Binding Motifs of Carboplatin and Oxaliplatin with Guanine: A Combined MS/MS, IRMPD, and Theoretical Study

Complexes generated in the gas phase involving the purine nucleobase guanine bound to second and third generation platinum drugs, namely, carboplatin (CarboPt) and oxaliplatin (OxaliPt), were investigated by combining tandem mass spectrometry, collision-induced dissociation (CID), infrared multiple photon dissociation spectroscopy (IRMPD), and density functional theory (DFT) calculations. As the first step, a spectroscopic characterization of the protonated platinum drugs was accomplished. Protonation of both CarboPt and OxaliPt in the gas phase occurs on one of the two carbonyl groups of the cyclobutanedicarboxylate and oxalate ligand, respectively. Such protonation has been postulated by several theoretical studies as a key preliminary step in the hydrolysis of Pt drugs under acidic conditions. Subsequently, the protonated drugs react with guanine in solution to generate a complex of general formula [Pt drug + H + guanine]+, which was then mass-selected. CID experiments provided evidence of the presence of strong binding between guanine and platinum-based drugs within the complexes. The structures of the two complexes have also been examined by comparing the experimental IRMPD spectra recorded in two spectral regions with DFT-computed IR spectra. For each system, the IRMPD spectra agree with the vibrational spectra calculated for the global minimum structures, which present a monodentate complexation of Pt at the N7 position of canonical guanine. This binding scheme is therefore akin to that observed for cisplatin, while other coordination sites yield substantially less stable species. Interestingly, in the case of oxaliplatin, the IRMPD spectra are consistent with the presence of two isomeric forms very close in energy.

DFT study of trialkylborohydride-catalysed hydrosilylation of alkenes – the mechanism and its implications

A detailed DFT study reveals the mechanism of trialkylborohydride-catalysed Markovnikov hydrosilylation of aromatic alkenes.

Mass Spectrometric and Computational Investigation of the Protonated Carnosine-Carboplatin Complex Fragmentation

Platinum(II)-based anticancer drugs are square-planar d(8) complexes that, activated by hydrolysis, cause cancer cell death by binding to nuclear DNA and distorting its structure. For that reason, interactions of platinum anticancer drugs with DNA have been extensively investigated, aiming at disentangling the mechanism of action and toxicity. Less attention, however, has been devoted to the formation of adducts between platinum drugs with biological ligands other than DNA. These adducts can cause the loss and deactivation of the drug before it arrives at the ultimate target and are also thought to contribute to the drug's toxicity. Here are reported the outcomes of electrospray ionization mass spectrometry experiments and density functional theory (DFT) computations carried out to investigate the fragmentation pathways of the protonated carnosine-carboplatin complex, [Carnosine + CarbPt + H](+). DFT calculations at the B3LYP/LANL2DZ level employed to probe fragmentation mechanisms account for all experimental data. Because of the relative rigidity of the structure of the most stable 1A conformer, stabilized by three strong hydrogen bonds, the first step of all of the examined fragmentation pathways is the interconversion of the 1A conformer into the less stable structure 1B. Formation of the [Carnosine + H](+) fragment from the precursor ion, [Carnosine + CarbPt + H](+), is calculated to be the lowest-energy process. At slightly higher energies, the loss of two amino groups is observed to produce the [Carnosine + (CarbPt - NH3) + H](+) and [Carnosine + (CarbPt - 2NH3) + H](+) ions. At significantly higher energies, the loss of CO2 occurs, yielding the final [Carnosine + (CarbPt - NH3) - CO2 + H](+) and [Carnosine + (CarbPt - 2NH3) - CO2 + H](+) products. Formation of the [CarbPt + H](+) fragment from [Carnosine + CarbPt + H](+), even if not hampered by a high activation barrier, is calculated to be very unfavorable from a thermodynamic point of view.

Fragmentation pathways analysis for the gas phase dissociation of protonated carnosine-oxaliplatin complexes

Collision-induced dissociation (CID) experiments on the protonated carnosine-oxaliplatin complex were shown to yield nine different fragment ions. Density functional calculations were employed to probe the fragmentation mechanisms that account for all experimental data.

The mechanism of catalytic methylation of 2-phenylpyridine using di-tert-butyl peroxide

The mechanism of palladium chloride-catalyzed direct methylation of arenes with peroxides is elucidated by using the energetics computed at the M06 density functional theory. The introduction of a methyl group by tert-butyl peroxides at the ortho-position of a prototypical 2-phenyl pyridine, a commonly used substrate in directed C-H functionalization reactions, is examined in detail by identifying the key intermediates and transition states involved in the reaction sequence. Different possibilities that differ in terms of the site of catalyst coordination with the substrate and the ensuing mechanism are presented. The important mechanistic events involved are (a) an oxidative or a homolytic cleavage of the peroxide O-O bond, (b) C-H bond activation, (c) C-C bond activation, and (d) reductive elimination involving methyl transfer to the aromatic ring. We have examined both radical and non-radical pathways. In the non-radical pathway, the lowest energy pathway involves C-H bond activation prior to the coordination of the peroxide to palladium, which is subsequently followed by the O-O bond cleavage of the peroxide and the C-C bond activation. Reductive elimination in the resulting intermediate leads to the vital C-C bond formation between methyl and aryl carbon atoms. In the non-radical pathway, the C-C bond activation is higher in energy and has been identified as the rate-limiting step of this reaction. In the radical pathway, however, the activation barrier for the C-C bond cleavage is lower than for the peroxide O-O bond cleavage. A combination of a radical pathway up to the formation of a palladium methyl intermediate and a subsequent non-radical pathway has been identified as the most favored pathway for the title reaction. The predicted mechanism is in good agreement with the experimental observations on PdCl2 catalyzed methylation of 2-phenyl pyridine using tert-butyl peroxide.