Two new ruthenium(II) polypyridyl dimers containing the large planar aromatic bridging ligands 9,11,20,22-tetraazatetrapyrido[3,2-a:2'3'-c:3' ',2''-l:2''',3'''-n]pentacene (tatpp) and 9,11,20,22-tetraazatetrapyrido[3,2-a:2'3'-c:3'',2' '-l:2''',3'''-n]pentacene-10,21-quinone (tatpq) have been synthesized and characterized by (1)H and (13)C NMR, MALDI mass spectrometry, and elemental analyses. The electronic properties (UV-vis, redox, photophysical) of these dimers are analyzed in the context of orbital calculations (PM3 level) on the bridging ligands. A localized orbital model is proposed in which low-lying acceptor orbitals on the center portion of the ligands effectively quench the Ru(II)-based MLCT emission via a mechanism that can be viewed as intramolecular electron transfer to specific subunits of the bridges.

Condensation reactions between the enantiopure, substitutionally inert building blocks Δ-[Ru(1,10-phenanthroline-5,6-dione)3][PF6]2 (2) and Δ-[Ru(phen)2(1,10-phenanthroline-5,6-diamine)][PF6]2 (3) gave the D 3-symmetric tetramer [(Δ-(phen)2Ru(tpphz))3-Δ-Ru]8+ (ΔΔ3-1) in 68% isolated yield as the PF6 - salt (where tpphz is tetrapyrido[3,2-a:2‘,3‘-c:3‘‘,2‘‘-h:2‘‘,3‘‘-j]phenazine). Reactions between appropriate enantiomers of 2 and 3 also yielded the remaining D 3 isomers ΛΔ3-1, ΔΛ3-1, and ΛΛ3-1 which collectively represent the highest nuclearity Ru oligomers based on bidentate type ligands prepared in a stereospecific fashion to give diasteromerically and enantiomerically pure products. The complexes were purified via cation-exchange HPLC. 1H and 13C NMR, COSY and HMQC NMR, and UV−visible spectroscopy were employed to characterize these supramolecular assemblies. MALDI-TOF mass spectrometry gave parent molecular ion peaks corresponding to the species m/z 2638 [1 − 8PF6]+, 2783 [1 − 7PF6]+, 2928 [1 − 6PF6]+, 3073 [1 − 5PF6]+, 3218 [1 − 4PF6]+, and 3363 [M − 3PF6]+, proving the macromolecular structure. Circular dichroism spectroscopy was used to determine the absolute stereochemistry and optical purity of each of these stereoisomers.

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Literature values for the gas-phase basicity (GB) and proton affinity (PA) of glutamic acid range from 216 to 224 kcal/mol (GB) and 218 to 241 kcal/mol (PA). In this paper, a high-level theoretical study aimed at resolving the apparent disagreement among the experimental values is presented. Hartree−Fock, MP2, and DFT calculations with large basis sets were carried out on the neutral and protonated forms of glutamic acid. Nine protonated and 21 neutral conformers were located at the HF/3-21G and B3LYP/6-31+G** levels with full geometry optimization and characterization of stationary points. The energetics were subsequently reevaluated at the MP2(full)/6-311+G(2d,p)//B3LYP/6-31+G** level. Thermodynamic data in the harmonic approximation were obtained at the B3LYP/6-31+G** level. This data was used to estimate the gas-phase distribution of conformers at 298 K. The lowest energy structures of protonated and neutral glutamic acid both exhibit cyclic structures due to the formation of intramolecular hydrogen bonds. The calculated PA and GB are 224.4 and 214.4 kcal/mol, respectively. It is shown that, when certain empirical corrections for the entropy of cyclization are omitted and appropriate adjustments are made to thermodynamic scales, the GB and gas-phase PA values reported here are in excellent agreement with a variety of previous experimental measurements.

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The influence of polymer surface-protein binding affinity on protein ion signals in matrix-assisted laser desorption/ionization (MALDI) mass spectrometry is examined. The surfaces of poly(vinylidene fluoride) and poly(ethylene terephthalate) polymer substrates are modified by pulsed rf plasma deposition of allylamine. By varying the on/off duty cycle of the pulsed rf plasma, the polymer substrate surfaces are coated with thin films having varying densities of surface amine groups. The varying surface amine density is shown to lead to systematic changes in the surface binding affinity for the 125I-radiolabeled peptides angiotensin I and porcine insulin. Unlabeled angiotensin I and porcine insulin are then deposited on the pulsed rf plasma-modified substrates and analyzed by MALDI mass spectrometry. The experimental approach involves applying the peptide to the modified polymer surface in an aqueous phosphate-buffered saline solution and allowing the peptide solution to dry completely under ambient conditions. Subsequently, the MALDI matrix alpha-cyano-4-hydroxycinnamic acid in methanol and 10% trifluoroacetic acid in water are added to the peptide-coated modified polymer surfaces. The results of these studies demonstrate that, for the sample preparation method employed, increases in the surface peptide binding affinity lead to decreases in the peptide MALDI ion signal.

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The photoionization of (pro)(n)DHB (pro = proline, DHB = 2,5-dihydroxybenzoic acid, n = 0, 1, 2 or 4) clusters was studied both experimentally and computationally. Experimentally the (pro)(n)DHB clusters are generated in the gas phase by laser desorption and supersonic jet entrainment. The photoionization thresholds are then determined by the mass-selective measurement of both one- and two-color photoionization efficiency curves. These experiments demonstrate that the ionization energies (IEs) of the (pro)(n)DHB clusters are substantially reduced in comparison with the IE of free DHB. Computational studies of the (pro)(n)DHB clusters provide insights into the mechanism of IE reduction. For the (pro)DHB system the IE reduction results from spin delocalization in the ion state of the cluster. In contrast, for the (pro)(2)DHB and (pro)(4)DHB clusters the IE reduction results from an inductive delocalization of electron density from pro to DHB in the ground state of the cluster. This latter effect, which is a result of the specific hydrogen-bonding interactions occurring in the mixed clusters, leads to IE reductions of >1 eV. Finally, determination of the energetics of the (pro)(2)DHB radical cation demonstrate that the DHB-to-proline proton transfer reaction is a barrierless, exoergic process in the ion state and that energetic demands for cluster dissociation to protonated (pro)(2) plus a deprotonated DHB radical are substantially lower than those for cluster dissociation to (pro)(2) plus DHB(+*). Cumulatively, these studies provide new energetic and mechanistic insights into both primary and secondary MALDI ionization processes.