To understand the diffusion mechanism, we designed a Fickian diffusion model in which mucin solution acts as a polymer matrix and bile salt solution serves as the diffusant. A bile salt mixture of sodium taurodeoxycholate and L-α-phosphatidylcholine was layered atop the mucin solution resting on a silicon ATR crystal. As the bile salt diffused into the mucin and toward the ATR surface, spectral changes in the mucin-bile salt mixture were monitored in real time. 2DCOS enabled enhanced spectral resolution through identification of synchronous and asynchronous cross-peaks, revealing the sequence of molecular changes within the mucin matrix.
Control experiments showed that mucin’s functional groups interact internally and reach a conformational equilibrium over time. Upon bile salt exposure, however, 2DCOS analysis revealed a sequential breakdown of hydrogen bonding and disulfide linkages, along with conformational rearrangements that permitted hydrophobic interactions between mucin and bile salt micelles. These interactions disrupted mucin’s structural integrity—initially maintained through hydrogen bonding and electrostatic interactions at physiological pH (6.0–6.5)—ultimately transforming the mucus into a more permeable interface.
Together, these findings highlight the utility of ATR-FTIR and 2DCOS as powerful analytical tools for real-time, sequence-resolved characterization of mucosal barrier disruption. This work provides a mechanistic understanding of bile salt-induced mucin remodeling, with implications for drug absorption and gastrointestinal physiology.