Deuterium MAS NMR and Local Molecular Dynamic Model to Study Adsorption–Desorption Kinetics of a Dipeptide at the Inner Surfaces of SBA-15

• Published in: Journal of physical chemistry. C Vol. 120; no. 5; pp. 2797 - 2806
• Main Authors: Jayanthi, Sundaresan, Kababya, Shifi, Schmidt, Asher, Vega, Shimon
• Format: Journal Article
• Language: English
• Published: American Chemical Society 11-02-2016
• DOI: 10.1021/acs.jpcc.5b11429
• EISSN: 1932-7455
• ISSN: 1932-7447

This work presents a deuterium magic angle spinning (MAS) NMR study of the adsorption–desorption dynamics of glycine-(2,2)-d 2-alanine dipeptide adsorbed at the inner surfaces of mesoporous SBA-15 silica under different hydration levels and temperatures. The experimental and theoretical challenges posed by the strong quadrupolar interaction of the rigid CD2 group, 3-fold bigger than that of the rotating methyl CD3, were addressed. Deuterium MAS NMR spectra modulated by exchange were analyzed using theoretically calculated exchange spectra based on the two-site Bloch–McConnel exchange equation represented in Floquet space. To solve this equation, which is composed of a high dimensional Floquet exchange matrix, our former computational approach was modified to reduce the overall computation time by orders of magnitude so as to yield more accurate exchange parameters from the spectral analysis. The adsorption–desorption kinetics of minutely hydrated silica surfaces is understood to originate from the diffusion of water molecules into and out of adsorbate binding sites, thereby gating the dynamic behavior of the adsorbate via increase or reduction of the size of the surrounding water cluster. Molecular dynamic (MD) simulations were employed to model the dynamic behavior of the adsorbate at the two states. Deviations between the MD and experimental observations are attributed to the simplified surface modeling, thereby highlighting the importance of experimental MAS NMR data to improve future modeling of realistic functional surfaces.