Gyemin Kwon

One-Step Peptide Backbone Dissociations in Negative-Ion Free Radical Initiated Peptide Sequencing Mass Spectrometry

Peptide dissociation behavior in TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl)-based FRIPS (free radical initiated peptide sequencing) mass spectrometry was analyzed in both positive- and negative-ion modes for a number of peptides including angiotensin II, kinetensin, glycoprotein IIb fragment (296-306), des-Pro(2)-bradykinin, and ubiquitin tryptic fragment (43-48). In the positive mode, the ·Bz-C(O)-peptide radical species was produced exclusively at the initial collisional activation of o-TEMPO-Bz-C(O)-peptides, and two consecutive applications of collisional activation were needed to observe peptide backbone fragments. In contrast, in the negative-ion mode, a single application of collisional activation to o-TEMPO-Bz-C(O)-peptides produced extensive peptide backbone fragmentations as well as ·Bz-C(O)-peptide radical species. This result indicates that the duty cycle in the TEMPO-based FRIPS mass spectrometry can be reduced by one-half in the negative-ion mode. In addition, the fragment ions observed in the negative-ion experiments were mainly of the a-, c-, x-, and z-types, indicating that radical-driven tandem mass spectrometry was mainly responsible for the TEMPO-based FRIPS even with a single application of collisional activation. Furthermore, the survival fraction analysis of o-TEMPO-Bz-C(O)-peptides was made as a function of the applied normalized collision energy (NCE). This helped us to better understand the differences in FRIPS behavior between the positive- and negative-ion modes in terms of dissociation energetics. The duty-cycle improvement made in the present study provides a cornerstone for future research aiming to achieve a single-step FRIPS in the positive-ion mode.

Three-Dimensional Monte Carlo Simulation of the Electrical Conductivity of Carbon Nanotube/Polymer Composites

Monte Carlo simulations are performed to investigate the effects of the attractive intermolecular interaction, flexibility and size dispersity of carbon nanotubes (CNTs) on the electrical conductivity of CNT/polymer composites. We show that a flexibility plays an important role in the CNT agglomeration; rigid CNTs agglomerate readily for sufficiently strong attractive intermolecular interaction, while flexible CNTs do not. This is because more flexible CNTs are less likely to connect to neighbor CNTs. For weak attractive intermolecular interactions, however, the electrical percolation threshold concentration increases with increases in the flexibility and size dispersity, thus decreasing the electrical conductivity.

Mixing molecular junctions with carbon nanotube-polymer composites: A strategy to enhance the conductivity and the transparency

The effect of carbon nanotube (CNT) molecular junctions on the percolation behavior of CNTs is studied using computer simulations. A CNT is modeled as a rigid rod comprised of hard spheres. Molecular junctions of CNTs are constructed by joining segments of CNTs. The equilibrated configurations of the mixture of CNTs and molecular junctions are obtained using Monte Carlo simulations and are used to locate the percolating clusters of connected molecules. When CNT molecular junctions are mixed with CNTs, the minimum concentration of CNTs for a finite electric conductivity is significantly decreased; thus increasing both the conductivity and the transparency.

Dynamics and mechanism of flame retardants in polymer matrixes: experiment and simulation.

We investigate the dynamics and the mechanism of flame retardants in polycarbonate matrixes to explore for a way of designing efficient and environment-friendly flame retardants. The high phosphorus content of organic phosphates has been considered as a requirement for efficient flame retardants. We show, however, that one can enhance the efficiency of flame retardants even with a relatively low phosphorus content by tuning the dynamics and the intermolecular interactions of flame retardants. This would enable one to design bulkier flame retardants that should be less volatile and less harmful in indoor environments. UL94 flammability tests indicate that even though the phosphorus content of 2,4-di-tert-butylphenyl diphenyl phosphate (DDP) is much smaller with two bulky tertiary butyl groups than that of triphenyl phosphate (TPP), DDP should be as efficient of a flame retardant as TPP, which is a widely used flame retardant. On the other hand, the 2-tert-butylphenyl diphenyl phosphate (2-tBuDP), with a lower phosphorus content than TPP but with a greater phosphorus content than DDP, is less efficient as a flame retardant than both DDP and TPP. Dynamic secondary ion mass spectrometry and molecular dynamics simulations reveal that the diffusion of DDP is slower by an order of magnitude at low temperature than that of TPP but becomes comparable to that of TPP at the ignition temperature. This implies that DDP should be much less volatile than TPP at low temperature, which is confirmed by thermogravimetric analysis. We also find from Fourier transform infrared spectroscopy that Fries rearrangement and char formation are suppressed more by DDP than by TPP. The low volatility and the suppressed char formation of DDP suggest that the enhanced flame retardancy of DDP should be attributed to its slow diffusivity at room temperature and yet sufficiently high diffusivity at high temperature.