Direct Observation of Atoms Entering and Exiting a Dipeptide Nanotube System: A Hyperpolarized Xenon

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 129Xe 2D-EXSY spectra of
Xe in AV at -10oC, acquired in (a) IF mode and
(b) CF mode, each with a mixing time of tm1s.
The gas peak in (a) was truncated to facilitate
comparison of the cross-peaks in each spectrum.