Probing the heterogeneous structure of eumelanin using ultrafast vibrational fingerprinting

Grieco, Christopher; Kohl, Forrest R; Hanes, A.; Kohler, Bern

doi:10.1038/s41467-020-18393-w

“…From the structural point of view,w ef ind at hermodynamic preference for oxidized structures which is consistent with the fact that DHI undergoes oxidative polymerization under mild conditions.Moreover cyclic structures are favored compared to linear ones.This speaks in favor of the polycyclic graphite-like structures, [10] which have been also favored by ar ecent Raman spectroscopic study of dopa melanin. [18] We have also found an energetically accessible electrocyclic path for the formation of cyclic dimers which may complement the usually assumed radical oligomerization mechanism.…”

Section: Discussionsupporting

confidence: 61%

Stability and Optical Absorption of a Comprehensive Virtual Library of Minimal Eumelanin Oligomer Models**

Wang

¹

,

Blancafort

²

2021

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Eumelanin is responsible for photoprotection in living organisms. It is made of 5,6-dihydroxyindole (DHI) oligomers. However, lack of detailed structural knowledge limits understanding its function and exploiting its potential in material science. To uncover the relationship between structural stability and optical properties, we have studied a virtual library of minimal oligomer models made of 830 DHI dimers. We find a preference for oxidized, polycyclic structures which speaks in favor of graphite-like structures for the larger oligomers, and propose an electrocyclic formation mechanism. Besides widely considered quinone oxidation patterns, also structures with one or three oxidized sites per fragment and an interfragment double bond are stable, and we introduce a new representative set of 49 stable dimers that considers this diversity. Some stable oxidized dimers have absorption energies as low as 1.3 eV. They may be present as substructures in the naturally found oligomers and contribute to the absorption spectrum of the biopolymer.

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“…This speaks in favor of the polycyclic graphite-like structures, 14 which have been also favored by a recent Raman spectroscopic study of dopa melanin. 29 While our calculated C6 structures may be present as substructures in melanin oligomers, the C5 structures are probably not relevant because they are too high in energy. We have also found an energetically accessible electrocyclic path for the formation of cyclic dimers which may complement the usually assumed radical oligomerization mechanism.…”

Section: Discussionmentioning

confidence: 87%

Structure, Stability and Optical Absorption of a Comprehensive Li-Brary of Minimal Eumelanin Oligomer Models

wang¹,

Blancafort

²

2021

Preprint

0

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Eumelanin is the black biopolymer responsible for photoprotection in living organisms. Lack of knowledge of the atomic-level structure limits understanding of its function and exploitation of its potential in material science. To overcome these limitations, we present a systematic density functional theory study of the stability and optical properties of a library of 830 dimers of 5,6-dihydroxyindole (DHI), which are minimal models of eumelanin oligomers. Our aim is to understand the principles that govern the formation of DHI oligomers, relate the optical properties of the dimers with their stability, and establish their possible role in the photophysics of the biopolymer. From the structural point of view, we find a preference for oxidized over reduced and cyclic over linear structures, which speaks in favor of polycyclic graphite-like structures for the larger oligomers. We present an electrocyclization mechanism leading to the cyclic structures. We also find that besides the widely considered quinone and quinone methide oxidation patterns where two heteroatoms per DHI fragment are oxidized, dimers with one or three oxidized sites per fragment and an interfragment double bond are also stable and may be present in eumelanin. As far as the optical properties are concerned, some oxidized dimers combine relative stability with absorption energies as low as 1.3 eV. Such fragments may be present as substructures in the naturally found oligomers and might have a relevant contribution to the absorption spectrum of the biopolymer. In addition to these insights into the struc-tural and optical properties of the oligomers, we introduce a new classification scheme and a representative set of 53 dimers combining thermodynamic stability with chemical diversity.<br>

show abstract

“…This speaks in favor of the polycyclic graphite-like structures, [11] which have been also favored by a recent Raman spectroscopic study of dopa melanin. [19] While our calculated C6 structures may be present as substructures in melanin oligomers, the C5 structures are probably not relevant because they are too high in energy. We have also found an energetically accessible electrocyclic path for the formation of cyclic dimers which may complement the usually assumed radical oligomerization mechanism.…”

Section: Discussionmentioning

confidence: 87%

Stability and Optical Absorption of a Comprehensive Virtual Library of Minimal Eumelanin Oligomer Models

Wang¹,

Blancafort

²

2021

Preprint

0

View full text Add to dashboard Cite

Eumelanin is the black biopolymer responsible for photoprotection in living organisms. Lack of knowledge of the atomic-level structure limits understanding of its function and exploitation of its potential in material science. To overcome these limitations, we present a systematic density functional theory study of the stability and optical properties of a library of 830 dimers of 5,6-dihydroxyindole (DHI), which are minimal models of eumelanin oligomers. Our aim is to understand the principles that govern the formation of DHI oligomers, relate the optical properties of the dimers with their stability, and establish their possible role in the photophysics of the biopolymer. From the structural point of view, we find a preference for oxidized over reduced and cyclic over linear structures, which speaks in favor of polycyclic graphite-like structures for the larger oligomers. We present an electrocyclization mechanism leading to the cyclic structures. We also find that besides the widely considered quinone and quinone methide oxidation patterns where two heteroatoms per DHI fragment are oxidized, dimers with one or three oxidized sites per fragment and an interfragment double bond are also stable and may be present in eumelanin. As far as the optical properties are concerned, some oxidized dimers combine relative stability with absorption energies as low as 1.3 eV. Such fragments may be present as substructures in the naturally found oligomers and might have a relevant contribution to the absorption spectrum of the biopolymer. In addition to these insights into the struc-tural and optical properties of the oligomers, we introduce a new classification scheme and a representative set of 53 dimers combining thermodynamic stability with chemical diversity.<br>

show abstract

Probing the heterogeneous structure of eumelanin using ultrafast vibrational fingerprinting

Cited by 46 publications

References 53 publications

Stability and Optical Absorption of a Comprehensive Virtual Library of Minimal Eumelanin Oligomer Models**

Stability and Optical Absorption of a Comprehensive Virtual Library of Minimal Eumelanin Oligomer Models**

Structure, Stability and Optical Absorption of a Comprehensive Li-Brary of Minimal Eumelanin Oligomer Models

Stability and Optical Absorption of a Comprehensive Virtual Library of Minimal Eumelanin Oligomer Models

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