Friday, February 26th, 2016

Brian W. Williams – Breffke, Jens; Williams, Brian W.; and Maroncelli, Mark. “The Photophysics of Three Naphthylmethylene Malononitriles.” Journal of Physical Chemistry B 119, no. 29 (2015) : 9254-9267.

Brian W. Williams, Professor of Chemistry

The,solvent dependence of the photophysical properties of three naphthylmethylene malononitriles, 1-(1-naphthalenylmethylene)-propanedinitrile (I-MENT), 2-(2-naphthalenylmethylene)-propanedinitrile (2-MN), and 2(3,4-dihydro-1-(2H)-phenathrenylidene)-propanedinitile (r2-MN), was studied in order to determine their potential utility as fluidity probes and to make comparisons to the better, studied, benzylidene malononitriles. Density functional calculations Were used to understand the possible conformational states related to, rotation, about the vinyl-aromatic bond (“tau”). Absorption and emission frequencies, extinction coefficients, fluorescence quantum yields, and, fluorescence lifetimes were measured in 11 representative solvents. Both the computational and experimental results indicate that the So -> Si transitions of these molecules have substantial charge-transfer character and produce highly polar excited states. Emission appears to result from relaxed SI states which do not differ qualitatively from the Franck-Condon states reached by absorption. In 2-MN, time resolved emission reveals the presence of two ground-state conformers (“a” and “b” differing by similar to 180 degrees rotation, about tau) coexisting in low-polarity solvents. In contrast, 1-MN appears to exist primarily as a single dominant ground-state conformer. Fluorescence lifetimes vary from ps In 1-MN to similar to 200 ps in 2-MN(a) at room temperature. With the exception of 2-MN(a), the lifetimes vary Systematically with solvent in a manner similar to what is observed in the benzylidene malononitriles. Both solvent polarity and fluidity appear to be important determinants of lifetime. The primary mechanism of fluorescence-decay in naphthylmethylene malononitriles, is likely to be the same as that of the benzylidene malononitriles twisting about the double bond in S-1 which leads to rapid internal conversion via a conical intersection with So.

Breffke, Jens; Williams, Brian W.; and Maroncelli, Mark. “The Photophysics of Three Naphthylmethylene Malononitriles.” Journal of Physical Chemistry B 119, no. 29 (2015) : 9254-9267.

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Friday, February 26th, 2016

Eric Tillman – Blackburn, Scott C. and Tillman, Eric. “Synthesis of Cyclic Poly(methyl methacrylate) Directly from Dihalogenated Linear Precursors.” Macromolecular Chemistry and Physics 216, no. 12 (2015) : 1282-1290.

Eric Tillman, Professor of Chemistry

Cyclic poly(methyl methacrylate) (PMMA) is prepared by intramolecular radical trap-assisted atom transfer radical coupling (RTA-ATRC) of dihalogenated PMMA precursors. The inclusion of the radical trap nitrosobenzene (NBz) in the coupling sequence affords high yields of cyclic polymers, as observed by gel permeation chromatography and confirmed by H-1 NMR and electrospray ionization mass spectra, which show the presence of the aromatic group from the NBz incorporated into the cycle. Analogous coupling reactions in the absence of the radical trap do not lead to appreciable cyclization or even intermolecular elongation, consistent with chain-end sterics preventing radical-radical coupling as the predominant termination pathway. Thermolysis of the cyclic PMMA, possible because of the labile C-O bond in the alkoxyamine linkage contained in the macrocycle, causes a reversion back to the linear form and is consistent with the role of the radical trap in the coupling sequence. Differential scanning calorimetry is also used to compare cyclic PMMA with its linear analog, with a marked increase in glass transition temperatures found after cyclization.

Blackburn, Scott C. and Tillman, Eric. “Synthesis of Cyclic Poly(methyl methacrylate) Directly from Dihalogenated Linear Precursors.” Macromolecular Chemistry and Physics 216, no. 12 (2015) : 1282-1290.

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Friday, February 26th, 2016

Eric Tillman – Blackburn, Scott C.; Myers, Kenneth D.; and Tillman, Eric. “Macrocyclic Poly(Methyl Acrylate) and Macrocyclic Poly(Methyl Acrylate-Block-Styrene) Synthesized by Radical Trap-Assisted Atom Transfer Radical Coupling.” Polymer 68, (2015) : 284-292.

