Robert W. Holman

A molecular dynamics investigation of the structural characteristics of amorphous and annealed poly(vinylidene fluoride) and vinylidene fluoride-trifluoroethylene copolymers

Abstract Molecular dynamics (MD) computations predict that the extent of gauche-character for poly(vinylidene fluoride) (PVDF) is greater than that of its 50 mol% copolymer with trifluoroethylene, at both high temperatures and after annealing, PVDF is enriched in gauche-conformations upon cooling whereas the copolymer is enriched in trans-conformations. The MD results are consistent with the experimental trends observed for these polymers.

Bicyclobutonium ion versus cyclopropylcarbinyl cation chemistry: the cycloaddition reaction with ethene in the dilute gas phase

The C4H7+ cations were formed from cyclopropylcarbinyl, cyclobutyl and homoallyl chlorides via chloride atom loss from the radical cations in a chemical ionization source. Their structures were probed on a short time-scale (of the order of 10−6 s) as a function of internal energy by utilizing collisionally activated dissociation with tandem mass spectrometric methods. The results show that the radical cations of cyclopropylcarbinyl and homoallyl chlorides generate primarily the cyclopropylcarbinyl cation, 3, whereas the radical cation of cyclobutyl chloride generates a substantial amount of bicyclobutonium ion, 4. Ion 4 reacts with nucleophiles via multiple competing reaction pathways, unlike 3. Ion 3 undergoes a cycloaddition reaction with ethene to generate the cyclopentylcarbinyl cation. Ion 4, although reactive with ethene, does not generate, to any appreciable degree, the cyclopentylcarbinyl cation. Copyright

The initial noncovalent binding of glucose to human hemoglobin in nonenzymatic glycation

Mechanisms for nonenzymatic protein glycation have been extensively studied albeit with an emphasis at the later stages that gives rise to advanced glycation end products. No detailed investigation of the initial, noncovalent binding of d-glucose to human hemoglobin A (HbA) exists in the literature. Although anionic molecules 2,3-bisphosphoglycerate (BPG), inorganic phosphate (Pi) and HCO3(-) have been implicated in the latter stages of glycation, their involvement at the initial binding of glucose to HbA has not yet been assessed. Results from this computational study involving crystal structures of HbA predict that the transient, ring-opened glucose isomer, assumed to be critical in the later stages of glycation, is not directly involved in initial binding to the β-chain of HbA. All the five structures of glucose generated upon mutorotation will undergo reversible, competitive and slow binding at multiple amino acid residues. The ring-opened structure is most likely generated from previously bound pyranoses that undergo mutarotation while bound. BPG, Pi and HCO3(-) also reversibly bind to HbA with similar energies as glucose isomers (~3-5 kcal/mol) and share common binding sites with glucose isomers. However, there was modest amino acid residue selectivity for binding of certain anionic molecules (1-3 regions) but limited selectivity for glucose structures (≥ 7 regions). The clinical difference between average blood glucose and predicted HbA1c, and the presence of unstable HbA-glucose complexes may be more fully explained by initial noncovalent binding interactions and different concentrations of BPG, Pi and HCO3(-) in serum vs. erythrocytes.

A molecular mechanics and crystal packing study of the effects of fluorine content in poly(vinylidene fluoride) and vinylidene fluoride-trifluoroethylene copolymers

The tgtg′ conformation of poly(vinylidene fluoride) (PVDF) is ∼0.5–0.7 kcal mol−1 monomer−1 more stable than the all-trans conformation as calculated by molecular mechanics. For vinylidene fluoride-trifluoroethylene copolymer compositions

Calculations of the elastic compliance and piezoelectric constants of poly(vinylidene fluoride) and poly(vinylidene fluoride-trifluoroethylene) crystals

Abstract We present calculated compliance and piezoelectric constants for polyvinylidene fluoride (PVDF) and its copolymer with trifluoroethylene P(VDF/TrFE). The constants were derived by minimizing the energy of the unit cell under various states of stress, using a modified molecular mechanics force field. The model was first validated against experimental data and theoretical calculations for the properties of PVDF. The behavior of the compliance and piezoelectric constants of P(VDF/TrFE) are examined as a function of the molecular percentage of VDF. It was found that an optimum value for the dielectric constant d 33 of P(VDF-TrFE) exists between 55 and 65 mol % VDF.

Gas-phase electrophilic aromatic substitution of electron-rich and electron-deficient aromatic compounds

The first step in gas-phase electrophilic aromatic substitution (EAS) reactions of alkyl carbenium ions with fluoro-, chloro- and bromo-benzene, furan, thiophene and pyrrole, has been investigated by using high-pressure, chemical-ionization and tandem mass spectrometry (MS/MS). Collisionally activated dissociation (CAD) and tandem mass spectrometry, MS/MS, have been utilized for ion-structure determination. The coexistence of σ and π complexes in these reactions is a means of rationalizing the results. It has been established that the extent of π-complex formation decreases with an increased capability of the aromatic to donate electron density. Semi-empirical computations indicate that as π-electron density within the aromatic moiety increases, the energy difference between the more stable σ complex and the π complex increases, which is also consistent with experimental observations. The extent of bonding between the alkyl group and the aromatic of certain π complexes is sufficiently weak that isomerization of the alkyl moiety takes place.

