Amino Acids as Chelating Ligands for Platinum: Enhanced Stability in an Aqueous Environment Promoted by Biocompatible Molecules.

Platinum-based chemotherapeutics are a cornerstone in the treatment of many malignancies. However, their dose-limiting side effects have rooted efforts to develop new drug candidates with higher selectivity for tumor tissues and less problematic side effects. Here, we developed a cytotoxic platinum(II) complex based on Zeise's salt, containing the nonsteroidal anti-inflammatory drug acetylsalicylic acid and alanine as ligands (4). The previously developed complex (5) displayed high reactivity against sulfur-containing biomolecules; therefore, we put the focus on the optimization of the structure regarding its stability. Different amino acids were used as biocompatible chelating ligands to achieve this aim. Differences in the coordination sphere caused pronounced changes in the stability of Zeise-type precursors 1-3. Coordination with l-Ala through N in the trans position to ethylene showed the most promising results and was employed to stabilize 5. As a result, complex 4 showed improved stability and cytotoxicity, outperforming both 5 and 1.

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.

IRMPD spectroscopy and quantum-chemical simulations of the reaction products of cisplatin with the dipeptide CysGly.

The inorganic antineoplastic drug cisplatin was made to react in solution with the dipeptide cysteinylglycine (CysGly), chosen as a functional model of glutathione, and the reaction products were analyzed using electrospray ionization mass spectrometry (ESI-MS). Selected complexes, i.e., the primary substitution product cis-[PtCl(NH3)2(CysGly)]+ and the chelate cis-[PtCl(NH3)(CysGly)]+, were submitted to IR multiple photon dissociation (IRMPD) spectroscopy obtaining their vibrational features. The experimental IR ion spectra were compared with the calculated IR absorptions of different plausible isomeric families, finding CysGly to bind preferentially platinum(II) via its deprotonated thiolic group in the monovalent complex, cis-[PtCl(NH3)2(CysGly)]+, and to evolve in the S,N-bound chelate structure cis-[PtCl(NH3)(CysGly)]+ through the SH and NH2 functionality of the cysteine residue. Moreover, our findings indicate that the platination reaction does not affect the CysGly peptide bond, which remains in its trans configuration. These results provide additional insights into the reactivity of Pt(II)-complexes with glutathione which is involved in cellular cisplatin resistance.

Protonated Forms of Naringenin and Naringenin Chalcone: Proteiform Bioactive Species Elucidated by IRMPD Spectroscopy, IMS, CID-MS, and Computational Approaches.

Naringenin (Nar) and its structural isomer, naringenin chalcone (ChNar), are two natural phytophenols with beneficial health effects belonging to the flavonoids family. A direct discrimination and structural characterization of the protonated forms of Nar and ChNar, delivered into the gas phase by electrospray ionization (ESI), was performed by mass spectrometry-based methods. In this study, we exploit a combination of electrospray ionization coupled to (high-resolution) mass spectrometry (HR-MS), collision-induced dissociation (CID) measurements, IR multiple-photon dissociation (IRMPD) action spectroscopy, density functional theory (DFT) calculations, and ion mobility-mass spectrometry (IMS). While IMS and variable collision-energy CID experiments hardly differentiate the two isomers, IRMPD spectroscopy appears to be an efficient method to distinguish naringenin from its related chalcone. In particular, the spectral range between 1400 and 1700 cm-1 is highly specific in discriminating between the two protonated isomers. Selected vibrational signatures in the IRMPD spectra have allowed us to identify the nature of the metabolite present in methanolic extracts of commercial tomatoes and grapefruits. Furthermore, comparisons between experimental IRMPD and calculated IR spectra have clarified the geometries adopted by the two protonated isomers, allowing a conformational analysis of the probed species.

Binding Modes of a Cytotoxic Dinuclear Copper(II) Complex with Phosphate Ligands Probed by Vibrational Photodissociation Ion Spectroscopy.

