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Mass-balanced [sup.1]H/[sup.2]H isotope dipeptide tags (MBITs) are presented for simultaneous protein quantitation and identification. MB1T is derived from N-acetyl-Ala-Ala dipeptide and conjugated to primary amines of target peptides. [sup.1]H/[sup.2]H isotopes are encoded in the methyl groups of N-acetylated dipeptide: one tag deuterated on the N-acetyl group and another on the C-terminal alanine. MBIT-linked peptides comigrate in reversed-phase liquid chromatography without significant [sup.1]H/[sup.2]H isotope effects and provide 2-plex quantitation signals at 114 and 117 Th as well as peptide sequence information upon MS/ MS analysis with MALDI TOF/TOF. MBIT shows good quantitation linearity in a concentration range of 20-250 fmol. The performance of MB1T on protein quantitation and identification is further tested with yeast heat-shock protein (Hsp82p) obtained from three different physiological states. MB1T using nanogram-scale samples produces the relative abundance ratios comparable to those obtained from optical imaging of microgram-scale samples visualized with SYPRO Ruby stain. The MBIT strategy is a simple and low-cost alternative for 2-plex quantitation of proteins and offers possibilities of tuning the 2-plex signal mass window by replacing the N-terminal alanine with other amino acid residues.

Issues concerning the solvation of the Li+ ion with tetrahydrofuran, acetone and diethyl ester in the gas phase are discussed. Fourier transform ion cyclotron resonance spectrometry measurements produce absolute solvent-binding energies that can be used as reference values.

The solvation of the Li+-Br--Li+ triple ion with oxygen-donor Lewis bases was investigated in the gas phase through Fourier-transform ion cyclotron resonance spectrometry. In every step of solvation, vibrational predissociation competed with the association processes. The association-to-predissociation branching ratio varied with the salt binding energy, solvent binding energy, heat capacity, steric hindrance, and conformational change of solvent.

The spin-saturation transfer method can be used to determine the kinetics of internal rotation of N,N-Dimethylacetamide in the undergraduate physical chemistry laboratory. The experiment explores mutual-site exchange to determine the internal rotational barrier using spin-lattice relaxation processes instead of heating, allowing for a lower and narrower temperature range. The experimental procedures are included.

The formation of c-C6-SiH7+, bimolecular reactivities was examined using a Fourier transform ion cyclotron resonance spectrometry and ab initio technique. Results reveal that C6H6-SiH+ ion is not a silacycloheptateienyl cation but a C6H6-SiH+ adduct ion. The weak van der Waals interactions caused the unreactivity between hydride and chloride. This is how pi-donor complex ions modify their reactivities towards bases.

Despite intensive studies of the neutral tropyl radical, none of its structure, energetics, and vibrational modes are still clear. This system has puzzled scientists for over a decade since one vibrational mode frequency sharply varies from imaginary number 3000i cm-1 to the real number 6000 cm-1, depending on the calculation methods employed. We find that the origin of this peculiar mode is due to the pseudorotation (ωirot) involved in the interconversion of two nearly isoenergetic Jahn-Teller configurations (elongated structure 2B1 and compressed structure 2A2 with C2v symmetry). Here, we first report that this interconversion is not via D7h or C2v symmetry configuration but via Cs symmetry (i.e., by changing the C2v axis). This interconversion barrier is found negligibly small. Thus, the two conformers are considered to be not two different structures but a dynamically identical structure with partial quantum statistical distributions on the potential energy surface. Owing to the nearly barrierless pseudorotation, the overall structure in a short time scale (less than femtosecond) would be Cs-like between 2A2 and 2B1 configurations with small fluctuation of bond distances. However, the dynamical transitions between the 2B1 and 2A2 configurations via 14 different pseudorotation pathways would make the tropyl radical have the effective D7h structure in either a nonshort time scale (greater than femtosecond) or at nonlow temperatures, which explains the high temperature electron spin resonance experiments. [ABSTRACT FROM AUTHOR]

The potential energy surface for the Ar...CO van der Waals complex is calculated by the supermolecular approach using fourth-order Mo\ller–Plesset perturbation theory (MP4) with a large basis set containing bond functions. The Hartree–Fock potentials are repulsive for all configurations considered. The second-order correlation energy accounts for most of the dispersion interactions. The MP4 potential energy surface is characterized by a global minimum of -96.3 cm-1 at Re=3.743 Å and θe=98° with the argon atom closer to the oxygen end. There are no local minima in the linear configurations. The linear configurations provide shallow barriers at both of the carbon and oxygen ends. The barrier height at the oxygen end is 13.6 cm-1 at R=4.04 Å, while that at the carbon end is 28.0 cm-1 at R=4.58 Å. The rovibrational energies of Ar...CO are calculated by the discrete variable representation method. The Ar...CO complex undergoes large amplitude hindered rotations in the ground state with a zero-point energy of 21.8 cm-1. The ground state lies 7.2 cm-1 below the carbon-end barrier. The bending excited state lies 3.1 cm-1 above the carbon-end barrier, making the Ar...CO complex a nearly-free internal rotor. The calculated bending excitation frequency of 10.268 cm-1 for vCO=0 is in good agreement with the experimental value of 11.914 cm-1 for vCO=1. The A rotational constant of 2.638 cm-1 derived from the K-stack origins of the ground state is in reasonable agreement with the experimental result of 2.475 cm-1. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report singlet and triplet state splittings (ΔEST) for fluorine-substituted methylenes and silylenes using dissociation-consistent configuration interaction (CI) (based on generalized valence bond wave functions). These relatively simple CI calculations emphasize correlation consistency between the singlet and triplet states. Values of ΔEST for CH2, CF2, SiH2, and SiF2 are in excellent agreement with available experimental results, and we expect the predictions for the other cases CHF (14.5) and SiHF (41.3) to be equally accurate. This result strongly suggests that the correct choice among the experimental values for ΔEST of CHF is 14.7±0.2 kcal/mol. [ABSTRACT FROM AUTHOR]

Studies photodissociations of o-, m- and p-bromotoluene radical cations. Use of Fourier transform-ion cyclotron resonance mass spectrometry; Mechanism of the rearrangement process; Predominant formation of the benzyl cation.

Using the ab initio potential of Shin et al. (to be published), we have calculated the bound states and infrared absorption spectrum of the van der Waals complex Ar...CO. The results show that Ar...CO cannot be treated as a quasirigid rotor, nor as a molecule with a free internal rotor. In particular, a transition to the first excited van der Waals bending level is predicted to be present in the spectrum, and its frequency varies with Ω (the projection quantum number of the total angular momentum onto the intermolecular axis going from the center of mass of CO to the Ar atom). It is also shown that, although the spectrum cannot be analyzed by the use of a rigid rotor model, rotational ‘‘constants’’ can still be defined for each value of Ω. This is consistent with the available experimental data and the predicted bending excitation can account for unassigned transitions in the infrared spectrum of this complex. Finally, a sensitivity analysis of the calculated spectrum with respect to the potential anisotropy has been performed. [ABSTRACT FROM AUTHOR]