Probing Methionine Uptake in Live Cells by Deuterium Labeling and Stimulated Raman Scattering

Photocatalytic and Chemoselective H/D Exchange at α-Thio C(sp3)-H Bonds

Deuterated compounds used in drug discovery and live-cell imaging have recently gained the attention of various scientific fields. Although hydrogen-deuterium (H/D) exchange reactions are straightforward deuteration methods, achieving perfect chemoselectivity is challenging. We report the highly chemoselective deuteration of α-thio C(sp3)-H bonds using a thioxanthone or anthraquinone organic photocatalyst bearing an aromatic ketone skeleton and D2O as an inexpensive deuterium source under 390 nm irradiation. Notably, incorporation of deuterium at the α-positions of the O/N atoms, benzylic positions, and aromatic rings was not observed. The present chemoselectivity was accomplished via a single electron transfer mechanism between the photocatalyst and S-containing substrates, as proven by laser-induced time-resolved transient absorption spectroscopic measurements. Furthermore, the proposed deuteration method could be applied to various S-containing substrates, including pharmaceuticals and biologically active compounds with high regioselectivities. The available deuterated compounds as novel deuterated alkylation reagents for future drug discovery and materials for Raman imaging were also demonstrated.

Enzyme-Instructed Self-Assembly Reprograms Fatty Acid Metabolism for Cancer Therapeutics

Enzyme-instructed self-assembly (EISA) is actively explored as a promising therapeutic approach for cancer treatment. However, the metabolic response of cancer cells to EISA remains under-studied. Here, by stimulated Raman scattering (SRS) imaging in C─H, fingerprint, and silent windows, it is found that the formation of peptide assemblies within and around cancer cells significantly enhances both lipids catabolism and fatty acids (FAs) uptake. It is further found that the increased uptake of FAs aids the resistance of cancer cells under EISA treatment, likely to cope with the stress induced by the peptide assemblies. Combining EISA with FAs uptake inhibition leads to enhanced cancer suppression compared to EISA alone, while additional FAs supplementation rescue cancer cells from EISA treatment, both in vitro and in 3D-culture spheroid models. These findings shed new light on the impact of EISA on the metabolic activities of cancer cells and suggest a new approach for improved cancer therapy.

Illuminating life processes by vibrational probes

Vibration of chemical bonds can serve as imaging contrast. Vibrational probes, synergized with major advances in chemical bond imaging instruments, have recently flourished and proven valuable in illuminating life processes. Here, we review how the development of vibrational probes with optimal biocompatibility, enhanced sensitivity, multichromatic colors and diverse functionality has extended chemical bond imaging beyond the prevalent label-free paradigm into various novel applications such as imaging metabolites, metabolic imaging, drug imaging, super-multiplex imaging, vibrational profiling and vibrational sensing. These advancements in vibrational probes have greatly facilitated understanding living systems, a new field of vibrational chemical biology.

Multiple Deuterium Atom Transfer Perdeuteration of Unactivated Alkenes under Base-Assisted Cobalt/Photoredox Dual Catalysis

A radical approach for hydrogenative perdeuteration of unactivated alkenes under cobalt/photoredox dual catalysis is described. The addition of a suitable base plays a key role in controlling two competing pathways by switching the catalytic performance of cobalt/photoredox catalysis. Base-assisted cobalt/photoredox dual catalysis promoted a hydrogen isotope exchange reaction of alkenes to afford deuterated alkenes via multiple repeating deuterium atom transfer/hydrogen atom abstraction processes, while consecutive reductive deuteration of alkenes proceeded in the absence of a base to afford polydeuterated alkanes. One-pot hydrogenative perdeuteration and perdeutero-arylation were also developed, providing access to various polydeuterated aliphatic compounds.

Widely tunable fiber optical parametric oscillator synchronized with a Ti:sapphire laser for stimulated Raman scattering microscopy

Stimulated Raman scattering (SRS) microscopy is a powerful vibrational imaging technique with high chemical specificity. However, the insufficient tuning range or speed of light sources limits the spectral range of SRS imaging and, hence, the ability to identify molecular species. Here, we present a widely tunable fiber optical parametric oscillator with a tuning range of 1470 cm-1, which can be synchronized with a Ti:sapphire laser. By using the synchronized light sources, we develop an SRS imaging system that covers the fingerprint and C-H stretching regions, without balanced detection. We validate its broadband imaging capability by visualizing a mixed polymer sample in multiple vibrational modes. We also demonstrate SRS imaging of HeLa cells, showing the applicability of our SRS microscope to biological samples.