The dinuclear copper complex bearing a 2,7-disubstituted-1,8-naphthalenediol ligand, [(HtomMe){Cu(OAc)}2](OAc), a potential anticancer drug able to bind to two neighboring phosphates in the DNA backbone, is endowed with stronger cytotoxic effects and inhibition ability of DNA synthesis in human cancer cells as compared to cisplatin. In this study, the intrinsic binding ability of the charged complex [(HtomMe){Cu(OAc)}2]+ is investigated with representative phosphate diester ligands with growing chemical complexity, ranging from simple inorganic phosphate up to mononucleotides. An integrated method based on high-resolution mass spectrometry (MS), tandem MS, and infrared multiple photon dissociation (IRMPD) spectroscopy in the 600-1800 cm-1 spectral range, backed by quantum chemical calculations, has been used to characterize complexes formed in solution and delivered as bare species by electrospray ionization. The structural features revealed by IRMPD spectroscopy have been interpreted by comparison with linear IR spectra of the lowest-energy structures, revealing diagnostic signatures of binding modes of the dinuclear copper(II) complex with phosphate groups, whereas the possible competitive interaction with the nucleobase is silenced in the gas phase. This result points to the prevailing interaction of [(HtomMe){Cu(OAc)}2]+ with phosphate diesters and mononucleotides as a conceivable contribution to the observed anticancer activity.

Novel pyridine-containing histone deacetylase inhibitors strongly arrest proliferation, induce apoptosis and modulate miRNAs in cancer cells.

After over 30 years of research, the development of HDAC inhibitors led to five FDA/Chinese FDA-approved drugs and many others under clinical or preclinical investigation to treat cancer and non-cancer diseases. Herein, based on our recent development of pyridine-based isomers as HDAC inhibitors, we report a series of novel 5-acylamino-2-pyridylacrylic- and -picolinic hydroxamates and 2'-aminoanilides 5-8 as anticancer agents. The hydroxamate 5d proved to be quite HDAC3/6-selective exhibiting IC50 values of 80 and 11 nM, respectively, whereas the congener 5e behaved as inhibitor of HDAC1-3, -6, -8, and -10 (class I/IIb-selective inhibitor) at nanomolar level. Compound 5e provided a huge antiproliferative activity (nanomolar IC50 values) against both haematological and solid cancer cell lines. In leukaemia U937 cells, the hydroxamate 5d and the 2'-aminoanilide 8f induced remarkable cell death after 48 h, with 76% and 100% pre-G1 phase arrest, respectively, showing a stronger effect with respect to SAHA and MS-275 used as reference compounds. In U937 cells, the highest dose- and time-dependent cytodifferentiation was obtained by the 2'-aminoanilide 8d (up to 35% of CD11c positive/propidium iodide negative cells at 5 µM for 48 h). The same 8d and the hydroxamates 5d and 5e were the most effective in inducing p21 protein expression in the same cell line. Mechanistically, 5d, 5e, 8d and 8f increased mRNA expression of p21, BAX and BAK, downregulated cyclin D1 and BCL-2 and modulated pro- and anti-apoptotic microRNAs towards apoptosis induction. Finally, 5e strongly arrested proliferation in nine different haematological cancer cell lines, with dual-digit nanomolar potency towards MV4-11, Kasumi-1, and NB4, being more potent than mocetinostat, used as reference drug.

Unusually Chemoselective Photocyclization of 2-(Hydroxyimino)aldehydes to Cyclobutanol Oximes: Synthetic, Stereochemical, and Mechanistic Aspects.

Photocyclization of carbonyl compounds (known as the Norrish-Yang reaction) to yield cyclobutanols is, in general, accompanied by fragmentation reactions. The latter are predominant in the case of aldehydes so that secondary cyclobutanols are not considered accessible via the straightforward Norrish-Yang reaction. A noteworthy exception has been reported in our laboratory, where cyclobutanols bearing a secondary alcohol function were observed upon UV light irradiation of 2-(hydroxyimino)aldehydes (HIAs). This reaction is here investigated in detail by combining synthesis, spectroscopic data, molecular dynamics, and DFT calculations. The synthetic methodology is generally applicable to a series of HIAs, affording the corresponding cyclobutanol oximes (CBOs) chemoselectively (i.e., without sizable fragmentation side-reactions), diastereoselectively (up to >99:1), and in good to excellent yields (up to 95%). CBO oxime ether derivatives can be purified and diastereomers isolated by standard column chromatography. The mechanistic and stereochemical picture of this photocyclization reaction, as well as of the postcyclization E/Z isomerization of the oxime double bond is completed.

