Peptides as Model Systems for Biofunctionalizations of Cellulose─Synthesis and Structural Characterization by Advanced Solid-State Nuclear Magnetic Resonance Techniques

Abstract

The tailored design of bioactive materials based on cellulose or paper is still a challenging task. It requires detailed knowledge of the structure and interaction of the biofunctionalization with the carrier material at the nanoscale. In this work, the small peptide sequence Acetyl-Pro-Ala-Phe-Gly-OH (peptide 1) that can serve as a model for biofunctionalization is synthesized via solid-phase peptide synthesis, purified, and characterized by high-performance liquid chromatography (HPLC) and mass spectrometry (MS). The as-obtained peptide is bound to microcrystalline cellulose (MCC) via a wet chemical approach. Quantification of the peptide on the MCC carrier is performed by replacing l-proline (Pro) in the peptide sequence by 4-fluoro-l-proline (Pro(19F)) (peptide 2) and applying 19F magic angle spinning nuclear magnetic resonance (MAS NMR). Detailed characterization of the model system is provided by using 1H → 13C cross-polarization magic angle spinning (CP MAS NMR) combined with dynamic nuclear polarization (DNP) to enhance sensitivity. Analysis of the binding of the peptide on MCC necessitates the replacement of l-glycine (Gly) in the sequence by 13C-labeled l-glycine (Gly(13C)) (peptide 3). DNP-enhanced 13C-13C correlation experiments carried out with dipolar assisted rotational resonance (DARR) are then used to analyze the proximity between the model peptide and the MCC carrier. The strength of the dipolar coupling is estimated from the DNP-enhanced 1H → 13C CP MAS double-quantum rotational resonance (DQrotres) experiment. The obtained dipolar coupling between the 13C═O carbon of peptide 3 and the C6 carbon of the cellulose is equal to a carbon-carbon distance of about two C-O bond lengths, which strongly suggests the binding of significant amounts of the peptide on MCC via an ester bond.