SONICC^®

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What is SONICC Protein Crystal Detection technology?
SONICC (Second Order Nonlinear Imaging of Chiral Crystals) is an imaging method that combines SHG and UV-TPEF to detect protein crystals with high sensitivity. It eliminates background from non-crystalline material, allowing even very small or hidden crystals, such as those in LCP or peptide experiments, to be identified, where traditional brightfield or UV imaging may fail.

What imaging technologies are combined in SONICC (SHG and UV-TPEF)?
SONICC combines two powerful imaging technologies: Second Harmonic Generation (SHG) and Ultraviolet Two-Photon Excited Fluorescence (UV-TPEF).

What is SHG?
SHG stands for Second Harmonic Generation and is a nonlinear optical process. In intense electric fields (i.e., in the presence of a femtosecond laser), the distance between the electrons and the nucleus is distorted (anharmonicity), resulting in non-linear optical effects such as SHG, where the frequency of the outgoing light is twice that of the incident light (i.e., 1064 nm incident results in 532 nm exiting).

What does “chiral” mean?
A chiral molecule, or in this case a chiral crystal, is a crystal that lacks an internal plane of symmetry, and thus its mirror image is nonsuperimposable. Achiral crystals are symmetric and therefore produce SHG in equal and opposite directions that sum to a net zero signal.

Are all protein crystals detectable?
Almost all molecules that have a chiral center form a chiral crystal. Therefore, most proteins will form chiral crystals that are detectable via SONICC (Second Order Non-linear Imaging of Chiral Crystals). Over 99% of the proteins in the Protein Data Bank have a space group that is detectable with SONICC. Crystals with extremely high symmetry classes will generate less SHG signal. Although theoretically all protein crystals should be detected with SHG, that is not always experimentally the case. The higher the space group, the less SHG is generated. Also, the hyperpolarizability (i.e., SHG efficiency) of the protein significantly impacts the amount of light generated. Some proteins interact more favorably with light and therefore produce a higher SHG signal, while others hold their electrons closely, have a low hyperpolarizability, and do not give a strong, detectable SHG response.

How does SONICC detect protein crystals invisible to the human eye?
SONICC uses two-photon scattering, which suppresses background signals from randomly oriented molecules while generating a strong response from chiral molecules in an ordered crystal lattice. This enables the detection of crystals that are hidden within precipitate or are submicron in size—crystals that would otherwise remain invisible under conventional microscopy.

Can SONICC distinguish protein crystals from salt crystals?
Yes. SONICC can distinguish protein crystals from salt crystals.

Will salts produce signal?
They can if they are chiral, but the majority of salts are achiral and therefore do not generate an SHG signal.

How does SONICC improve protein crystal detection in lipidic cubic phase (LCP)?
SONICC’s extremely low detection limit makes it highly effective for LCP experiments, allowing clear visualization of small crystals even when they are buried within the turbid lipidic cubic phase.

How is SONICC different from fluorescent imaging?
Fluorescent imaging takes advantage of either the endogenous fluorescence of the protein or the use of fluorescent tags. Although fluorescence is bright and easily detectable, it is generated from solubilized and aggregated proteins as well as crystallized proteins. The background from solubilized protein decreases the signal-to-noise ratio significantly, resulting in false positives. SONICC, on the other hand, is only sensitive to crystallized proteins.

Why is SONICC better than visible or UV imaging for microcrystal detection?
SONICC outperforms visible and traditional UV imaging because it can detect submicron crystals and those buried in precipitate, which are often missed by conventional methods. Moreover, SONICC’s UV-TPEF mode uses multiphoton excitation with longer wavelengths, offering greater plate compatibility and significantly reducing protein sample damage compared to traditional UV.

How does UV-TPEF compare to traditional UV imaging?
Both imaging methods probe the amino acids present in proteins that are excited by UV light (~280 nm). However, with UV-TPEF, the incident wavelength is 532 nm instead of UV and has less energy. UV imaging can cause the breakage of disulfide bonds, but by using green light instead to excite, damage does not occur.

How small of a crystal can SONICC detect?
Theoretically, the lower limit of detection can be estimated by the forward-to-backward ratio of the SHG signal. Based on the coherence length of the generated SHG signal and the refractive index of the material, this lower limit ranges from 90 nm–300 nm in thickness. In practice, 1 μm³ crystals can be routinely detected. Two-dimensional crystals have also been routinely imaged with a signal-to-noise ratio greater than 30.

What is the spatial resolution?
Crystals can be resolved to 2 μm (detection limit is much lower).

What is the laser’s Z-resolution, and how deeply can it penetrate?
The laser focuses to a width of approximately 50 μm and can image drops greater than 3 mm tall with multiple Z-steps, called “slices.”

How fast is SONICC?
The current electronic package allows 512 × 512 image acquisition for one Z-slice in 500 ms. This corresponds to eight traces of the fast-scanning mirror per line. A one-drop 96-well plate can be imaged with visible light in 3 minutes and with SHG in 15 minutes with eight Z-slices per drop.

Will the laser damage my crystals?
Experiments show no detectable damage to protein crystals. In one experiment, a protein crystal was imaged on one half with excessive laser input. X-ray diffraction was obtained from both the exposed and unexposed halves of the crystal. Both sides diffracted to within expected resolution (~2 Å) and within statistical variation (i.e., there was no statistical difference between the diffraction of either side). SONICC has also imaged live cells with no observed impact (they remained adhered to a polylysine-coated slide).

Can I use SONICC if my sample is fluorescent?
Yes. As long as fluorescence is Stokes-shifted by 10 nm, the fluorescence will not be detected nor interfere with the SHG signal.

With which platforms is SONICC compatible?
SONICC is compatible with all optically accessible plates and seals.

Can SONICC be integrated into automated Rock Imager systems?
Yes. SONICC can be integrated with Rock Imager 1000 and can also be operated as a standalone system.

Is SONICC suitable for high-throughput protein crystallization projects?
Yes. SONICC is well-suited for high-throughput protein crystallization when integrated with the Rock Imager 1000, enabling automated imaging and storage of up to 940 plates.

How does SONICC accelerate research in Structural Biology?
SONICC accelerates research in Structural Biology by enabling the reliable detection of protein crystals that may be overlooked with visible or UV imaging. Its ability to identify microcrystals and crystals buried in precipitate speeds up the discovery process and helps researchers move more quickly toward structure determination.

What research applications benefit most from SONICC’s protein crystal detection?
Any research application requiring the detection of protein crystals at the nanometer scale can benefit from SONICC, including crystals buried under precipitates, in LCP, or within turbid environments. Key applications include nano-crystal identification for serial femtosecond crystallography (SFX) and detection of crystals in living cells.