Chiral recognition of d- and l-amino acids is achieved in the gas phase on the basis of the kinetics of competitive fragmentations of trimeric Cu(II)-bound complexes. The singly charged copper(II)-amino acid trimeric cluster ions [A(2)BCu(II) - H](+) dissociate to form [A(2)Cu(II) - H](+) and [ABCu(II) - H](+) upon collision-induced dissociation (CID) in a quadrupole ion trap. The abundance ratios of these fragments depend strongly on the stereochemistry of the ligands in the [A(2)BCu(II) - H](+) complex ion. The kinetic method was used to calculate relative Cu ion affinities (ΔCu(II)') for homo- and heterochiral copper(II)-bound dimeric cluster ions as the indicator of chiral discrimination. Six amino acids of four different types showed chiral distinctions which ranged from 0 to 6.5 kJ/mol in terms of values of ΔCu(II)' with abundance ratios, referenced to the other enantiomer, ranging from 1 to 9.2. Amino acids with aromatic substituents displayed the largest chiral distinction, which correlates well with reported chromatographic results. The methodology presented here provides a sensitive means to study enantiomers by mass spectrometry, and initial results show that it is applicable to measurement of enantiomeric excess.

Summary Single particle cryo-EM typically discards close-packed particle images as unusable data. Here, we report an image processing strategy and case study of obtaining near-atomic resolution 3-D reconstructions from close-packed particles. Multiple independent de novo initial models were constructed to determine and cross-validate the particle parameters. The particles with consistent views were further refined including not only Euler angles and center positions but also defocus, astigmatism, beam tilt, overall and anisotropic magnification. We demonstrated this strategy with a 2.9 Å resolution reconstruction of a 1.67 MDa virus-like particle of a circovirus, PCV2, recorded on 86 photographic films. The map resolution was further validated with phase randomization test, local resolution assessment, and the atomic model was validated with MolProbility and EMRinger. Close-packed virus particles were thus shown here not only useful for high-resolution 3-D reconstructions but also allow data collection at significantly improved throughput for near-atomic resolution reconstructions.

Urea undergoes O-protonation in the gas phase to yield a product that is thermodynamically more stable than the N-protonated isomer, as also is the case in aqueous solution. The proton affinity and gas-phase basicity of urea, determined by using the kinetic method, are 873.5 ± 5.0 and 841.6 ± 5.0 kJ/mol, respectively. These values are in excellent agreement with G2(MP2) calculations, which give PA = 872.4 kJ/mol. The entropy requirements for the competitive dissociation channels of the proton-bound heterodimers of urea and the chosen reference compounds are measured and lead to the conclusion that urea and these references have almost equal protonation entropies (Δ(ΔS°) = 0.80 J/kmol). In comparison with proton affinities of acetone and acetamide, the proton affinity of urea is understandably enhanced by resonance stabilization in both neutral and protonated urea, and an upper limit to the PA value is established by a resonance saturation effect. These considerations provide a basis for the explanation of some aspects of the reactivity of urea.

Efficiency and structural specificity earmark the reaction of phosphonium ions 1 with cyclic acetals and ketals to yield 1,3,2-dioxaphospholanium ions 2 [Eq. (1)]. Potential applications of this reaction are in monitoring trace levels of organophosphorus esters and in developing novel carbonyl deprotection agents. R=OCH , CH ; R =H, CH ; R =CH , C H ; R =H, CH .

Tricoordinated cyclic boron cations result from gas-phase ion/molecule reactions of dicoordinated borinium ions with neutral acetals and ketals and thiazolidine. The reaction, which proceeds via initial cationic binding to a heteroatom followed by a consecutive ring-opening and ring-reclosing process, resembles the Eberlin transacetalization of acylium ions (Eberlin, M. N.; Cooks, R. G. Org. Mass Spectrom. 1993, 28, 679). The cyclic structure of the tricoordinated boron cation is demonstrated by tandem mass spectrometry and further confirmed by comparison with authentic cyclic tricoordinated boron cations. The five-membered cyclic boron cations dissociate by ethylene oxide loss to thus reform the reactant-dicoordinated borinium ion; the six-membered boron cations fragment instead by ethylene loss. Consistent with the proposed mechanism, the ion/molecule reaction efficiency falls in the order CH(3)OB(+)C(2)H(5) > CH(3)OB(+)OCH(3) >> CH(3)B(+)CH(3); i.e., the higher the nucleophilicity of the borinium ion, the higher the reaction efficiency. A potential energy surface is calculated for the reaction of CH(3)OB(+)OCH(3) with 2-methyl-1,3-dioxolane, and the reaction is found to be 43.3 kcal/mol exothermic due to initial formation of a strong B-O bond. The analogous reactivity displayed by phosphonium ions is also investigated by both experiment and ab initio calculations. In contrast to the borinium ions, the phosphonium ions exhibit higher regioselectivity for sulfur compared to nitrogen and oxygen. Finally, the present findings indicate that the reaction exothermicity and the regioselectivity are controlled by both the Lewis acidity of the reactant cations and the leaving ability of the released neutrals in the rate-limiting nucleophilic-induced recyclization step.