A Perspective on Reagent Diversity and Non-covalent Binding of Reactive Carbonyl Species (RCS) and Effector Reagents in Non-enzymatic Glycation (NEG): Mechanistic Considerations and Implications for Future Research

This perspective focuses on illustrating the underappreciated connections between reactive carbonyl species (RCS), initial binding in the nonenzymatic glycation (NEG) process, and nonenzymatic covalent protein modification (here termed NECPM). While glucose is the central species involved in NEG, recent studies indicate that the initially-bound glucose species in the NEG of human hemoglobin (HbA) and human serum albumin (HSA) are non-RCS ring-closed isomers. The ring-opened glucose, an RCS structure that reacts in the NEG process, is most likely generated from previously-bound ring-closed isomers undergoing concerted acid/base reactions while bound to protein. The generation of the glucose RCS can involve concomitantly-bound physiological species (e.g., inorganic phosphate, water, etc.); here termed effector reagents. Extant NEG schemes do not account for these recent findings. In addition, effector reagent reactions with glucose in the serum and erythrocyte cytosol can generate RCS (e.g., glyoxal, glyceraldehyde, etc.). Recent research has shown that these RCS covalently modify proteins in vivo via NECPM mechanisms. A general scheme that reflects both the reagent and mechanistic diversity that can lead to NEG and NECPM is presented here. A perspective that accounts for the relationships between RCS, NEG, and NECPM can facilitate the understanding of site selectivity, may help explain overall glycation rates, and may have implications for the clinical assessment/control of diabetes mellitus. In view of this perspective, concentrations of ribose, fructose, Pi, bicarbonate, counter ions, and the resulting RCS generated within intracellular and extracellular compartments may be of importance and of clinical relevance. Future research is also proposed.

An investigation of gaseous α-halogenated carbocations and isomeric halonium, halenium, and allylhalonium ions

We investigated with tandem mass spectrometric methods (MS/MS) the nature and extent of stabilization of gas-phase alkyl, vinyl and 2-allyl carbenium ions caused by halogen participation of neighboring chlorine and bromine atoms. The extent of halogen atom stabilization is greatest for alkyl ions, followed closely by that for vinyl ions, and is significantly less for the 2-halosubstituted allyl ions. The data is consistent with bridged halonium ion formation in alkyl systems and bridged halenium ion formation in vinyl systems. Our results for the 2-chloro allyl system are in accord with an earlier NMR interpretation rather than with recent theory, indicating that a bridged allyl halonium ion species is involved.

Cyclopropane as a propagating reagent in gas-phase radical cation oligomerization

Abstract Fourier transform (FTMS), collisionally activated dissociation, and tandem mass spectrometry, were utilized for investigating gas-phase radical-cation initiated reactions where neutral cyclopropane functions as the propagating reagent. When cyclopropane is reacted in a FTMS trap with the radical cations of ethylene, propylene, and cyclopropane as initiators, the cyclopropane propagating species undergoes successive reactions that proceed by the addition of three carbons followed by the rapid expulsion of ethylene, resulting in the sequential addition of a methylene unit. The mechanism of these successive addition reactions, whereas potentially being either radical- or cation-based, is consistent with a cationic addition processes. The resulting radical cations that now contain an additional methylene unit addition undergo extensive isomerization. The isomerized species may react further with the cyclopropane propagating reagent to yield higher-order oligomeric radical cations. With the cyclopropane radical cation as the initiator, neutral cyclopropane adds, in successive reactions, three methylene units resulting in a mixture of C 6 H 12 ·+ ions, the highest-order oligomeric products observed. These ions do not react further with cyclopropane, hence causing the oligomerization process to stop.

Calculations of the elastic compliance and piezoelectric constants of PVDF and P(VDF‐TrFE) crystals using molecular mechanics

The selection and/or design of projector materials for ultrasonic transducers requires knowledge of their elastic, piezoelectric, and dielectric properties. These properties can be measured directly; however, in order to minimize the fabrication and measurement of many different materials, it would be useful to compute these properties from the chemical composition and structure of the material. In molecular mechanics, a force field is used to describe short‐ and long‐range interactions between atoms in a crystal. The most favored state occurs when the total energy, i.e., the sum of the interaction energies, is minimized. This paper presents calculated compliance and piezoelectric constants for β‐phase crystals of polyvinylidene fluoride (PVDF) and its copolymer with trifluoroethylene, P(VDF/TrFE). The constants were derived by minimizing the energy of the unit cell while varying the stress. The model was validated using experimental and theoretical values for the properties of PVDF. The behavior of the c...