Spectroscopic Discrimination of Diastereomeric Complexes Involving an Axially Chiral Receptor.

The infrared multiphoton dissociation (IRMPD) spectra of electrospray ionization (ESI)-formed proton-bound complexes between the axially chiral multifunctional macrocycle MaR and d- and l-phenylalanine (PD and PL ) or d- and l-3,4-dihydroxyphenylalanine (DD and DL ) are recorded in the ν˜ =2800-3700 cm-1 region. Whereas the diastereomeric [MaR ⋅H⋅PD ]+ and [MaR ⋅H⋅PL ]+ complexes do not show any significant spectral differences, the spectrum of [MaR ⋅H⋅DD ]+ clearly diverges from that of its [MaR ⋅H⋅DL ]+ diastereomer. A comparison of the experimental spectra with the B3LYP/6-31G**-calculated harmonic vibrational frequencies of a number of stable structures indicates the formation, in the ESI source, of very similar spectroscopically active structures, with the protonated amino acid placed outside the flat cavity of the macrocycle and hydrogen-bonded at its O2 center. Different spectral signatures of the [MaR ⋅H⋅DD ]+ and [MaR ⋅H⋅DL ]+ complexes are attributed to the coexistence of several stable rotamers in the ESI source.

Chlorine Para-Substitution of 1-Phenylethanol: Resonant Photoionization Spectroscopy and Quantum Chemical Calculations of Hydrated and Diastereomeric Complexes.

The conformational landscape of (S)-1-(4-chlorophenyl)ethanol, its monohydrated complex, and its diastereomeric adducts with R- and S-butan-2-ol, have been investigated by resonant two-photon ionization (R2PI) spectroscopy coupled with time-of-flight mass spectrometry. Theoretical calculations at the D-B3LYP/6-31++G** level of theory have been performed to assist in the interpretation of the spectra and in the assignment of the structures. The R2PI spectra and the predicted structures have been compared with those obtained on the analogous non-halogenated and fluorinated systems, i.e., (R)-1-phenylethanol and (S)-1-(4-fluorophenyl)ethanol, respectively. It appears that the presence of chlorine atom in the para position of the aromatic ring does not influence the overall geometry of bare molecule and its complexes with respect to the non-halogenated analogous systems. Anyway, it affects the electron density in the π system, and in turn the strength of OH···π and CH···π interactions. A spectral chiral discrimination is evident from the R2PI spectra of the diastereomeric adducts of (S)-1-(4-chlorophenyl)ethanol with the two enantiomers of butan-2-ol.

Boyer's Reaction and Transetherification: Mechanism and New Perspectives.

The progress and stereochemistry of Boyer's reaction were analyzed using several simple, chiral, alcoholic substrates, a variable amount of BiBr3 and different solvents. Basic solvents inhibit the reaction, while cyclohexane works very well; thus, it was our choice for the present study. In contrast to previous works, BiBr3 behaves as a true catalyst, being not consumed during the reaction. Although poisoning of the catalyst occurs to some extent, it does not prejudice the reaction yields (>90%). Gas chromatography/mass spectrometry (GC-MS) monitoring of the reaction revealed that, for example, in the presence of alcohol , isomeric ethers transetherificate to . We propose a unifying mechanistic model for both Boyer's and transetherification reactions, in which the electronic properties of n-adducts intermediates, formed by combination of bismuth(III) of BiBr3 and oxygen atoms of alcohols and ethers, play the key role for both the reactivity and the stereochemical outcome of the reaction.

Contact Ion Pairs on a Protonated Azamacrocycle: the Role of the Anion Basicity.

A potassium-containing hexaazamacrocyclic dication, [M•H•K](2+), is able to add in the gas phase mono- and dicarboxylate anions as well as inorganic anions by forming the corresponding monocharged adducts, the structure of which markedly depends on the basicity of the anion. With anions, such as acetate or fluoride, the neutral hexaazamacrocycle M acts as an acceptor of monosolvated K(+) ion. With less basic anions, such as trifluoroacetate or chloride, the protonated hexaazamacrocycle [M•H](+) performs the unusual functions of an acceptor of contact K(+)/anion pairs. Graphical Abstract ᅟ.

