Exploring the ground and excited states structural diversity of levosimendan, a cardiovascular calcium sensitizer

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.