Exploring the Ground and Excited States Structural Diversity of Levosimendan, a Cardiovascular Calcium Sensitizer

Abstract

Exploring the relationship between the structure and dynamics of a molecular system is fundamental to a better understanding of its function. Here, we report on studies of femtosecond dynamics of the most stable molecular structures of a cardiovascular drug, levosimendan (LSM), in water at three different pHs, in chemical (β-cyclodextrin, β-CD) and biological (human serum albumin protein, HSA) nanocavities, and in two organic solvents with different viscosities. In the used organic solvents, the structural dynamics, ranging from 50 fs to 3 ps, depends on the viscosity of the solvent, reflecting the involvement of a twisting motion in the excited molecule. In water solutions at pH 3 and 5, the excited neutral form is decaying in a time of ∼0.4 ps, undergoing an ultrafast (<50 fs) intramolecular charge transfer (ICT) to generate charge transfer species decaying in ∼1 ps. In neutral (pH 7) and alkaline water (pH 12), the LSM is present in its anion structure at the ground state. In these media, the experiments reveal, in addition to the ultrafast decay of the anionic structure (1.3 ps), the formation of an ICT state having (n, π*) character, produced in ∼0.3 ps and decaying in ∼0.5 ps. Encapsulation by β-CD and HSA protein leads to a 1:1 stoichiometry complex, which shows longer decaying times (4 and 7 ps, respectively) of the caged anionic forms due to the nanoconfinement. Our results show a structural diversity of the LSM dynamics, reflecting its intimate interaction with its surrounding. We believe that the reported findings and the related discussion and conclusions bring new knowledge for a better understanding of the molecular activity of this drug, taking into account its rich structural dynamics. Furthermore, the results might be relevant for a better drug design and nanodelivery involving CDs and proteins.

Excited state interactions between flurbiprofen and tryptophan in drug–protein complexes and in model dyads. Fluorescence studies from the femtosecond to the nanosecond time domains

“…As previously described, a dramatic fluorescence quenching (>90%) was observed for the dyads. The fluorescence quantum yields of (S,S)-and (R,S)-FBP-TrpMe were found to be 0.028 and 0.015, respectively (Figure SI-2A), which are much lower than those of 30 the individual chromophores. 38 From the shape and the position of the fluorescence bands, it was confirmed that emission is dominated by 1 TrpMe*.…”

“…The kQ2 values of the dyads, calculated from the corresponding F values, underline the strong stereoselectivity. The TCSPC decays of the dyads were much faster at 310 nm 30 than at 340 nm ( Figure SI-4). This should not be surprising in view of the efficient FBP fluorescence quenching.…”

“…Actually, HSA is one of the most abundant proteins in blood and plasma and is responsible for the transport of different agents in the bloodstream, such as fatty acids, drugs, or metabolites. 1,2 30 Therefore, the binding of ligands to HSA constitutes a key process, relevant for the modulation of a number of properties (drug solubility in plasma, toxicity, susceptibility to oxidation, in vivo half-life, etc.). 3,4 Flurbiprofen (FBP, Chart 1) [2-(2-fluorobiphenyl-4- 35 yl)propanoic acid] is a nonsteroidal anti-inflammatory drug (NSAID) employed for the treatment of a broad spectrum of pathophysiological conditions, including fever, headache, etc.…”

“…In addition to steady-state fluorescence quenching, [16][17][18][19][20][21][22][23][24] time-resolved measurements [25][26][27] allow the characterisation of the involved dynamic processes. In particular, femtosecond emission [28][29][30][31][32] indole to act as an electron donor. 36 In view of the complexity of the FBP/HSA system and the potential difficulties to interpret the fluorescence properties, complementary information is necessary in order to investigate 20 45 nanosecond time-resolved measurements, the fluorescence lifetimes at em = 340 nm were much shorter in the dyads (F < 1 ns) than in (S)-TrpMe, indicating a dynamic quenching.…”

“…In addition to steady-state fluorescence quenching, [16][17][18][19][20][21][22][23][24] time-resolved measurements [25][26][27] allow the characterisation of the involved dynamic processes. In particular, femtosecond emission [28][29][30][31][32] constitutes a powerful tool for determining the very fast photo-initiated processes.…”