Communication
ChemComm
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conjugation or the bioorthogonal CuAAC reaction of proteins
that lack an endogenous photoexcitable group. For the first
time, LaserIMD distance determinations between a nitroxide
and a photoexcitable spin label exogenously introduced at a
user-defined protein site were performed, and modulation
depths comparable to the established porphyrin–nitroxide pair
were achieved. Compatibility with other spin labels and single-
frequency techniques such as LiRIDME has yet to be investi-
gated. For LaserIMD, AT12 outperforms RB and EO in terms of
modulation depth and photostability, but the long lifetime of
its triplet state requires longer measurement times. All four
dyes incorporate a carboxyl group for functionalization, making
it possible to compile a library with tethers of various lengths,
flexibility and bioconjugation moieties. The design of these
labels could be aided by an in silico rotamer library approach,39
18 M. Di Valentin, M. Albertini, E. Zurlo, M. Gobbo and D. Carbonera,
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¨
in which the reference point for distance determination, in 20 C. Hintze, D. Bu¨cker, S. Domingo Kohler, G. Jeschke and
agreement with the literature,18,23 is located in the center of the
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21 M. G. Dal Farra, S. Richert, C. Martin, C. Larminie, M. Gobbo,
molecule by a point-dipole approximation. In the bigger pic-
E. Bergantino, C. R. Timmel, A. M. Bowen and M. Di Valentin,
ture, the dual-addressable character of EO, RB and AT12 might
provide a gateway to correlating the results from luminescence
and EPR spectroscopy; and in turn, the field of bioimaging is
expected to offer a steady flow of inspiration for the design of
new labels for LiPDS.
This project has received funding from the European
Research Council (ERC) under the European Union’s Horizon
2020 research and innovation program (Grant Agreement number:
772027—SPICE—ERC-2017-COG). We thank M. Adam and
D. Brecht for experimental contributions, P. G. Schultz for the
plasmid pEVOL_pCN-Phe and D. Bu¨cker for helpful discussions.
ChemPhysChem, 2019, 20, 931–935.
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Conflicts of interest
There are no conflicts to declare.
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