Ruthenium compounds have become promising alternatives to platinum drugs by displaying specific activities against different cancers and favourable toxicity and clearance properties. Nonetheless, their molecular targeting and mechanism of action are poorly understood. Here we study two prototypical ruthenium-arene agents—the cytotoxic antiprimary tumour compound [(η6-p-cymene)Ru(ethylene-diamine)Cl]PF6 and the relatively non-cytotoxic antimetastasis compound [(η6-p-cymene)Ru(1,3,5-triaza-7-phosphaadamantane)Cl2]—and discover that the former targets the DNA of chromatin, while the latter preferentially forms adducts on the histone proteins. Using a novel ‘atom-to-cell’ approach, we establish the basis for the surprisingly site-selective adduct formation behaviour and distinct cellular impact of these two chemically similar anticancer agents, which suggests that the cytotoxic effects arise largely from DNA lesions, whereas the protein adducts may be linked to the other therapeutic activities. Our study shows promise for developing new ruthenium drugs, via ligand-based modulation of DNA versus protein binding and thus cytotoxic potential, to target distinguishing epigenetic features of cancer cells.

Bovine rhodopsin is the most extensively studied retinal protein and is considered the prototype of this important class of photosensitive biosystems involved in the process of vision. Many theoretical investigations have attempted to elucidate the role of the protein matrix in modulating the absorption of retinal chromophore in rhodopsin, but, while generally agreeing in predicting the correct location of the absorption maximum, they often reached contradicting conclusions on how the environment tunes the spectrum. To address this controversial issue, we combine here a thorough structural and dynamical characterization of rhodopsin with a careful validation of its excited-state properties via the use of a wide range of state-of-the-art quantum chemical approaches including various flavors of time-dependent density functional theory (TDDFT), different multireference perturbative schemes (CASPT2 and NEVPT2), and quantum Monte Carlo (QMC) methods. Through extensive quantum mechanical/molecular mechanical (QM/MM) molecular dynamics simulations, we obtain a comprehensive structural description of the chromophore-protein system and sample a wide range of thermally accessible configurations. We show that, in order to obtain reliable excitation properties, it is crucial to employ a sufficient number of representative configurations of the system. In fact, the common use of a single, ad hoc structure can easily lead to an incorrect model and an agreement with experimental absorption spectra due to cancelation of errors. Finally, we show that, to properly account for polarization effects on the chromophore and to quench the large blue-shift induced by the counterion on the excitation energies, it is necessary to adopt an enhanced description of the protein environment as given by a large quantum region including as many as 250 atoms.

Cells store energy in the form of neutral lipids packaged into micrometer-sized organelles named lipid droplets (LD). These structures emerge from the endoplasmic reticulum (ER) at sites marked by the protein seipin, but the mechanisms regulating their biogenesis remain poorly understood. Using a combination of molecular simulations, yeast genetics and fluorescence microscopy, we show that interactions between lipids' acyl-chains modulate the propensity of neutral lipids to be stored in LD, in turn preventing or promoting their accumulation in the ER membrane. Our data suggest that diacylglycerol, that is enriched at sites of LD formation, promotes the packaging of neutral lipids into LDs, together with ER-abundant lipids, such as phosphatidylethanolamine. On the opposite end, short and saturated acyl-chains antagonize fat storage in LD and promote accumulation of neutral lipids in the ER. Our results provide a new conceptual understanding of LD biogenesis in the context of ER homeostasis and function.

Horseradish peroxidase (HRP) is one of the most relevant peroxidase enzymes, used extensively in immunochemistry and biocatalysis applications. Unlike the closely related catalase enzymes, it exhibits a low activity to disproportionate hydrogen peroxide (H2O2). The origin of this disparity remains unknown due to the lack of atomistic information on the catalase-like reaction in HRP. Using QM(DFT)/MM metadynamics simulations, we uncover the mechanism for reduction of the HRP Compound I intermediate by H2O2 at atomic detail. The reaction begins with a hydrogen atom transfer, forming a peroxyl radical and a Compound II-like species. Reorientation of the peroxyl radical in the active site, concomitant with the transfer of the second hydrogen atom, is the rate-limiting step, with a computed free energy barrier (18.7 kcal/mol, ∼ 6 kcal/mol higher than the one obtained for catalase) in good agreement with experiments. Our simulations reveal the crucial role played by the distal pocket residues in accommodating H2O2, enabling formation of a Compound II-like intermediate, similar to catalases. However, out of the two pathways for Compound II reduction found in catalases, only one is operative in HRP. Moreover, the hydrogen bond network in the distal side of HRP compensates less efficiently than in catalases for the energetic cost required to reorient the peroxyl radical at the rate-determining step. The distal Arg and a water molecule in the "wet" active site of HRP have a substantial impact on the reaction barrier, compared to the "dry" active site in catalase. Therefore, the lower catalase-like efficiency of heme peroxidases compared to catalases can be directly attributed to the different distal pocket architecture, providing hints to engineer peroxidases with a higher rate of H2O2 disproportionation.