Elusive intermediates in cisplatin reaction with target amino acids: Platinum(II)-cysteine complexes assayed by IR ion spectroscopy and DFT calculations.

The reactivity of a widely used metal based antineoplastic drug, cisplatin, cis-PtCl2(NH3)2, with L-cysteine (Cys) has been investigated using a combination of electrospray ionization mass spectrometry (ESI-MS), IRMPD gas phase ion spectroscopy and DFT calculations. The cysteine lateral chain represents one of the main platination sites in proteins, which is believed to be related to the resistance mechanisms to cisplatin. The vibrational features of the mass-selected substitution product cis-[PtCl(NH3)2(Cys)]+ and the intercepted cis-[PtCl(NH3)2(H2O)(Cys)]+ intermediate complex were compared to calculated IR spectra, enabling the assessment of the sampled ions structures. In cis-[PtCl(NH3)2(Cys)]+, cysteine was found to bind platinum through the sulfur atom as a thiolate zwitterion, highlighting the enhanced acidity of the cysteine thiol group upon metal coordination. The cis-[PtCl(NH3)2(H2O)(Cys)]+ structure complies with the non-covalent encounter complex, formed by cis-[PtCl(NH3)2(H2O)]+ and neutral cysteine. This species is able to undergo the substitution process to produce cis-[PtCl(NH3)2(Cys)]+ when activated as a mass-isolated ion suggesting its participation in the reaction mechanism of cisplatin with cysteine in solution. Finally, the DFT-calculated energy profile for the substitution reaction was correlated with the peculiar gas-phase reactivity of this non-covalent complex, resulting to be 10-fold less reactive toward substitution than the corresponding methionine complex.

IRMPD Spectroscopy of Bare Monodeprotonated Genistein, an Antioxidant Flavonoid.

Genistein is a naturally occurring polyphenol belonging to the family of flavonoids with estrogenic properties and proven antioxidant, anti-inflammatory, and hormonal effects. Genistein and its derivatives are involved in radical scavenging activity by way of mechanisms based on sequential proton-loss electron transfer. In view of this role, a detailed structural characterization of its bare deprotonated form, [geni-H]-, generated by electrospray ionization, has been performed by tandem mass spectrometry and infrared multiple photon dissociation (IRMPD) spectroscopy in the 800-1800 cm-1 spectral range. Quantum chemical calculations at the B3LYP/6-311+G(d,p) level of theory were carried out to determine geometries, thermochemical data, and anharmonic vibrational properties of low-lying isomers, enabling to interpret the experimental spectrum. Evidence is gathered that the conjugate base of genistein exists as a single isomeric form, which is deprotonated at the most acidic site (7-OH) and benefits from a strong intramolecular H-bond interaction between 5-OH and the adjacent carbonyl oxygen in the most stable arrangement.

Ligation Motifs in Zinc-Bound Sulfonamide Drugs Assayed by IR Ion Spectroscopy.

The sulfonamide-zinc ion interaction, performing a key role in various biological contexts, is the focus of the present study, with the aim of elucidating ligation motifs in zinc complexes of sulfa drugs, namely sulfadiazine (SDZ) and sulfathiazole (STZ), in a perturbation-free environment. To this end, an approach is exploited based on mass spectrometry coupled with infrared multiple photon dissociation (IRMPD) spectroscopy backed by quantum chemical calculations. IR spectra of Zn(H2O+SDZ-H)+ and Zn(H2O+STZ-H)+ ions are consistent with a three-coordinate zinc complex, where ZnOH+ binds to the uncharged sulfonamide via N(heterocycle) and O(sulfonyl) donor atoms. Alternative prototropic isomers Zn(OH2)(SDZ-H)+ and Zn(OH2)(STZ-H)+ lie 63 and 26 kJ mol-1 higher in free energy, respectively, relative to the ground state Zn(OH)(SDZ)+ and Zn(OH)(STZ)+ species and do not contribute to any significant extent in the sampled population.

