Photochemistry of Eumelanin Precursors Role of Excited State Proton Transfer for UV Photoprotection
(2015)- 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) - 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)
Please use this url to cite or link to this publication:
https://lup.lub.lu.se/record/7756509
- author
- Corani, Alice LU
- supervisor
- opponent
-
- Prof. Huppert, Dan, Tel Aviv University
- organization
- publishing date
- 2015
- 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 10:00:00
- ISBN
- 978-91-7623-423-5
- 978-91-7623-422-8
- language
- English
- LU publication?
- yes
- additional info
- The information about affiliations in this record was updated in December 2015. The record was previously connected to the following departments: Chemical Physics (S) (011001060)
- id
- cfe8859f-0bd2-45d8-8ca8-222a299d42ff (old id 7756509)
- date added to LUP
- 2016-04-04 11:18:37
- date last changed
- 2020-04-29 12:40:33
@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}},
keywords = {{Time-resolved; Fluorescence spectroscopy; Excited State Proton Transfer (ESPT); Photochemistry; Eumelanin; Melanin}},
language = {{eng}},
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}},
url = {{https://lup.lub.lu.se/search/files/5743719/7766727.pdf}},
year = {{2015}},
}