Potnuru, Lokeswara Rao, Mesopotamia Nowotarski, Joshua Straub, Raj Chaklashiya, Toshihiko Shimasaki, Bholanath Pahari, Hunter Coffaro, Sheetal Jain, and Songi Han. “Dynamic Nuclear Polarization Enhanced Multiple Quantum Spin Counting of Molecular Assemblies in Vitrified Solutions,” May 31, 2024.
https://doi.org/10.26434/chemrxiv-2024-np4wp.
Crystallization pathways are essential to various industrial, geological, and biological processes. In nonclassical nucleation theory, prenucleation clusters (PNCs) form, aggregate and crystallize to produce higher order assemblies. Microscopy and X-Ray techniques have limited utility for PNC analysis due to small size (0.5 - 3 nm) and time stability constraints. We present a new approach for analyzing PNC formation based on 31P NMR spin counting of vitrified molecular assemblies. The use of glassing agents ensures that vitrification generates amorphous aqueous samples, and offers conditions to perform dynamic nuclear polarization (DNP) amplified NMR spectroscopy. We demon- strate that molecular adenosine triphosphate, along with crystalline, amorphous, and clustered calcium phosphate materials formed via a non-classical growth pathway can be differentiated from one another by the number of dipolar coupled 31P spins. We also present an innovative approach to examine spin counting data, demonstrating that a knowledge based fitting of integer multiples of cosine wave functions, instead of the traditional Fourier transform, provides a more physically meaningful retrieval of the existing frequencies. This is the first report of multi-quantum spin counting of assemblies formed in solution as captured under vitrified DNP conditions, which can be useful for future analysis of PNCs and other aqueous molecular clusters.