Travels of a young physicist
From basic physics to biomedicine
The Scuola Normale
Superiore in Pisa, Italy, founded by
Napoleon in 1810, is one of the world's
most exclusive institutions of higher
learning-and one of the most
demanding. Angelo Bifone, who got his
Ph.D. in physics there in 1995, calls it "a
very conservative institution whose
science programs are focused on the
fundamentals, not on applications."
Angelo Bifone made the journey from basic materials science to biomedicine. Click here for more photos.
It's an approach that has paid off in Nobel
Prizes for physicists such as Enrico Fermi
and Carlo Rubbia, as well as for some of
Italy's leading humanists. In whatever way
their success is measured, "Normalistas" are
expected to represent the very best that
Italy has to offer.
For Angelo Bifone, work on basic questions
about the properties of small clusters of
atoms, begun at the Scuola Normale,
unexpectedly led him to develop new
approaches for the diagnosis and treatment
First and most important, however, his
journey took him to the University of
California at Berkeley and the laboratories of
Alexander Pines, famed pioneer in nuclear
magnetic resonance (NMR).
Lighting up platinum in the lattice
"While I was at the Normale I read about Alex Pines using xenon NMR to
study surfaces," Bifone says. "At that time I was interested in the
properties of small metal clusters, aggregates of a few tens of metal
atoms deposited on surfaces or embedded in porous materials."
The central question was whether these tiny aggregates retained the
metallic properties of the bulk metal-for example, whether they would
retain large electron conductivity-or would behave like insulators, as
theory predicted for particles that were small enough.
"A size-induced metal/insulator transition should dramatically affect
cluster surfaces, because for small particles most of the atoms are on
the surface. Xenon NMR seemed ideal to me-because of its sensitivity
to the surface properties of materials, and because xenon is inert and would not
chemically react with the metal cluster."
When Bifone applied to finish his Ph.D. work in
Pines's lab, Pines not only agreed but also
arranged the necessary financial support. There
Bifone studied clusters of platinum atoms, each
with about 55 atoms and only a nanometer across.
By trapping the platinum inside the small pores of
a zeolite-a mineral with a regular lattice
structure-uniform cluster size would be assured.
At very low temperatures, the xenon stayed on
the surface of the metal cluster and was kept
from rapidly diffusing and smearing the NMR signal.
In this way Bifone could study highly localized
surface sites with different geometries, electronic
densities, and chemical reactivities-effectively
"lighting up" the molecular landscape with xenon
Bifone, Pines, and their colleagues observed distinct differences in the NMR signal,
depending on where the xenon atoms contacted the surface of the cluster. "An
unexpected result was that there were both metallic and nonmetallic sites on the
platinum," Bifone says, "which shows that the metal/insulator transition is not abrupt."
This discovery and other findings formed the basis of
Bifone's Ph.D. dissertation, as well as a letter published in
Physical Review Letters early in 1995. That same spring,
Alex Pines traveled to Pisa to be present for the ceremony
at which Bifone was granted his doctorate.
Pinenuts in pursuit
Meanwhile, Pines and his "Pinenuts"-the graduate
students, postdoctoral fellows, and staff members whom
he credits for much of his laboratory's success-had been
pursuing a number of other uses for hyperpolarized xenon.
Bifone, by now a seasoned Pinenut, found himself in the
thick of these discoveries.
"It appeared that the amazing sensitivity of xenon
NMR-until now applied to the study of materials-could be
used to study biological systems," says Bifone. Although other researchers had done MRI
of the lungs and brains of subjects who had inhaled hyperpolarized helium and xenon gas,
"to Alex it was clear that xenon-a proven, safe medical anesthetic-would also be
sensitive to the chemistry of biological
In collaboration with Thomas Budinger of Berkeley Lab's
Life Sciences Division, Pines, Bifone, and their teammates
studied methods of introducing laser-polarized xenon into
human blood and tissues. The catch was that xenon's
response in biological systems wanes rapidly, as its spin
polarization is damped by oxygen and other molecules.
Bifone became intrigued by the problem of introducing
laser-polarized xenon into the body in ways that would
allow it to maintain its high degree of polarization.
The team found that by dissolving the hyperpolarized gas
in fluids such as saline solution or perfluorocarbon (which
is used as a blood substitute) before introducing it into
the bloodstream, NMR signals could be detected for up to
13 seconds, several seconds longer than polarized xenon
introduced into the blood through respiration. In a
different approach, they trapped the polarized xenon
inside lipid vesicles, tiny biochemical balloons that kept
the gas from interacting with blood and could be used to
carry the xenon through the bloodstream to a targeted
organ or tissue.
"By injecting polarized xenon in solution we were able to
obtain strong NMR and MRI signals, which allowed us to
observe xenon penetrating red blood cells for the first
time," Bifone says. The achievement was reported in an article in the Proceedings of the
National Academy of Sciences late in 1996 and formed the basis of a patent application.