Cation-π Interactions between a Noble Metal and a Polyfunctional Aromatic Ligand: Ag+ (benzylamine).

The structure of an isolated Ag+ (benzylamine) complex is investigated by infrared multiple photon dissociation (IRMPD) spectroscopy complemented with quantum chemical calculations of candidate geometries and their vibrational spectra, aiming to ascertain the role of competing cation-N and cation-π interactions potentially offered by the polyfunctional ligand. The IRMPD spectrum has been recorded in the 800-1800 cm-1 fingerprint range using the IR free electron laser beamline coupled with an FT-ICR mass spectrometer at the Centre Laser Infrarouge d'Orsay (CLIO). The resulting IRMPD pattern points toward a chelate coordination (N-Ag+ -π) involving both the amino nitrogen atom and the aromatic π-system of the phenyl ring. The gas-phase reactivity of Ag+ (benzylamine) with a neutral molecular ligand (L) possessing either an amino/aza functionality or an aryl group confirms N- and π-binding affinity and suggests an augmented silver coordination in the product adduct ion Ag + ( benzylamine ) ( L ) .

Heterocycle-containing tranylcypromine derivatives endowed with high anti-LSD1 activity.

As regioisomers/bioisosteres of 1a, a 4-phenylbenzamide tranylcypromine (TCP) derivative previously disclosed by us, we report here the synthesis and biological evaluation of some (hetero)arylbenzoylamino TCP derivatives 1b-6, in which the 4-phenyl moiety of 1a was shifted at the benzamide C3 position or replaced by 2- or 3-furyl, 2- or 3-thienyl, or 4-pyridyl group, all at the benzamide C4 or C3 position. In anti-LSD1-CoREST assay, all the meta derivatives were more effective than the para analogues, with the meta thienyl analogs 4b and 5b being the most potent (IC50 values = 0.015 and 0.005 µM) and the most selective over MAO-B (selectivity indexes: 24.4 and 164). When tested in U937 AML and prostate cancer LNCaP cells, selected compounds 1a,b, 2b, 3b, 4b, and 5a,b displayed cell growth arrest mainly in LNCaP cells. Western blot analyses showed increased levels of H3K4me2 and/or H3K9me2 confirming the involvement of LSD1 inhibition in these assays.

Molecular Basis for the Remarkably Different Gas-Phase Behavior of Deprotonated Thyroid Hormones Triiodothyronine (T3) and Reverse Triiodothyronine (rT3): A Clue for Their Discrimination?

Thyroid hormones are biologically active small molecules responsible for growth and development regulation, basal metabolic rate, and lipid and carbohydrate metabolism. Liquid chromatography mass spectrometry (LC-MS) can be used to quantify thyroid hormones blood level with high speed and selectivity, aiming to improve the diagnosis and treatment of the severe pathological conditions in which they are implicated, i.e., hypo- and hyperthyroidism. In this work, the gas-phase behavior of the isomeric thyroid hormones triiodothyronine (T3) and reverse triiodothyronine (rT3) in their deprotonated form was studied at a molecular level using MS-based techniques. Previously reported collision-induced dissociation experiments yielded distinct spectra despite the high structural similarity of the two compounds, suggesting different charge sites to be responsible. Infrared multiple photon dissociation spectroscopy on [T3-H]- and [rT3-H]- was performed, and the results were interpreted using DFT and MP2 calculations, assessing the prevalence of T3 in the carboxylate form and rT3 as a phenolate isomer. The different deprotonation sites of the two isomers were also found to drive their ion-mobility behavior. In fact, [T3-H]- and [rT3-H]- were successfully separated. Drift times were correlated with collisional cross section values of 209 and 215 Å2 for [T3-H]- and [rT3-H]-, respectively. Calculations suggested the charge site to be the main parameter involved in the different mobilities of the two anions. Finally, bare [T3-H]- and [rT3-H]- were made to react with neutral acetylacetone and trifluoroacetic acid, confirming rT3 to be more acidic than T3 in agreement with the calculated gas-phase acidities of T3 and rT3 equal to 1345 and 1326 kJ mol-1, respectively.

From Preassociation to Chelation: A Survey of Cisplatin Interaction with Methionine at Molecular Level by IR Ion Spectroscopy and Computations.

