Despite the constraints of the Boltzmann factor, nuclear
magnetic resonance (NMR) has been enormously successful
using tiny (ppm) thermal polarizations to generate the
signal. By comparison, enormous non-equilibrium nuclear-spin
polarizations (of order 10%) can be achieved in ^3He and
^129Xe via spin-exchange optical pumping, greatly
enhancing the NMR sensitivity of these nuclei. These
hyperpolarized (HP) noble gases are being applied to a broad
range of problems in physics, chemistry, biology, and even
medicine. Perhaps the most dramatic example is magnetic
resonance imaging (MRI) of the air spaces of the lung, a
notoriously difficult organ to image conventionally. This
lecture will address the physics of optical pumping and spin
exchange, the application to lung MRI, including some recent
^3He lung-imaging results, and one particular aspect of
HP-gas physics that has concerned us most recently: the
interaction of ^3He nuclei with surfaces. The
understanding of surface interactions is crucial for
efficient production and handling of HP gases for
applications such as MRI, since these interactions cause the
nuclei to relax back to thermal equilibrium, destroying
their NMR sensitivity. For example, we recently discovered
that ferromagnetic sites at or near the glass surface of
^3He spin-exchange cells play a key role in surface
relaxation. These sites produce hysteresis in the measured
longitudinal spin relaxation time T_1 as a function of the
cell’s history of exposure to magnetic fields. In addition
to implications for HP-gas production, the exquisite
sensitivity of T_1 to the changing magnetic moments of the
sites suggests the use of ^3He as an inert probe of
surface magnetism.
[F1.002] Protein Flexibilty and Folding
Michael Thorpe (Arizona State University)
In this talk we apply a novel approach to the exploration of energy landscapes of macromolecules and proteins that uses constraint theory. Constraints fix the bond lengths and bond angles and allow the use of theorems from graph theory to perform a rigid region decomposition of the network of atoms, which identifies the rigid regions, the flexible joints between them and also the stressed regions. We will show movies of the diffusive motion of various proteins.
The protein unfolding transition is an example of a rigid to floppy transition and is shown to be more first order than second order because of the self-organized nature of the cross-linked polypeptide chain in the native protein. This approach emphasizes the universality in protein unfolding and allows the folding core and the transition state to be identified.
Useful reference are: M.F. Thorpe, Ming Lei, A.J. Rader, Donald J. Jacobs and Leslie A. Kuhn Protein Flexibility Predictions using Graph Theory, Proteins 44, 150 - 165, (2001).
A. J. Rader, Brandon M. Hespenheide, Leslie A. Kuhn and M. F. Thorpe Protein Unfolding: Rigidity Lost Proceedings of the National Academy of Sciences 99, 3540-3545 (2002).
More details of this work can be found via
http://physics.asu.edu/mfthorpe
[F1.003] Neutrino Masses and Mixing: An Overview
Steven Elliott (Los Alamos National Laboratory)
The past decade has seen a remarkable string of results in neutrino physics. The convincing evidence for neutrino oscillations in atmospheric, solar, and reactor neutrinos has indicated that neutrinos do have mass and they mix. However there is still much about the neutrino that we don’t know. In particular the absolute mass scale is still unknown. This presentation will summarize what we know about neutrino masses and mixing and motivate the directions for the future experimental efforts.