Eric Tillman, Professor of Chemistry

Dibrominated poly(methyl acrylate) (BrPMABr) was prepared by atom transfer radical polymerization (ATRP), and this unpurified reaction mixture was used directly in a radical trap-assisted atom transfer radical coupling (RTA-ATRC) reaction to achieve cyclic PMA by intramolecular chain-end coupling. When the polymerization-to-cyclization crossover was performed as a one pot-two step sequence simply by the addition of the radical trap 2-methyl-2-nitrosopropane (MNP) and increased amounts of ligand-catalyst complex, cyclic PMA could be formed in yields up to 65% by adjusting the total volume and reactant ratios during the intramolecular RTA-ATRC, based on gel permeation chromatography (GPC) analysis. In analogous intramolecular ATRC trials lacking the radical trap, cyclization was not successful and instead chain extension by continuation of ATRP was observed. When the ATRP reaction mixture containing the BrPMABr precursor was added drop-wise into a redox active solution containing MNP, yields of cyclic polymer were increased up to 85%. The effect of the ligand was studied, with the highest amounts of cyclic product obtained in intramolecular RTA-ATRC reactions using tris[2-(dimethylamino) ethyl]amine (Me6TREN) as the ligand. Cyclic diblock copolymers were also prepared by this method, using telechelic poly(methyl acrylate-block-styrene-block-methyl aciylate) (PMA-b-PS-b-PMA) linear precursors prepared by sequential ATRP reactions. (C) 2015 Elsevier Ltd. All rights reserved.

Blackburn, Scott C.; Myers, Kenneth D.; and Tillman, Eric. “Macrocyclic Poly(Methyl Acrylate) and Macrocyclic Poly(Methyl Acrylate-Block-Styrene) Synthesized by Radical Trap-Assisted Atom Transfer Radical Coupling.” Polymer 68, (2015) : 284-292.

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Friday, February 26th, 2016

Berhane Temelso – Swan, James; Findeis, Peter M.; Hilton, Sheena; Lebold, Kathryn M.; Temelso, Berhane; and Shields, George C. “Formation of Deprotonated 2-Imidazoline-4(5)-One Product Ions in the Collision-Induced Dissociation of Some Serine-Containing Dipeptides.” International Journal of Mass Spectrometry 381, (2015) : 25-32.

Berhane Temelso, Research Faculty Associate

A deprotonated 2-imidazoline-4(5)-one product ion was observed as a major fragment in the collision-induced dissociation (CID) of several dipeptides containing serine at the C-terminal and an amino acid with an alkyl substituent at the N-terminal. This fragment becomes predominant at high collision energies. The same type of product ion was seen in the CID of cyclo(GlyGly). Labeling GlySer with O-18 suggests that the fragmentation may proceed through a symmetrical intermediate such as a deprotonated diketopiperazine. Density functional theory calculations of GlySer provided a possible mechanism for the fragmentation. (C) 2015 Elsevier B.V. All rights’ reserved.

Swan, James; Findeis, Peter M.; Hilton, Sheena; Lebold, Kathryn M.; Temelso, Berhane; and Shields, George C. “Formation of Deprotonated 2-Imidazoline-4(5)-One Product Ions in the Collision-Induced Dissociation of Some Serine-Containing Dipeptides.” International Journal of Mass Spectrometry 381, (2015) : 25-32.

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Friday, February 26th, 2016

William D. Kerber – Kerber, William D.; Goheen, Joshua T.; Perez, Kaitlyn A.; and Siegler, Maxime A. “Enhanced Stability of the Fe(II)/Mn(II) State in a Synthetic Model of Heterobimetallic Cofactor Assembly.” Inorganic Chemistry 55, no. 2 (2016) : 848-857.