Structure and conformation of protonated D-(+)-biotin in the unsolvated state.

A combined computational and infrared multiphoton dissociation (IRMPD) spectroscopic investigation shows that protonated d-(+)-biotin, formed in the gas phase by ESI-MS, acquires a folded structure with proton bonding between the ureido and valeryl carbonyls, and that only a single conformer of such a structure predominates. A uniform frequency vs distance correlation function is proposed for the O(+)-H···O and N-H···O bonds involved in the folded conformers of O2'-protonated d-(+)-biotin in the gas phase which, therefore, depends exclusively on the corresponding geometric parameters.

Electronic structure and conformational flexibility of D-cycloserine.

The first comprehensive investigation of the effect of conformational flexibility of gaseous D-cycloserine on the valence and core electronic structures is reported here. The seven most stable conformers among the twelve structures calculated at the MP2/6-311++G** level of theory were assumed to properly describe the properties of the investigated compound. Taking into account the contribution of these isomers, the valence photoelectron spectrum (UPS) was simulated by the Outer Valence Green' s Function (OVGF) method. A different sensitivity towards the conformational flexibility of the outermost photoelectron bands was exhibited in the simulated spectrum. The comparison of the theoretical UPS with the experimental one allowed a detailed assignment of the outermost valence spectral region. The composition and bonding properties of the relevant MOs of the most stable conformers were analyzed in terms of leading Natural Bond Orbital (NBO) contributions to the HF/6-311++G** canonical MOs. The C1s, N1s, and O1s photoelectron spectra (XPS) were theoretically simulated by calculating the vertical Ionization Energies (IEs) of the relevant conformers using the ΔSCF approach. The different IE chemical shift spread of the XPS components associated with various conformers, which is expected to affect the experimental spectra, could be evaluated by simulated XPS, thus providing a new insight into the core electronic structure. The comparison of the theoretical results with the experimental ones unraveled that the atomic XPS components are not mixed by conformational flexibility of D-cycloserine, and that the specific vibronic structure of different spectral components should play a crucial role in determining different relative intensities and band shapes observed in the experiment.

Protonated Hexaazamacrocycles as Selective K(+) Receptors.

Protonated hexaazamacrocycle [M•H](+) is able to detect K(+) ions present at ppb level in methanolic solutions containing 10(-5) M of Na(+) ions. The high sensitivity and selectivity of [M•H](+) for K(+) is ascribed to the favorable energy balance between the K(+) ion desolvation and its coordination to the [M•H](+) macrocycle, which allows the formation of the corresponding adduct before the Coulombic explosion of the ESI-MS nanodroplets.

Role of the solvent on the stability of cycloserine under ESI-MS conditions.

The effects of methanol (M) and acetonitrile (A) on the stability of cycloserine (1) have been studied. InfraRed Multiphoton PhotoDissociation (IRMPD) spectroscopy of the ionic species from electrospray ionization tandem mass spectrometry (ESI-MS) of 1/M and 1/A solutions points to extensive dimerization of 1 to cis-3,6-bis(aminooxymethyl)-2,5-piperidinedione (2), while the same process is not observed in the ESI-MS of 1/M solutions. 1D and 2D nuclear magnetic resonance experiments confirmed these findings by showing that partial dimerization of 1 actually takes place at room temperature in acetonitrile even before ESI-MS analysis. Comparison of nuclear magnetic resonance and IRMPD spectroscopic data from the same 1/A solution suggests that dimerization of cycloserine is enhanced in the ESI source.

Isomerism of Cycloserine and Its Protonated Form.

