Eric Betzig and PALM: Seeing Inside Cells Beyond the Diffraction Limit

For over a century, textbooks said that light microscopes cannot see details smaller than about 200 nanometres. That is why the inside of a living cell, full of proteins only 5-20 nm in size, looked like a blur. In 2006, Eric Betzig and Harald Hess built a microscope in a living room that did exactly what the textbooks forbade: it produced images with resolution of tens of nanometres by switching fluorescent molecules on a few at a time. The method, called photoactivated localization microscopy (PALM), earned Betzig a share of the 2014 Nobel Prize in Chemistry with Stefan Hell and William Moerner "for the development of super-resolved fluorescence microscopy". This post explains the idea in simple steps.

The Scientific Problem Before the Discovery

In 1873 Ernst Abbe showed that a lens cannot focus light to a point. A point source appears as a blurred Airy disc, because light is a wave and diffracts. Two objects closer than the resolution limit merge. For a lens with numerical aperture NA, the lateral limit is:

d = λ / (2 · NA)

For green light (λ ≈ 500 nm) and a high-quality oil objective (NA ≈ 1.4), d ≈ 180-200 nm. Many cellular structures, such as microtubules (about 25 nm wide), synaptic vesicles and nuclear pores, are well below this size. Electron microscopy offered nanometre resolution, but it required dead, fixed samples and could not label specific proteins easily. Fluorescence microscopy, in contrast, can label a chosen protein with a fluorescent tag (like green fluorescent protein, GFP, Nobel Prize in Chemistry 2008) and work with living cells, but with only 200 nm resolution.

The challenge was to keep the advantages of fluorescence while beating Abbe's limit. Two routes appeared in the 1990s and 2000s: Stefan Hell's STED, which shrinks the effective spot with a second laser, and the single-molecule approach, which Betzig pursued.

The Discovery: Step by Step

1. Single molecules can be seen. William Moerner had shown in 1989 that a single molecule can be detected optically and, in the 1990s, that single fluorescent protein molecules can be switched between bright and dark states. Betzig had earlier worked on near-field scanning microscopy and had himself imaged single molecules in 1990s, and he knew the properties of individual emitters.
2. The key idea (1995). Betzig proposed in a paper that if the molecules in a sample could be distinguished from each other by some property, such as colour, each could be located precisely, since the centre of the blur of a single molecule can be found much more accurately than the blur's width.
3. Photoactivatable proteins. In 2002, George Patterson and Jennifer Lippincott-Schwartz developed a photoactivatable form of GFP (PA-GFP), a protein that is dark until a flash of violet light switches it on. This gave Betzig the "on/off" control he needed.
4. Building the microscope (2005). Betzig and Harald Hess, who had worked together at Bell Labs, assembled a microscope in Hess's living room in La Jolla, California, using a laser, a sensitive camera and a lens, in a few weeks.
5. PALM imaging (2006). A cell was labelled with photoactivatable proteins. A weak activation pulse switched on only a sparse random subset (so that the active molecules were farther apart than 200 nm). The camera recorded their images, each a blurred spot; the positions of centres were calculated; the molecules were bleached; and the cycle was repeated thousands of times. All the positions were then plotted to build one super-resolved image. The work was published in Science in 2006 with Lippincott-Schwartz's group at the NIH. Hess independently reported the related method FPALM at about the same time, and Xiaowei Zhuang's group published STORM.

The Chemistry Behind It

PALM works because of the photochemistry of fluorescent proteins. A photoactivatable protein is initially in a non-fluorescent form. Illumination with ~405 nm light causes a chemical change in the chromophore, such as decarboxylation of a nearby glutamate or a change in the protonation state of the chromophore, and the protein becomes fluorescent when excited by another wavelength. After a few thousand photons the chromophore bleaches irreversibly. In simple terms:

Protein (dark) + hν(405 nm) → Protein* (bright form)
Protein* + hν(488–561 nm) → fluorescence (λem, typically 510–600 nm)
Protein* + continued light → photobleached (non-fluorescent)

The accuracy of locating a single molecule depends on the number of photons N detected. If s is the standard deviation of the point spread function, the localisation precision is approximately:

σ(loc) ≈ s / √N

So collecting 400 photons from a spot with s = 100 nm gives about 5 nm precision. That is the central trick: accurate localisation of isolated emitters, not a better lens. The condition is that, at any moment, the active molecules are sparse. The resolution of the final image depends on localisation precision and on the density of labelled molecules (Nyquist-type sampling), so the label must be dense enough to outline the structure.

Applications Today

• Neuroscience. PALM and related methods reveal the nanostructure of synapses and the arrangement of receptors.
• Cell biology. Researchers see organisation of cytoskeleton, focal adhesions and membrane proteins at tens of nanometre scale.
• Virology and infection. The assembly of viruses such as HIV on cell membranes can be followed.
• Disease research. Clumping of proteins in neurodegenerative diseases such as Alzheimer's and Parkinson's can be studied in cells.
• Light-sheet imaging. Betzig later developed lattice light-sheet microscopy and adaptive optics to image living cells and organisms with low light damage.

Exam Corner

• Nobel Prize in Chemistry 2014: Eric Betzig, Stefan Hell and William Moerner, for super-resolved fluorescence microscopy.
• Abbe limit: d = λ/(2 NA), about 200 nm for visible light with a high-NA objective.
• PALM uses photoactivatable fluorescent proteins; a small random subset is activated, localised and bleached in repeated cycles.
• Localisation precision ≈ s/√N, where N is the number of detected photons.
• STED (Hell) uses a depletion laser; PALM/STORM use single-molecule localisation.

Common Misconceptions

• "Super-resolution breaks the laws of physics." It does not; it avoids the limit by separating emitters in time rather than space.
• "PALM uses a special super lens." The optics are ordinary; the gain comes from photochemistry and computation.
• "Betzig alone invented super-resolution." The Nobel was shared; Hell developed STED, Moerner pioneered single-molecule spectroscopy, and others, such as Zhuang with STORM, developed related techniques.

FAQ

Why does PALM need thousands of images?

Only a few molecules are active in each frame to keep them apart; thousands of frames are needed to collect positions of all molecules.

Can PALM image living cells?

Yes, but it is slow, because many frames are needed. Newer versions and other methods improve speed.

What is a point spread function?

It is the blurred image formed by an ideal point source in the microscope. Its width sets the diffraction limit.

How is this connected to chemistry?

The method relies on photochemistry of fluorophores and on single-molecule spectroscopy, both central to physical chemistry.

Published by ABC Chemistry — IIT-JAM | CSIR-NET | GATE Chemistry Coaching. Visit abcchemistry.com