The results presented herein demonstrate the key role played by the boron clusters, closo-dodecaborate and cobaltabisdicarbollide, in the cell staining properties of organotin dyes. Whilst those bearing the closo-dodecaborate produce nucleoli and cytoplasmic staining, cobaltabisdicarbollide-containing dyes only produce cytoplasm staining. The remarkable fluorescence staining properties showed by these compounds make them excellent candidates as cell-staining fluorescent probes
Boron clusters are three-dimensional rigid compounds with high thermal and chemical stability. Due to their intriguing electronic properties and their biological compatibility, our group and others have been interested in the development of carborane-containing dyes with potential applications in biomedicine and material sciences [1-3]. Amongst the wide variety of biomedical techniques, Fluorescence Bioimaging has emerged as an important tool for cell and tissue visualisation, with organotin Schiff-base compounds proving to act as cytoplasm markers in vitro .
Therefore, incorporation of boron clusters in the structure of organotin complexes is an attractive chemical strategy, not only to modulate their photophysical properties, but also to change their cellular internalization behavior. Herein the design of novel fluorescent organotin compounds bearing closo-dodecaborate ([B12H12]2-) and cobaltabisdicarbollide (COSAN) is described, and their cytoplasm and nucleoli staining in vitro (B16F10 cells) is studied.
Organotin compounds bearing an aliphatic chain displayed fluorescent quantum yields (FF) around 24-34 %. Solution-state photophysical studies showed a considerably increase in the FF up to 49 % when the aliphatic chain is replaced by the [B12H12]2- moiety, whereas those bearing COSAN moieties showed lower quantum yield values (3-7 %). Despite this, the fluorescence intensity still allows the analysis of all the compounds by confocal microscopy (see Figure). Two noteworthy conclusions arise from the study of these confocal images. Firstly, there is a striking improvement in solubility for those boron cluster-containing complexes, and no aggregation is observed, neither in the cell media nor on the cell membrane. Secondly, the presence of each boron cluster in the structure produces a different staining effect; those bearing [B12H12]2- produce nucleoli and cytoplasmic staining, while the COSAN-containing dyes are only detected in the cytoplasm.
The striking results obtained for the in vitro studies, highlight the suitability of these boron cluster-containing organotin complexes as potential candidates for cell labelling agents towards medical diagnosis in clinical biopsies.
Justo Cabrera‐González,1 Blanca M. Muñoz‐Flores,2 Clara Viñas,1 Arturo Chávez‐Reyes,3 H. V. Rasika Dias,5 Víctor M. Jiménez‐Pérez 2 and Rosario Núñez1
1Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Spain
2Universidad Autónoma de Nuevo León, Facultad de Ciencias Químicas, Ciudad Universitaria, México.
3Centro de Investigación y de Estudios Avanzados del IPN, Unidad Monterrey, México.
4Department of Chemistry and Biochemistry, The University of Texas at Arlington, USA
Organotin dyes bearing anionic boron clusters as cell-staining fluorescent probes
Chemistry A-European Journal 24, 5601–5612 (2018)
 Núñez, R., M. Tarrès, A. Ferrer-Ugalde, F. Fabrizi de Biani & F. Teixidor (2016) Electrochemistry and Photoluminescence of Icosahedral Carboranes, Boranes, Metallacarboranes, and Their Derivatives. Chemical Reviews, 116, 14307-14378.
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 Chaari, M., N. Gaztelumendi, J. Cabrera-González, P. Peixoto-Moledo, C. Viñas, E. Xochitiotzi-Flores, N. Farfan, A. Ben Salah, C. Nogues & R. Nuñez (2018) Fluorescent BODIPY-anionic boron cluster conjugates as potential agents for cell tracking. Bioconjugate Chemistry, 29, 1763-1773.
 Jiménez-Pérez, V. M., M. C. García-López, B. M. Muñoz-Flores, R. Chan-Navarro, J. C. Berrones-Reyes, H. V. R. Dias, I. Moggio, E. Arias, J. A. Serrano-Mireles & A. Chavez-Reyes (2015) New application of fluorescent organotin compounds derived from Schiff bases: synthesis, X-ray structures, photophysical properties, cytotoxicity and fluorescent bioimaging. Journal of Materials Chemistry B, 3, 5731-5745.
Celullar localization by confocal microscopy of organotin complexes bearing closo-dodecaborate (left) and cobaltabisdicarbollide (right)
The work at ICMAB was supported by Spanish Ministerio de Economía y Competitividad, MINECO (CTQ2016-75150-R and “Severo Ochoa” Program for Centers of Excellence in R&D SEV- 2015-0496); and Generalitat de Catalunya (2014/SGR/149).