William D. Kerber, Assistant Professor of Chemistry

Heterobimetallic Mn/Fe cofactors are found in the R2 subunit of class Ic ribonucleotide reductases (R2c) and R2-like ligand binding oxidases (R2lox). Selective cofactor assembly is due at least in part to the thermodynamics of M(II) binding to the apoprotein. We report here equilibrium studies of Fe(II)/Mn(II) discrimination in the biomimetic model system H5(F-HXTA) (5-fluoro-2-hydroxy-1,3-xylene-alpha,alpha’-diamine-N,N,N’,N’-tetraacetic acid). The homobimetallic F-HXTA complexes [Fe(H2O)6][1]2·14H2O and [Mn(H2O)6][2]2·14H2O (1 = [Fe(II)2(F-HXTA)(H2O)4](-); 2 = [Mn(II)2(F-HXTA)(H2O)4](-)) were characterized by single crystal X-ray diffraction. NMR data show that 1 retains its structure in solution (2 is NMR silent). Metal exchange is facile, and the heterobimetallic complex [Fe(II)Mn(II)(F-HXTA)(H2O)4](-) (3) is formed from mixtures of 1 and 2. (19)F NMR was used to quantify 1 and 3 in the presence of excess M(II)(aq) at various metal ratios, and equilibrium constants for Fe(II)/Mn(II) discrimination were calculated from these data. Fe(II) is preferred over Mn(II) with K1 = 182 ± 13 for complete replacement (2 ⇌ 1). This relatively modest preference is attributed to a hard-soft acid-base mismatch between the divalent cations and the polycarboxylate ligand. The stepwise constants for replacement are K2 = 20.1 ± 1.3 (2 ⇌ 3) and K3 = 9.1 ± 1.1 (3 ⇌ 1). K2 > K3 demonstrates enhanced stability of the heterobimetallic state beyond what is expected for simple Mn(II) Fe(II) replacement. The relevance to Fe(II)/Mn(II) discrimination in R2c and R2lox proteins is discussed.

Kerber, William D.; Goheen, Joshua T.; Perez, Kaitlyn A.; and Siegler, Maxime A. “Enhanced Stability of the Fe(II)/Mn(II) State in a Synthetic Model of Heterobimetallic Cofactor Assembly.” Inorganic Chemistry 55, no. 2 (2016) : 848-857.

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Friday, February 26th, 2016

Peter M. Findeis – Swan, James; Findeis, Peter M.; Hilton, Sheena; Lebold, Kathryn M.; Temelso, Berhane; and Shields, George C. “Formation of Deprotonated 2-Imidazoline-4(5)-One Product Ions in the Collision-Induced Dissociation of Some Serine-Containing Dipeptides.” International Journal of Mass Spectrometry 381, (2015) : 25-32.

Peter M. Findeis, Senior Mass Spectrometry & Instrumentation Specialist

A deprotonated 2-imidazoline-4(5)-one product ion was observed as a major fragment in the collision-induced dissociation (CID) of several dipeptides containing serine at the C-terminal and an amino acid with an alkyl substituent at the N-terminal. This fragment becomes predominant at high collision energies. The same type of product ion was seen in the CID of cyclo(GlyGly). Labeling GlySer with O-18 suggests that the fragmentation may proceed through a symmetrical intermediate such as a deprotonated diketopiperazine. Density functional theory calculations of GlySer provided a possible mechanism for the fragmentation. (C) 2015 Elsevier B.V. All rights’ reserved.

Swan, James; Findeis, Peter M.; Hilton, Sheena; Lebold, Kathryn M.; Temelso, Berhane; and Shields, George C. “Formation of Deprotonated 2-Imidazoline-4(5)-One Product Ions in the Collision-Induced Dissociation of Some Serine-Containing Dipeptides.” International Journal of Mass Spectrometry 381, (2015) : 25-32.

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Friday, February 26th, 2016

Karen Castle – Cecchini, Michael R. and Castle, Karen. “Vibrational Relaxation of (CO2)-C-13(v(2)) by Atomic Oxygen.” Chemical Physics Letters 638, (2015) : 149-152.

Karen Castle, Associate Professor of Chemistry

The rate coefficient for the vibrational relaxation of isotopically-pure (CO2)-C-13 by ground state oxygen atoms was measured between 239 and 358 K. 03 was photolyzed in a slow-flowing mixture of (CO2)-C-13, O-3, and Xe to simultaneously create O atoms and stimulate a temperature-jump, shifting population into the first vibrationally-excited state of (CO2)-C-13. The population redistribution was monitored via transient diode laser absorption spectroscopy and the rate coefficient was extracted from the dependence of this redistribution on quencher concentration. The results are in good agreement with previous measurements of the (CO2)-C-12-O system, but with decreased uncertainty due to experimental improvements. (C) 2015 Elsevier B.V. All rights reserved.

Cecchini, Michael R. and Castle, Karen. “Vibrational Relaxation of (CO2)-C-13(v(2)) by Atomic Oxygen.” Chemical Physics Letters 638, (2015) : 149-152.

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Friday, February 26th, 2016

Berhane Temelso – Temelso, Berhane; Renner, Carla R.; and Shields, George C. “Importance and Reliability of Small Basis Set CCSD(T) Corrections to MP2 Binding and Relative Energies of Water Clusters.” Journal of Chemical Theory and Computation 11, no. 4 (2015) : 1439-1448.