A comprehensive ab initio investigation has been performed on the structure and stability of the isomers of cycloserine and its protonated forms in the unsolvated state. Many conformers of cycloserine in the ketonic (K), enolic (E4 and E2 ), and zwitterionic (Z7 and Z2 ) forms have been characterized. Enols E2 are only a few kilocalories per mole less stable than the K isomers. Enols E4 , as well as Z7 and Z2 zwitterions, are several tens of kilocalories per mole less stable than K. All the above isomeric structures exhibit pronounced isoxazolidine ring puckering, which generates very rich conformeric landscapes. The relative stability of the conformers of K, E2 , and E4 responds essentially to a complex balance between the attractive and repulsive electrostatic interactions among their functional groups. The preferred site of protonation of cycloserine in the gas phase has been also investigated computationally and experimentally by IR multiphoton dissociation (IRMPD) spectroscopy. The most basic center of cycloserine is the N(7) nitrogen atom (proton affinity (PA)=215.3 kcal mol-1 ). Another important basic site is the O(6) oxygen atom (PA=213.0 kcal mol-1 ). Their most populated conformers have been identified by IRMPD spectroscopy. Their predominance responds to the electrostatic interactions among the functional groups of the protonated molecule.

The effect of fluorine substitution on chiral recognition: interplay of CH···π, OH···π and CH···F interactions in gas-phase complexes of 1-aryl-1-ethanol with butan-2-ol.

The molecular diastereomeric complexes between R-1-phenyl-1-ethanol, S-1-(4-fluorophenyl)ethanol and S-1-(2-fluorophenyl)ethanol and R and S-butan-2-ol, isolated under molecular beam conditions in the gas phase, have been investigated by mass-selective resonant two-photon ionization (R2PI) and infrared depleted R2PI (IR-R2PI). The comparison of the three systems allowed us to highlight the significance of specific intermolecular interactions in the chiral discrimination process. The interpretation of the results is based on theoretical predictions mainly at the D-B3LYP/6-31++G** level of theory. The homo and heterochiral complexes are endowed with fine differences in intermolecular interactions, namely strong OH···O, and weaker CH···π, OH···π, CH···F as well as repulsive interactions. The presence of a fluorine atom in the para position of the aromatic ring does not influence the overall geometry of the complex whilst it affects the electron density in the π system and the strength of CH···π and OH···π interactions. The role and the importance of CH···F intermolecular interactions are evident in the complexes with fluorine substitution in the ortho position. While the ortho hetero complex is structurally analogous to the hetero para and non-fluorinated structures, butan-2-ol in the ortho homo adduct adopts a different conformation in order to establish a CH···F intermolecular interaction.

Conformational sensitivity in photoelectron circular dichroism of 3-methylcyclopentanone.

A study of (R)-3-methylcyclopentanone [(R)-3-MCP] by photoelectron spectroscopy and photoelectron circular dichroism (PECD) is presented. The synchrotron radiation gas-phase photoelectron spectra of (R)-3-MCP were measured and are discussed on the basis of different theoretical methodologies. The experimental dichroism of (R)-3-MCP for selected deconvoluted valence states and for the carbonyl carbon 1s core state are reported and reproduced well by calculated dispersions generated by considering the contributions of two different conformers. The theoretical dichroic parameters are calculated by employing a multicentre basis set of B-spline functions and a Kohn-Sham Hamiltonian. Temperature-dependent PECD studies of the HOMO state and the carbonyl carbon 1s core level allowed the separation of the contributions of each conformer by photoelectron dichroism. This new approach clearly shows how the PECD methodology is sensitive to conformational and structural changes of unoriented (R)-3-MCP in the gas phase, opening up new perspectives in the characterisation of chiral molecular systems.

Unexpected behavior of diastereomeric ions in the GasPhase: a stimulus for pondering on ee measurements by ESI-MS.

The most common protocols for the quantitative determination of the enantiomeric excess (ee) of raw mixtures by ESI-MS reveal inadequate in cases where the distribution of diastereomeric derivatives diverges from the ee of original solutions. This phenomenon is attributable to a matrix effect, i.e., to the stereospecific formation of high order noncovalent adducts in the ESI droplets, which alters the actual availability of the diastereomeric species under MS analysis. In this frame, the assumption of classic protocols that the ionization correction factor q is independent on the composition of the mixture submitted to analysis is questionable. An alternative methodology is presented in this paper, which is aimed at circumventing the problem by excluding any chemical derivatization of the original raw mixture. It is based on the measurement of the actual distribution of ESI-formed proton-bound diastereomeric complexes from the enantiomeric mixture through a careful analysis of their reaction kinetics with a suitable reactant.