Before long the technique had been extended to imaging a variety of specific
physiological structures including the brain, liver, and muscles, and was soon licensed to
Nycomed Amersham, a world leader in diagnostic medical imaging. John Padfield, head of
the company's imaging division, notes that the technology "has the potential to
revolutionize the way physicians image organ function, and ultimately the way they
Possible biological applications of hyperpolarized xenon had also attracted the attention
of medical institutions, including the University of London's Institute of Cancer Research
(ICR) in England. Soon after giving a talk there on his ideas for using NMR to measure
cancerous tumors, Bifone was invited to join the staff as a lecturer.
The ICR is associated with the teaching hospital of the Royal Marsden National Health
Service Trust, and together they form one of the largest centers in the world dedicated
to translating basic research directly into patient care and treatment. One of the most
challenging problems facing cancer diagnosis and treatment is to understand the
vascularization of tumors.
"Rapidly dividing tumor cells need oxygen," Bifone explains. "They secrete blood-vessel
growth factors, which promote a form of development different from the vascularization
of normal tissues." A tangled and inefficient network of blood vessels sprouts throughout
the tumor as it grows. "One result is that some regions of the tumor are hypoxic-starved
for oxygen. Paradoxically, these regions are more resistant to radiotherapy and
chemotherapy, so it's important to identify them."
While imaging methods such as PET scans have been used to study tumor perfusion
(blood flow) and vascularization, they have limited spatial resolution and are not sensitive
to the oxygenation level of the tumors. Measuring tumor oxygenation often involves such
invasive procedures as inserting electrodes.
Hyperpolarized xenon, however, if dissolved in a medium such as perfluorocarbon, is an
excellent way of tracing the perfusion of blood through the vascular network of a tumor
using MRI. In addition, xenon NMR is highly sensitive to oxygen in the blood and tissues.
Bifone explains that "xenon doesn't detect oxygen
directly but binds to specific cavities in
hemoglobin," the protein that transports oxygen
and carbon dioxide in the blood. "Oxygen changes
the conformation of hemoglobin and reduces the
degree of xenon binding."
At ICR, Bifone and his colleagues are developing
methods to exploit xenon's sensitivity to the
presence of oxygen, both to map hypoxic areas of
tumors and for studies of oxygen concentration in
regions of the brain as well, work that has already
resulted in several publications. Their research
benefits from Bifone's continued close ties with
Out of curiosity, progress
"My work at ICR in extending the applications of hyperpolarized-xenon NMR to biomedicine
is a straightforward development of previous work with Alex," Bifone says. "I've visited
him several times, sometimes for long periods, and he's shared his ideas and methods.
He's very open."
Says Pines, "The world of science is very international; it is both competitive and
cooperative. There's a common language that asks, what does this mean? How does this
work? What are the scientific principles that dictate it? Because these questions are
universal, there is a shared enthusiasm that tends to forge friendships."
Science crosses not only international boundaries but the artificial boundaries imposed by
departments and disciplines as well-although Angelo Bifone looks at his cross-disciplinary
career so far with bemusement.
"It's amazing that xenon's physical and chemical properties make it sensitive to the
surface states of metal clusters and also make it a powerful probe for blood and tissue
oxygenation," he says. "When I went off to Pisa to be a physicist, I never imagined that
research based purely on curiosity about nature could lead to applications that might
have a direct impact on people's lives."
NMR depends on the fact that the nuclei of some atoms have a quantized property called
spin, which gives rise to a tiny magnetic moment. In a strong magnetic field, these
miniature magnets attempt to line up along the field lines-either along the applied field,
"spin up," or in the opposite direction, "spin down." Because spin-up nuclei slightly
outnumber more energetic spin-down nuclei, the net effect is a weak overall
If, says Alex Pines, "the nuclear spins are subjected to pulses of radio waves at the
frequencies corresponding to the up-versus-down energy differences, they are neither up
nor down, but in a superposition, a peculiarly quantum combination of the two basic
In classical terms, it's as if their spins cause the nuclei to precess-to wobble on a tilted
axis around the field lines. Each element has its characteristic wobble rate: the single
proton of a hydrogen nucleus precesses four times faster than a nucleus of carbon 13,
When a system of nuclear spins returns to its
equilibrium state, it reemits the radio energy it
absorbed as a detectable NMR signal. Like a telltale
fingerprint, the resonance frequency of this signal is
a giveaway signature of the chemical species of the
nuclei; it carries information about both the relative
positions of atoms and their chemical environment.
Xenon gas is particularly useful in NMR studies
because the spins of its nuclei are readily polarized
by a process called "optical pumping." When a
circularly polarized beam of laser light is shone
through a mixture of rubidium vapor and xenon, the
spin of the photons is transferred first to the
rubidium atoms and thence to the xenon atoms.
Instead of just a slight majority of spin-up nuclei, which even at the highest magnetic
fields available in the laboratory is normally only one in 100,000 (.00001 percent), in
"hyperpolarized" xenon there are as many as 20 percent or more, yielding a much
stronger NMR signal. Moreover, this spin can be transferred to atoms the xenon comes in
contact with, enhancing their NMR signals as well and leaving an indelible NMR fingerprint.