OPTICAL RESPONSE OF VOLTAGE SENSITIVE DYES EMBEDDED IN POLARIZED LIPID MEMBRANES: AN INSIGHT FROM MOLECULAR DYNAMICS SIMULATIONS

GALASSI VANESA; SOSA, MICAELA; ANDRES BERTONI; SANCHEZ, CRISTIÁN; MARIO G DEL POPOLO

Campinas

Congreso; 47th Annual Meeting of the Brazilian Biophysical Society; 2023

SBBF, UNICAMP

Voltage sensitive dyes (VSDs) are amphiphilic molecules capable of sensing the polarization state of biological membranes. Their relevance due to their application in experimental science is boosted by their potential use in diagnostic imaging of excitable tissues. Its scope is limited by the sensitivity and spatiotemporal resolution achieved by available probes. A thorough understanding of the biophysical basis underlying their chromophoric behavior is the cornerstone towards the rational design of dyes with improved response. The aim of our work is to understand the action of VSDs in terms of elucidating configurational and electrochromic contributions to the optical response. We used indocyanine green (ICG) as study case, which is a molecule with peculiar optical activity, being excited in the near infrared, allowing to be irradiated and detected transdermally up to 2 cm. ICG is also approved for human clinical use by the FDA and its applications have been reported in various biomedical research articles [1].Electrochromic response of ICG was evaluated by linear response calculations with the TD-DFT formalism, for the molecule in vacuum with an external homogeneous electric field applied, obtaining a shift of 0.5 nm in the absorption maximum upon polarization with a field of 50 V/nm. To evaluate configurational modulation in the probe-membrane interaction due to the electric field, molecular mechanics models of ICG embedded in bilayers with different polarization states were built. The dye was localized in the hemilayer with positive polarity, mimicking the extracellular environment. Potential of mean force (PMF) for the partition of the dye in the membrane was assessed, accounting for the affinity. The effect of polarization on the affinity, location and orientation with respect to the membrane, were computed. Slightly different regions of this configurational space were spanned in different polarization states. Absorption spectra were obtained by TD-DFT calculations of both, randomly selected configurations and homogeneously distributed in the three-dimensional configurational space. The dye described with ab-initio formalism was embedded in a grid of classical charges of the environment (solvent and membrane). While the first configuration ensemble selection was much larger than the second, consistent results were obtained: the mean spectrum of the polarized state was 5-10 nm red-shifted regarding the non-polarized system. This not only is in agreement with experimental results [2], but also evidences a synergic effect between the configurational modulation in the probe-membrane interaction by the transmembrane electric field and the electrochromic effect, that lead to the differential optical response.Furthermore, we assessed the orientation of the susceptibility of the molecule, which is not the one oriented with the transmembrane field, but across the aromatic-conjugated system. We performed a first attempt at a rational design of potential sensors with improved response, proposing derivatives ICG molecule, in order to induce an orientation in membrane that maximizes the optical response to the transmembrane electric field.[1] Alander, J. T.; Kaartinen, I.; Laakso, A.; Pätilä, T.; Spillmann, T.; Tuchin, V. V.; Venermo, M.; Välisuo, P. Journal of Biomedical Imaging 2012 (2012) 7 [2] Treger, Jeremy S., et al. Biophysical journal 107.6 (2014) L09-L12