Insight of N-Lauroyl-L-alanine Molecular Assembly Using Solid-State Nuclear Magnetic Resonance Technique

Amino acid surfactants (AAS) have excellent properties such as low toxicity, antibacterial, mildness, and corrosion resistance. In recent years, they have been widely applied in the field of daily chemicals, food and pharmaceutical industry. It is pointed out that the structure at molecular level largely affects the physical and chemical properties of AAS materials. In this work, the structures of solid -state N-lauroyl-L-alanine (NLLA) including intermolecular interactions, molecular local dynamics and molecular assembly were investigated by a variety of solid -state nuclear magnetic resonance (SSNMR) techniques. Based on 2D( 1)H- H-1 double quantum -single quantum (2D H-1 - H-1 DQ-SQ), H-1 - H-1 DQ sideband pattern and molecular simulation, it was found that there was a stable intermolecular hydrogen bond formed between the carboxyl units of two NLLA molecules. Combined with the study of 2D frequency switched Lee -Goldberg heteronuclear correlation (2D C-13 - H-1 FSLG-HETCOR) and C-13 T- i , another type of hydrogen bond was probed, which existed between amide units of neighboring molecules. In addition, the conformation of alkyl chain ends was investigated. According to 1D C-13{H-1} cross polarization/magic angle spinning (CP/MAS) and 2D C-13 - H-1 FSLG-HETCOR spectra, it was revealed that the alkyl-chain ends had both gauche and trans conformations. Moreover, the trans conformation showed two distinct C-13 chemical shifts, originated from two NLLA molecular assembly forms. Accordingly, two NLLA molecular assembly structures were suggested which were transoid form and cisoid form. This work provided a SSNMR investigation strategy for characterizing the molecular local structure and dynamics of AAS materials, which helped task of researching and developing materials with improved chemical and physical properties.