br/><br>
The work presented here focuses on eumelanin and its interaction with UV-light. With help of fluorescence<br/><br>
steady state and time-resolved methods we have investigated eumelanin photochemistry. We present here a<br/><br>
model of the energy dissipation mechanism of the pigment after UV absorption. Our method is based first on<br/><br>
the study of synthetic samples, which allows us to have control over the heterogeneity and thus identify the<br/><br>
function of each molecule involved in the whole melanin structure. Secondly, we have performed a bottom<br/><br>
up approach, starting with the study of monomer constituents up to the polymer. Moreover, we have<br/><br>
developed a method to solubilize the polymer, which does not interfere with the photodynamics of the<br/><br>
molecules.<br/><br>
We demonstrate that the main dissipation channel of eumelanin after UV absorption in aqueous solution is<br/><br>
controlled by Excited State Proton Transfer (ESPT). The surrounding solvent is essential to have a rapid and<br/><br>
efficient UV dissipation on the order of hundreds of femtoseconds. We show that the melanin precursor<br/><br>
DHICA, in its polymeric form, is much more efficient than the DHI precursor in the dissipation mechanism.<br/><br>
Our approach brings new insight to the eumelanin photochemistry and shows that one of the eumelanin<br/><br>
components has great photoprotection properties against UV-light, while the other one present longer excited<br/><br>
state lifetimes that leave more time to the molecule to produce radicals and reactive species, possibly<br/><br>
responsible of melanoma formation. We hope to have brought a better understanding to the property of the<br/><br>
black polymer and opened a way to deepen the study of melanin and its interaction with UV-light.},
  author       = {Corani, Alice},
  isbn         = {978-91-7623-423-5 (pdf)},
  keyword      = {Time-resolved,Fluorescence spectroscopy,Excited State Proton Transfer (ESPT),Photochemistry,Eumelanin,Melanin},
  language     = {eng},
  pages        = {156},
  publisher    = {Division of Chemical Physics, Department of Chemistry, Lund University},
  school       = {Lund University},
  title        = {Photochemistry of Eumelanin Precursors Role of Excited State Proton Transfer for UV Photoprotection},
  year         = {2015},
}