Berhane Temelso, Research Faculty Associate

MP2 describes hydrogen-bonded systems well, yet a higher-order electron correlation correction in the form of a CCSD(T) calculation is usually necessary to achieve benchmark quality energies. We evaluated the importance and reliability of small basis set CCSD(T) corrections to MP2 (delta(CCSD(T))(MP2)) both on the binding (Delta E) and relative (Delta Delta E) energies for a large number of systems including four water dimer stationary points and 57 other clusters up to undecamers, (H2O)11. By comparing the MP2 energies with CCSD(T) and the explicitly correlated MP2-F12 energies with variants of CCSD(T)-F12 using different basis sets, we were able to establish that the correction to the binding energy (Delta E) is sensitive to basis set size, especially for small double-zeta basis sets. On the other hand, the basis set sensitivity of the correction to the relative energy (Delta Delta E) within each cluster size is very small. While the delta(CCSD(T))(MP2) correction to the binding energy might vary in magnitude with basis set size, its effect on relative energy (and hence the stability of isomers) is remarkably consistent. Therefore, we recommend the inclusion of this correction to obtain the relative stability of closely spaced isomers using a double-zeta basis set with polarization and diffuse functions such as aug-cc-pVDZ.

Temelso, Berhane; Renner, Carla R.; and Shields, George C. “Importance and Reliability of Small Basis Set CCSD(T) Corrections to MP2 Binding and Relative Energies of Water Clusters.” Journal of Chemical Theory and Computation 11, no. 4 (2015) : 1439-1448.

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Friday, February 26th, 2016

James Swan – Swan, James; Findeis, Peter M.; Hilton, Sheena; Lebold, Kathryn M.; Temelso, Berhane; and Shields, George C. “Formation of Deprotonated 2-Imidazoline-4(5)-One Product Ions in the Collision-Induced Dissociation of Some Serine-Containing Dipeptides.” International Journal of Mass Spectrometry 381, (2015) : 25-32.

James Swan, Associate Professor of Chemistry

A deprotonated 2-imidazoline-4(5)-one product ion was observed as a major fragment in the collision-induced dissociation (CID) of several dipeptides containing serine at the C-terminal and an amino acid with an alkyl substituent at the N-terminal. This fragment becomes predominant at high collision energies. The same type of product ion was seen in the CID of cyclo(GlyGly). Labeling GlySer with O-18 suggests that the fragmentation may proceed through a symmetrical intermediate such as a deprotonated diketopiperazine. Density functional theory calculations of GlySer provided a possible mechanism for the fragmentation. (C) 2015 Elsevier B.V. All rights’ reserved.

Swan, James; Findeis, Peter M.; Hilton, Sheena; Lebold, Kathryn M.; Temelso, Berhane; and Shields, George C. “Formation of Deprotonated 2-Imidazoline-4(5)-One Product Ions in the Collision-Induced Dissociation of Some Serine-Containing Dipeptides.” International Journal of Mass Spectrometry 381, (2015) : 25-32.

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Friday, February 26th, 2016

David Rovnyak – Palmer, Melissa R.; Suiter, Christopher L.; Henry, Geneive E.; Rovnyak, James; Hoch, Jeffrey C.; Polenova, Tatyana; and Rovnyak, David. “Sensitivity of Nonuniform Sampling NMR.” Journal of Physical Chemistry B 119, no. 22 (2015) : 6502-6515.

David Rovnyak, Professor of Chemistry

Many information-rich multidimensional experiments in nuclear magnetic resonance spectroscopy can benefit from a signal-to-noise ratio (SNR) enhancement of up to about 2-fold if a decaying signal in an indirect dimension is sampled with nonconsecutive increments, termed nonuniform sampling (NUS). This work provides formal theoretical results and applications to resolve major questions about the scope of the NUS enhancement. First, we introduce the NUS Sensitivity Theorem in which any decreasing sampling density applied to any exponentially decaying signal always results in higher sensitivity (SNR per square root of measurement time) than uniform sampling (US). Several cases will illustrate this theorem and show that even conservative applications of NUS improve sensitivity by useful amounts. Next, we turn to a serious limitation of uniform sampling: the SNR by US decreases for extending evolution times, and thus total experimental times, beyond 1.26T(2) (T-2 = signal decay constant). Thus, SNR and resolution cannot be simultaneously improved by extending US beyond 1.26T(2). We find that NUS can eliminate this constraint, and we introduce the matched NUS SNR Theorem: an exponential sampling density matched to the signal decay always improves the SNR with additional evolution time. Though proved for a specific case, broader classes of NUS densities also improve SNR with evolution time. Applications of these theoretical results are given for a soluble plant natural product and a solid tripeptide (u-C-13,N-15-MLF). These formal results clearly demonstrate the inadequacies of applying US to decaying signals in indirect nD-NMR dimensions, supporting a broader adoption of NUS.