Multifunctional Macrocyclic Receptors as Templates for Aromatic Amino Acids: A Rare Example of a Highly Selective Multi-Input Multi-Output Chemo-"Logic Gate".

Proton-bound [M⋅H⋅G]+ diastereomeric complexes between some chiral aromatic amino acids or dipeptides (G) and a chiral multifunctional macrocyclic receptor (M=Chirabite-A) undergo, in the gas phase, highly selective substitution and addition reactions by amines, such as 2-aminobutane and piperidine. All the [M⋅H⋅G]+ complexes follow time-dependent monoexponential decays. In some cases, the kinetic curves exhibit a plateau revealing the presence of unreactive [M⋅H⋅G]+ structures. In them, the amino acid is accommodated in the macrocycle cavity in the zwitterionic form by sharing its acidic hydrogen atoms with the pyridine nitrogen atoms of the host. The same interactions are structurally inaccessible to G=dipeptides or monofunctional amines, which then can be readily released from [M⋅H⋅G]+ . When the amino acid interacts with the amidocarbonyl oxygen atoms pointing outside the macrocycle cavity, it saves the canonical structure and can be readily displaced by the amine. The Chirabite-A may act as an efficient template for aromatic amino acids by releasing them or not depending upon the amino acid configuration and the basicity of the amine. These unique properties confer to the gas-phase diastereomeric [M⋅H⋅G]+ complexes the features of multi-input multi-output chemo-"logic gates".

Enantioselective supramolecular carriers for nucleoside drugs. A thermodynamic and kinetic gas phase investigation.

The enantioselective interactions between chiral tetra-amidic receptors and nucleosides have been investigated by the ESI-IT-MS and ESI-FT-ICR-MS methodologies. Configurational effects on the CID fragmentation of diastereomeric [M(H)(2)•H•A](+) aggregates (A = 2'-deoxycytidine dC, citarabine (ara-C) were found to be mostly offset by isotope effect in [S(X)(2)•H•A](+) (X = H, D) differently from the results obtained on the analogues (A = cytidine C and gemcitabine G). This result points the involvement of two different nucleoside/tetraamide isoforms. The structural differences of the [M(H)(2)•H•A](+) (A = C and G) complexes vs. the [M(H)(2)•H•A](+) (dC and ara-C) ones is fully confirmed by the kinetics of their uptake of the 2-aminobutane enantiomers, measured by FT-ICR mass spectrometry. Indeed, uptake of the 2-aminobutane enantiomers by [M(H)(n)•H•A](+) (n = 1,2; A = dC and ara-C) complexes is reversible, while that by [M(H)(n)•H•A](+) (n = 1,2; A = C and G) is not. The most encouraging result concerning the measured fragmentation and kinetic differences between C and ara-C, that are just epimers, indicates the possibility to subtly modulate the non-covalent drug/receptor interactions, through the electronic properties of the 2'-substituent on the nucleoside furanose ring, and furthermore on its three-dimensional position.

Chirality effects on the IRMPD spectra of basket resorcinarene/nucleoside complexes.

The IRMPD spectra of the ESI-formed proton-bound complexes of the R,R,R,R- and S,S,S,S-enantiomers of a bis(diamido)-bridged basket resorcin[4]arene (R and S) with cytosine (1), cytidine (2), and cytarabine (3) were measured in the region 2800-3600 cm(-1). Comparison of the IRMPD spectra with the corresponding ONIOM (B3LYP/6-31(d):UFF)-calculated absorption frequencies allowed the assessment of the vibrational modes that are responsible for the observed spectroscopic features. All of the complexes investigated, apart from [R⋅H⋅3](+), showed similar IRMPD spectra, which points to similar structural and conformational landscapes. Their IRMPD spectra agree with the formation of several isomeric structures in the ESI source, wherein the N(3)-protonated guest establishes noncovalent interactions with the host amidocarbonyl groups that are either oriented inside the host cavity or outside it between one of the bridged side-chains and the upper aromatic nucleus. The IRMPD spectrum of the [R⋅H⋅3](+) complex was clearly different from the others. This difference is attributed to the effect of intramolecular hydrogen-bonding interactions between the C(2')-OH group and the aglycone oxygen atom of the nucleosidic guest upon repulsive interactions between the same oxygen atom and the aromatic rings of the host.