Methionine (Met) plays an important role in the metabolism of cisplatin anticancer drug. Yet, methionine platination in aqueous solution presents a highly complex pattern of interconnected paths and intermediates. This study reports on the reaction of methionine with the active aqua form of cisplatin, cis-[PtCl(NH3)2(H2O)]+, isolating the encounter complex of the reactant pair, {cis-[PtCl(NH3)2(H2O)]+·Met}, by electrospray ionization. In the unsolvated state, charged intermediates are characterized for their structure and photofragmentation behavior by IR ion spectroscopy combined with quantum-chemical calculations, obtaining an outline of the cisplatin-methionine reaction at a molecular level. To summarize the major findings: (i) the {cis-[PtCl(NH3)2(H2O)]+·Met} encounter complex, lying on the reaction coordinate of the Eigen-Wilkins preassociation mechanism for ligand substitution, is delivered in the gas phase and characterized by IR ion spectroscopy; (ii) upon vibrational excitation, ligand exchange occurs within {cis-[PtCl(NH3)2(H2O)]+·Met}, releasing water and cis-[PtCl(NH3)2(Met)]+, along the calculated energy profile; (iii) activated cis-[PtCl(NH3)2(Met)]+ ions undergo NH3 departure, forming a chelate complex, [PtCl(NH3)(Met)]+, whose structure is congruent with overwhelming S-Met ligation as the primary coordination step. The latter process involving ammonia loss marks a difference with the prevailing chloride replacement in protic solvent, pointing to the effect of a low-polarity environment.

Design and Synthesis of Piperazine-Based Compounds Conjugated to Humanized Ferritin as Delivery System of siRNA in Cancer Cells.

Gene expression regulation by small interfering RNA (siRNA) holds promise in treating a wide range of diseases through selective gene silencing. However, successful clinical application of nucleic acid-based therapy requires novel delivery options. Herein, to achieve efficient delivery of negatively charged siRNA duplexes, the internal cavity of "humanized" chimeric Archaeal ferritin (HumAfFt) was specifically decorated with novel cationic piperazine-based compounds (PAs). By coupling these rigid-rod-like amines with thiol-reactive reagents, chemoselective conjugation was efficiently afforded on topologically selected cysteine residues properly located inside HumAfFt. The capability of PAs-HumAfFt to host and deliver siRNA molecules through human transferrin receptor (TfR1), overexpressed in many cancer cells, was explored. These systems allowed siRNA delivery into HeLa, HepG2, and MCF-7 cancer cells with improved silencing effect on glyceraldehyde-3-phosphate dehydrogenase (GAPDH) gene expression with respect to traditional transfection methodologies and provided a promising TfR1-targeting system for multifunctional siRNA delivery to therapeutic applications.

Molecular Properties of Bare and Microhydrated Vitamin B5-Calcium Complexes.

Pantothenic acid, also called vitamin B5, is an essential nutrient involved in several metabolic pathways. It shows a characteristic preference for interacting with Ca(II) ions, which are abundant in the extracellular media and act as secondary mediators in the activation of numerous biological functions. The bare deprotonated form of pantothenic acid, [panto-H]-, its complex with Ca(II) ion, [Ca(panto-H)]+, and singly charged micro-hydrated calcium pantothenate [Ca(panto-H)(H2O)]+ adduct have been obtained in the gas phase by electrospray ionization and assayed by mass spectrometry and IR multiple photon dissociation spectroscopy in the fingerprint spectral range. Quantum chemical calculations at the B3LYP(-D3) and MP2 levels of theory were performed to simulate geometries, thermochemical data, and linear absorption spectra of low-lying isomers, allowing us to assign the experimental absorptions to particular structural motifs. Pantothenate was found to exist in the gas phase as a single isomeric form showing deprotonation on the carboxylic moiety. On the contrary, free and monohydrated calcium complexes of deprotonated pantothenic acid both present at least two isomers participating in the gas-phase population, sharing the deprotonation of pantothenate on the carboxylic group and either a fourfold or fivefold coordination with calcium, thus justifying the strong affinity of pantothenate for the metal.