Palmer, Melissa R.; Suiter, Christopher L.; Henry, Geneive E.; Rovnyak, James; Hoch, Jeffrey C.; Polenova, Tatyana; and Rovnyak, David. “Sensitivity of Nonuniform Sampling NMR.” Journal of Physical Chemistry B 119, no. 22 (2015) : 6502-6515.

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Friday, February 26th, 2016

David Rovnyak – Wenrich, Broc; Sonstrom, Reilly; Gupta, Riju A.; and Rovnyak, David. “Enhanced Biosynthetically Directed Fractional Carbon-13 Enrichment of Proteins for Backbone NMR Assignments.” Protein Expression and Purification 115, (2015) : 1-10.

David Rovnyak, Professor of Chemistry

Routes to carbon-13 enrichment of bacterially expressed proteins include achieving uniform or positionally selective (e.g. ILV-Me, or (13)C’, etc.) enrichment. We consider the potential for biosynthetically directed fractional enrichment (e.g. carbon-13 incorporation in the protein less than 100%) for performing routine n-(D)dimensional NMR spectroscopy of proteins. First, we demonstrate an approach to fractional isotope addition where the initial growth media containing natural abundance glucose is replenished at induction with a small amount (e.g. 10%(w/w)u-(13)C-glucose) of enriched nutrient. The approach considered here is to add 10% (e.g. 200mg for a 2g/L culture) u-(13)C-glucose at the induction time (OD600=0.8), resulting in a protein with enhanced (13)C incorporation that gives almost the same NMR signal levels as an exact 20% (13)C sample. Second, whereas fractional enrichment is used for obtaining stereospecific methyl assignments, we find that (13)C incorporation levels no greater than 20%(w/w) yield (13)C and (13)C-(13)C spin pair incorporation sufficient to conduct typical 3D-bioNMR backbone experiments on moderate instrumentation (600MHz, RT probe). Typical 3D-bioNMR experiments of a fractionally enriched protein yield expected backbone connectivities, and did not show amino acid biases in this work, with one exception. When adding 10% u-(13)C glucose to expression media at induction, there is poor preservation of (13)Calpha-(13)Cbeta spin pairs in the amino acids ILV, leading to the absence of Cbeta signals in HNCACB spectra for ILV, a potentially useful editing effect. Enhanced fractional carbon-13 enrichment provides lower-cost routes to high throughput protein NMR studies, and makes modern protein NMR more cost-accessible.Copyright 2015 Elsevier Inc. All rights reserved.

Wenrich, Broc; Sonstrom, Reilly; Gupta, Riju A.; and Rovnyak, David. “Enhanced Biosynthetically Directed Fractional Carbon-13 Enrichment of Proteins for Backbone NMR Assignments.” Protein Expression and Purification 115, (2015) : 1-10.

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Friday, February 26th, 2016

Molly M. McGuire – McGuire, Molly M. and Herman, Ellen K. “A Novel ATR-FTIR Technique for Identifying Colloid Composition in Natural Waters.” Hydrological Processes 29, no. 6 (2015) : 1314-1323.

Molly M. McGuire, Associate Professor of Chemistry

Although understanding colloid composition has been frequently cited as essential to predicting contaminant transport in natural waters, most current methods to collect and identify colloid composition chemically alter the colloids prior to analysis and fail to identify colloid mineralogy and organic components. This paper presents a new, low-cost method employing attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) to identify colloids including organic material in concentrated suspensions. The concentration method employing tangential ultrafiltration at a steady temperature prevents redistribution of dissolved phase and suspended sediments into the colloidal fraction through post-sampling reactions. ATR-FTIR allows for direct analysis of concentrated suspensions rather than requiring drying that may alter composition in the colloidal phase, for example, by precipitating carbonates in samples from karst waters. The ability of this technique to monitor variation in colloidal composition is demonstrated through the examination of colloids under two different flow conditions in a karst aquifer and the West Branch of the Susquehanna River in Central Pennsylvania. Copyright (c) 2014 John Wiley & Sons, Ltd.

McGuire, Molly M. and Herman, Ellen K. “A Novel ATR-FTIR Technique for Identifying Colloid Composition in Natural Waters.” Hydrological Processes 29, no. 6 (2015) : 1314-1323.

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