Frequency-Swept Integrated and Stretched Solid Effect Dynamic Nuclear Polarization.

We investigate a new time domain approach to dynamic nuclear polarization (DNP), the frequency-swept integrated solid effect (FS-ISE), utilizing a high power, broadband 94 GHz (3.35 T) pulse EPR spectrometer. The bandwidth of the spectrometer enabled measurement of the DNP Zeeman frequency/field profile that revealed two dominant polarization mechanisms, the expected ISE, and a recently observed mechanism, the stretched solid effect (S2E). At 94 GHz, despite the limitations in the microwave chirp pulse length (10 µs) and the repetition rate (2 kHz), we obtained signal enhancements up to ∼70 for the S2E and ∼50 for the ISE. The results successfully demonstrate the viability of the FS-ISE and S2E DNP at a frequency 10 times higher than previous studies. Our results also suggest that these approaches are candidates for implementation at higher magnetic fields.

Dawning of the N=32 Shell Closure Seen through Precision Mass Measurements of Neutron-Rich Titanium Isotopes.

A precision mass investigation of the neutron-rich titanium isotopes ^{51-55}Ti was performed at TRIUMF's Ion Trap for Atomic and Nuclear science (TITAN). The range of the measurements covers the N=32 shell closure, and the overall uncertainties of the ^{52-55}Ti mass values were significantly reduced. Our results conclusively establish the existence of the weak shell effect at N=32, narrowing down the abrupt onset of this shell closure. Our data were compared with state-of-the-art ab initio shell model calculations which, despite very successfully describing where the N=32 shell gap is strong, overpredict its strength and extent in titanium and heavier isotones. These measurements also represent the first scientific results of TITAN using the newly commissioned multiple-reflection time-of-flight mass spectrometer, substantiated by independent measurements from TITAN's Penning trap mass spectrometer.

The use of the Rx spin label in orientation measurement on proteins, by EPR.

The bipedal spin label Rx is more restricted in its conformation and dynamics than its monopodal counterpart R1. To systematically investigate the utility of the Rx label, we have attempted to comprehensively survey the attachment of Rx to protein secondary structures. We have examined the formation, structure and dynamics of the spin label in relation to the underlying protein in order to determine feasibility and optimum conditions for distance and orientation measurement by pulsed EPR. The labeled proteins have been studied using molecular dynamics, CW EPR, pulsed EPR distance measurement at X-band and orientation measurement at W-band. The utility of different modes and positions of attachment have been compared and contrasted.

NMR and molecular orbital studies of isomerism and tautomerism in oximes of 2-acyl cyclic 1,3-diones.

Molecular orbital calculations and 13C and 15N NMR experiments have been performed on the O-methyl oximes of two types of 2-acyl cyclic 1,3-diones. One (III; X = CH2) was based on cyclohexane, and the other (X = O) on pyran. The data indicate that both the cyclohexane and pyran compounds prefer to exist as the oxime, rather than the enamine isomer. Two equivalent, interconverting, keto-enol tautomers exist in the cyclohexane compound. The pyran compound exists as the lactone-enol tautomer. Our results have implications in the design of herbicidal compounds and drugs containing similar tautomeric systems.