Visualization of Modified Bisarylbutadiyne-Tagged Small Molecules in Live-Cell Nuclei by Stimulated Raman Scattering Microscopy

Visualizing the distribution of small-molecule drugs in living cells is an important strategy for developing specific, effective, and minimally toxic drugs. As an alternative to fluorescence imaging using bulky fluorophores or cell fixation, stimulated Raman scattering (SRS) imaging combined with bisarylbutadiyne (BADY) tagging enables the observation of small molecules closer to their native intracellular state. However, there is evidence that the physicochemical properties of BADY-tagged analogues of small-molecule drugs differ significantly from those of their parent drugs, potentially affecting their intracellular distribution. Herein, we developed a modified BADY to reduce deviations in physicochemical properties (in particular, lipophilicity and membrane permeability) between tagged and parent drugs, while maintaining high Raman activity in live-cell SRS imaging. We highlight the practical application of this approach by revealing the nuclear distribution of a modified BADY-tagged analogue of JQ1, a bromodomain and extra-terminal motif inhibitor with applications in targeted cancer therapy, in living HeLa cells. The modified BADY, methoxypyridazyl pyrimidyl butadiyne (MPDY), revealed intranuclear JQ1, while BADY-tagged JQ1 did not show a clear nuclear signal. We anticipate that the present approach combining MPDY tagging with live-cell SRS imaging provides important insight into the behavior of intracellular drugs and represents a promising avenue for improving drug development.

Biomedical applications, perspectives and tag design concepts in the cell - silent Raman window

Spectroscopic studies increasingly employ Raman tags exhibiting a signal in the cell - silent region of the Raman spectrum (1800-2800 cm-1), where bands arising from biological molecules are inherently absent. Raman tags bearing functional groups which contain a triple bond, such as alkyne and nitrile or a carbon-deuterium bond, have a distinct vibrational frequency in this region. Due to the lack of spectral background and cell-associated bands in the specific area, the implementation of those tags can help overcome the inherently poor signal-to-noise ratio and presence of overlapping Raman bands in measurements of biological samples. The cell - silent Raman tags allow for bioorthogonal imaging of biomolecules with improved chemical contrast and they have found application in analyte detection and monitoring, biomarker profiling and live cell imaging. This review focuses on the potential of the cell - silent Raman region, reporting on the tags employed for biomedical applications using variants of Raman spectroscopy.

Measuring Drug Response with Single-Cell Growth Rate Quantification

Intratumoral heterogeneity is a substantial cause of drug resistance development during chemotherapy or other drug treatments for cancer. Therefore, monitoring and measuring cell exposure and response to drugs at the single-cell level are crucial. Previous research suggested that the single-cell growth rate can be used to investigate drug-cell interactions. However, currently established methods for quantifying single-cell growth are limited to isolated or monolayer cells. Here, we introduce a technique that accurately measures both 2D and 3D cell growth rates using label-free ratiometric stimulated Raman scattering (SRS) microscopy. We use deuterated amino acids, leucine, isoleucine, and valine, as tracers and measure the C-D SRS signal from deuterium-labeled proteins and the C-H SRS signal from unlabeled proteins simultaneously to determine the cell growth rate at the single-cell level. The technique offers single-cell level drug sensitivity measurement with a shorter turnaround time (within 12 h) than most traditional assays. The submicrometer resolution of the imaging technique allows us to examine the effects of chemotherapeutic drugs, including kinase inhibitors, mitotic inhibitors, and topoisomerase II inhibitors, on both the cell growth rate and morphology. The capability of quantifying 3D cell growth rates provides insight into a deeper understanding of the cell-drug interaction in the actual tumor environment.

Multiple deuteration of triphenylphosphine and live-cell Raman imaging of deuterium-incorporated Mito-Q

All aromatic C-H bonds of triphenylphosphine (PPh3) were efficiently replaced by C-D bonds using Ru/C and Ir/C co-catalysts in 2-PrOH and D2O, an inexpensive deuterium source. Furthermore, non-radioactive and safe deuterium-incorporated Mito-Q (drug candidate) was prepared from deuterated PPh3 and used for the live-cell Raman imaging to evaluate the mitochondrial uptake.

Imaging the uptake of deuterated methionine in Drosophila with stimulated Raman scattering

Introduction: Visualizing small individual biomolecules at subcellular resolution in live cells and tissues can provide valuable insights into metabolic activity in heterogeneous cells, but is challenging.

Methods: Here, we used stimulated Raman scattering (SRS) microscopy to image deuterated methionine (d-Met) incorporated into Drosophila tissues in vivo.

Results: Our results demonstrate that SRS can detect a range of previously uncharacterized cell-to-cell differences in d-Met distribution within a tissue at the subcellular level.

Discussion: These results demonstrate the potential of SRS microscopy for metabolic imaging of less abundant but important amino acids such as methionine in tissue.