Engineered gold nanoparticles can be like ice cream scoops covered in chocolate sprinkles

There are many ways to interrogate molecular phenomena. You might think that this is restricted to physical measurements such as direct observation with a microscope, a laser, or more seemingly arcane observation with nuclear magnetic resonance (NMR). But I’m happy to report that observation of computer simulations is yet another, as long as our models are sufficiently accurate that they mimic reality perfectly. In today’s age when it’s hard to see the difference between CGI and real humans, this may not sound surprising. Nevertheless, the question is what can we learn from observation of real and simulated systems in tandem?

Computer-generated images are getting more and more realistic. By using realistic computer-generated molecular models, we can learn about how nanoparticles interact with lipid-like molecules attached to their surface. (images by ChildishGiant (left) and MaxPixel (right))

I’m happy to report that my student Gene Chong and Cathy Murphy’s student Meng Wu did precisely this parallel study.¹ Gene made simulations of a simpler system, involving nanoparticles covered by lipid-like molecules called MUTABs. Meng made NMR measurements of actual nanoparticles covered by a similar but somewhat longer lipid-like molecule called MTABs. (Note that if you are worried about the term “nuclear” in NMR, as in nuclear energy, don’t be. We are just looking at the positions of the nuclei, not spitting them apart. It was the concern over this misunderstanding that led to the use of such a device to look at your body in detail to be called MRI instead of NMR!)

Simulation snapshot of a gold nanoparticle 8nm across, with 4.0 MUTAB molecules per square nm on its surface. Gold atoms are shown in yellow, sulfur in green, carbon in gray, hydrogen in white, and nitrogen in blue. Bromide counterions are removed for clarity. (image from Wu et al. 2019¹, used with permission)

The happy result was that the two observations agreed. But only together did Meng’s and Gene’s observations show clearly that the lipid-like molecules didn’t always cover the nanoparticle smoothly like melted chocolate on ice cream, but rather assembled like sprinkles all pointing out in the same direction packed together in different islands on the surface. This structure means that lipid-decorated nanoparticles will have shapes and responses to other systems that you might not otherwise anticipate. And this opens the question to our next set of investigations as we chart a course to understand the interactions between nanoparticles and biological components such as membranes.

*It turns out that lipid-like molecules behave more like sprinkles (left) than sauce (right) when they coat gold nanoparticles. (images by Alison Fayre (L) and sciencefreak (R))*

Note: This post originally appeared on the EveryWhereChemistry blog² and is reproduced here with the author’s permission. We also hope to publish a blog post focusing more on the NMR aspects of the study described here, so stay tuned. To learn more about NMR, check out this episode of our podcast.

References

Wu, M., Vartanian, A., Chong, G., Kumar, A., Hamers, R., Hernandez, R., & Murphy, C. Solution NMR Analysis of Ligand Environment in Quaternary Ammonium-Terminated Self-Assembled Monolayers on Gold Nanoparticles: The Effect of Surface Curvature and Ligand Structure. Journal of the American Chemical Society, 2019,141(10), 4316-4327. DOI: 10.1021/jacs.8b11445
Hernandez, R. Engineered gold nanoparticles can be like ice cream scoops covered in chocolate sprinkles. EveryWhereChemistry, March 17, 2019.