Chi-Yuan Cheng and Clifford R. Bowers, Department of Chemistry

1. Direct Observation of Atoms Entering and Exiting a Dipeptide Nanotube System: A Hyperpolarized Xenon-129 2D Exchange Study Chi-Yuan Cheng and Clifford R. Bowers, Department of Chemistry In 2000, the NHMFL IHRP funded the construction of a Rb-129Xe spin exchange gas polarizer utilizing a 210W fiber array coupled laser diode array system. A world record 129Xe polarization of 68% in an LDA based SEOP polarizer was achieved [1]. The 104 fold NMR sensitivity gain has facilitated unprecedented studies of diffusion and exchange inside a dipeptide nanotube system. The channels of of L-Alanyl-L-Valine (AV) have an inner diameter of 5.13Å, while Xe has a diameter of 4.5Å. Xenon-129 NMR reveals interactions with the channel interiors and other Xe atoms in AV nanotubes. The density of Xe inside the channels can be determined from the chemical shielding, allowing diffusion and exchange processes to be investigated as a function of the nanotube occupancy. The figure shows an exemplary hyperpolarized 129Xe 2D exchange spectrum. Cross-peaks representing atoms entering and exiting the channels are clearly visible. By acquiring such spectra as a function of the mixing time and fitting the cross-peak signals to an appropriate kinetic model, the exchange rate has been measured. This study demonstrates how hyperpolarized 129Xe NMR can be applied to the investigation of gas exchange dynamics and diffusion in nanotubes, and should serve as a starting point for future hyperpolarized NMR studies of adsorption, diffusion and exchange processes in nanotubes and other nanoporous materials. C.-Y. Cheng and C.R. Bowers, Direct observation of atoms entering and exiting AV nanotubes: a hyperpolarized 129Xe study, JACS, accepted for publication (2007); C.-Y. Cheng and C.R. Bowers, Observation of single file diffusion in a dipeptide nanotube system by hyperpolarized 129Xe NMR, ChemPhysChem, in press (2007).

2. Cross-peak enhancement by interruption of flow in 2D EXSY NMR Schematic of the NHMFL-UF continuous-flow hyperpolarized 129Xe generator Experimental layout for continuous flow and interrupted flow hyperpolarized 129Xe NMR. Flow is controlled by a solenoid valve (SN) at the outlet of the sample holder. Two three-way valves (V) allow isolation of the optical pumping system from the sample holder. (P: pressure gauge, OT: O2 trap, GP: gas purifier, RP: re-circulation pump.) 3D representation of HP 129Xe2D-EXSY spectra of Xe in AV at -10oC, acquired in (a) IF mode and (b) CF mode, each with a mixing time of tm=1s. The gas peak in (a) was truncated to facilitate comparison of the cross-peaks in each spectrum.