Enantioselective supramolecular devices in the gas phase. Resorcin[4]arene as a model system.

This review describes the state-of-art in the field of the gas-phase reactivity of diastereomeric complexes formed between a chiral artificial receptor and a biologically active molecule. The presented experimental approach is a ligand-displacement reaction carried out in a nano ESI-FT-ICR instrument, supported by a thermodynamic MS-study and molecular-mechanics and molecular-dynamics (MM/MD) computational techniques. The noncovalent ion-molecule complexes are ideal for the study of chiral recognition in the absence of complicating solvent and counterion effects.

Cyclochiral resorcin[4]arenes as effective enantioselectors in the gas phase.

The effect of cyclochirality of rccc-2,8,14,20-tetra-n-decyl-4,10,16,22-tetra-O-methylresorcin[4]arene (C) on the enantiodiscrimination of a number of chiral bidentate and tridentate aromatic and aliphatic biomolecules (G) has been investigated by nano-electrospray ionization (nano-ESI)-Fourier transform ion cyclotron resonance mass spectrometry. The experimental approach is based on the formation of diastereomeric proton-bound [C·H·G](+) complexes by nano-ESI of solutions containing an equimolar amount of quasi-enantiomers (C) together with the chiral guest (G) and the subsequent measurement of the rate of the G substitution by the attack of several achiral and chiral amines. In general, the heterochiral complexes react faster than the homochiral ones, except when G is an aminoalcoholic neurotransmitter whose complexes, beyond that, exhibit the highest enantioselectivity. The kinetic results were further supported by both collision-induced dissociation experiments on some of the relevant [C(2) ·H·G](+) three-body species and Density functional theory (DFT) calculations performed on the most selective systems.

Unprecedented gas-phase chiroselective logic gates.

The gas-phase encounters between 2-aminobutane and proton-bound chiral resorcin[4]arene/nucleoside complexes behave in the gas phase as supramolecular "chiroselective logic gates" by releasing the nucleoside depending on the resorcin[4]arene and the 2-aminobutane configurations.

The "bridge" game: role of the fourth player in chiral recognition.

A new team player: The "three-point interaction" model, which is usually employed to rationalize chiral recognition, does not account for the amazing enantioselectivity measured for the receptors of many proteic acceptors. Gas-phase experiments have indicated that at least a fourth "player" must be considered: the rigidity that a receptor opposes to distortions of its cavity resulting from noncovalent interactions with a chiral molecule (see picture).

Chiral recognition between 1-(4-fluorophenyl)ethanol and 2-butanol: higher binding energy of homochiral complexes in the gas phase.

Diastereomeric adducts between (S)-1-(4-fluorophenyl)-ethanol and R and S 2-butanol, formed by supersonic expansion, have been investigated by means of a combination of mass selected resonant two-photon ionization-spectroscopy and infrared depletion spectroscopy. Chiral recognition is evidenced by the specific spectroscopic signatures of the S(1)← S(0) electronic transition as well as different frequencies and intensities of the OH stretch vibrational mode in the ground state. D-DFT calculations have been performed to assist in the analysis of the spectra and the determination of the structures. The homochiral and heterochiral complexes show slight structural differences, in particular in the interaction of the alkyl groups of 2-butanol with the aromatic ring. The experimental results show that the homochiral [FE(S)·B(S)] complex is more stable than the heterochiral [FE(S)·B(R)] diastereomer in both the ground and excited states. The binding energy difference has been evaluated to be greater than 0.60 kcal mol(-1).