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Prevalent intrinsic emission from nonaromatic amino acids and poly(amino acids)

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  • ReceivedJul 22, 2017
  • AcceptedJul 23, 2017
  • PublishedSep 6, 2017

Abstract

Nonaromatic amino acids are generally believed to be nonemissive, owing to their lack of apparently remarkable conjugation within individual molecules. Here we report the intrinsic visible emission of nonaromatic amino acids and poly(amino acids) in concentrated solutions and solid powders. This unique and widespread luminescent characteristic can be well rationalized by the clustering-triggered emission (CTE) mechanism, namely the clustering of nonconventional chromophores (i.e. amino, carbonyl, and hydroxyl) and subsequent electron cloud overlap with simultaneous conformation rigidification. Such CTE mechanism is further supported by the single crystal structure analysis, from which 3D through space electronic communications are uncovered. Besides prompt fluorescence, room temperature phosphorescence (RTP) is also detected from the solids. Moreover, persistent RTP is observed in the powders of exampled poly(amino acids) of ε-poly-L-lysine (ɛ-PLL) after ceasing UV irradiation. These results not only illustrate the feasibility of employing the building blocks of nonaromatic amino acids in the exploration of new luminescent biomolecules, but also provide significant implications for the emissions of peptides and proteins at aggregated or crystalline states. Meanwhile, they may also shed lights on further understanding of autofluorescence from biological systems.


Funded by

National Natural Science Foundation of China(51473092)

Shanghai Rising-Star Program(15QA1402500)


Acknowledgment

This work was supported by the National Natural Science Foundation of China (51473092), and the Shanghai Rising-Star Program (15QA1402500). The authors appreciate Ms Xiaoli Bao and Ms Lingling Li at the Instrumental Analysis Center of Shanghai Jiao Tong University for the single-crystal structure determination of L-Ile.


Interest statement

The authors declare that they have no conflict of interest.


Supplement

The supporting information is available online at http://chem.scichina.com and http://link.springer.com/journal/11426. The supporting materials are published as submitted, without typesetting or editing. The responsibility for scientific accuracy and content remains entirely with the authors.


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  • Figure 1

    Exampled nonaromatic amino acids and photographs of their recrystallized solids taken under 365 nm UV light. Emission efficiencies of the recrystallized solids are given in brackets (color online).

  • Figure 2

    (a) Emission (λex=365 nm) and (b) absorption spectra of varying L-Lys aqueous solutions; (c) emission spectra of 0.1 M L-Lys aqueous solution with different λexs; (d, e) photographs of varying L-Lys aqueous solutions taken under 365 nm UV light or ceasing the irradiation at room temperature and 77 K (color online).

  • Figure 3

    Emission spectra of recrystallized solids of (a) L-Lys, (b) L-Ser, and (c) L-Ile with td of 0 (solid line) and 0.1 ms (dash line). (d) Microscope images of L-Lys solids taken under illumination of UV (330–385 nm, left), blue (460–495 nm, middle) and green (530–550 nm, right) lights. (e) Photographs of L-Lys, L-Ser, and L-Ile solids taken at 77 K under 365 nm UV light or after ceasing the UV irradiation (color online).

  • Figure 4

    (a, b) Crystal structure of L-Ser with denoted intermolecular interactions around one molecule; (c) N···O and O···O intermolecular interactions around one molecule; (d) fragmental 3D through space electronic communication channel in the L-Ser crystals (color online).

  • Figure 5

    Photographs of (a) different ε-PLL aqueous solutions and (b) solid powders taken under 365 nm UV light or after ceasing the UV irradiation. Emission spectra of (c) different ε-PLL aqueous solutions (λex=336 nm) and (d) 15 mg/mL solution with varying λexs. (e) Normalized emission spectra of ε-PLL solids with td of 0 (solid line) and 0.1 ms (dash line) under varying λexs (color online).

  • Figure 6

    Confocal luminescent images of HeLa cells after incubation with 0.1 M L-Ile in DMEM for 1.5 h. (a) Confocal image recorded under excitation at 405 nm, (b) bright field image, and (c) corresponding overlayed image (color online).

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