Binding Motifs in the Naked Complexes of Target Amino Acids with an Excerpt of Antitumor Active Biomolecule: An Ion Vibrational Spectroscopy Assay.

The structures of proton-bound complexes of 5,7-dimethoxy-4H-chromen-4-one (1) and basic amino acids (AAs), namely, histidine (His) and lysine (Lys), have been examined by means of mass spectrometry coupled with IR ion spectroscopy and quantum chemical calculations. This selection of systems is based on the fact that 1 represents a portion of glabrescione B, a natural small molecule of promising antitumor activity, while His and Lys are protein residues lining the cavity of the alleged receptor binding site. These species are thus a model of the bioactive adduct, although clearly the isolated state of the present study bears little resemblance to the complex biological environment. A common feature of [1+AA+H]+ complexes is the presence of a protonated AA bound to neutral 1, in spite of the fact that the gas-phase basicity of 1 is comparable to those of Lys and His. The carbonyl group of 1 acts as a powerful hydrogen-bond acceptor. Within [1+AA+H]+ the side-chain substituents (imidazole group for His and terminal amino group for Lys) present comparable basic properties to those of the α-amino group, taking part to a cooperative hydrogen-bond network. Structural assignment, relying on the comparative analysis of the infrared multiple photon dissociation (IRMPD) spectrum and calculated IR spectra for the candidate geometries, derives from an examination over two frequency ranges: 900-1800 and 2900-3700 cm-1 . Information gained from the latter one proved especially valuable, for example, pointing to the contribution of species characterized by an unperturbed carboxylic OH or imidazole NH stretching mode.

Can an Elusive Platinum(III) Oxidation State be Exposed in an Isolated Complex?

Platinum(IV) complexes are extensively studied for their activity against cancer cells as potential substitutes for the widely used platinum(II) drugs. PtIV complexes are kinetically inert and need to be reduced to PtII species to play their pharmacological action, thus acting as prodrugs. The mechanism of the reduction step inside the cell is however still largely unknown. Gas-phase activation of deprotonated platinum(IV) prodrugs was found to generate products in which platinum has a formal +3 oxidation state. IR multiple photon dissociation spectroscopy is thus used to obtain structural information helping to define the nature of both the platinum atom and the ligands. In particular, comparison of calculations at DFT, MP2 and CCSD levels with experimental results demonstrates that the localization of the radical is about equally shared between the dxz orbital of platinum and the pz of nitrogen on the amino group, the latter acting as a non-innocent ligand.

A multi-methodological inquiry of the behavior of cisplatin-based Pt(IV) derivatives in the presence of bioreductants with a focus on the isolated encounter complexes.

The study of Pt(IV) antitumor prodrugs able to circumvent some drawbacks of the conventional Pt(II) chemotherapeutics is the focus of a lot of attention. This paper reports a thorough study based on experimental methods (reduction kinetics, electrochemistry, tandem mass spectrometry and IR ion spectroscopy) and quantum-mechanical DFT calculations on the reduction mechanism of cisplatin-based Pt(IV) derivatives having two hydroxido (1), one hydroxido and one acetato (2), or two acetato ligands (3) in axial position. The biological reductants glutathione and ascorbic acid were taken into consideration. The presence of a hydroxido ligand resulted to play an important role in the chemical reduction with ascorbic acid, as verified by 15N-NMR kinetic analysis using 15N-enriched complexes. The reactivity trend (1 > 2 > 3) does not reflect the respective reduction peak potentials (1 < 2 < 3), an inverse relationship already documented in similar systems. Turning to a simplified environment, the Pt(IV) complexes associated with a single reductant molecule (corresponding to the encounter complex occurring along the reaction coordinate in bimolecular reactions in solution) were characterized by IR ion spectroscopy and sampled for their reactivity under collision-induced dissociation (CID) conditions. The complexes display a comparable reduction reactivity ordering as that observed in solution. DFT calculations of the free energy pathways for the observed fragmentation reactions provide theoretical support for the CID patterns and the mechanistic hypotheses on the reduction process are corroborated by the observed reaction paths. The bulk of these data offers a clue of the intricate pathways occurring in solution.Graphic abstract.