Probing effects of bile salt on lipase adsorption at air/solution interface by sum frequency generation vibrational spectroscopy

• SFG is a useful tool to probe the competitive adsorption of bile salt and lipase at the air–water interface.
• Lipase could be activated by NaDC at low NaDC concentration.
• Lipase could be replaced by NaDC gradually with increasing concentration of NaDC.

By using sum frequency generation vibrational spectroscopy (SFG), the structure of sodium deoxycholate (NaDC, a kind of bile salts) at air/solution interface was analyzed at molecular level, and its effects on Candida rugosa lipase (CRL) and Porcine pancreatic lipase (PPL) adsorption were revealed. When NaDC was incubated with lipase first, lipase was activated at low NaDC concentration (≤CMCNaDC, critical micelle concentration of NaDC). While when NaDC was incubated with the substrate p-nitrophenyl palmitate (p-NPP) first, the activity of lipase was almost completely inhibited, since NaDC adsorbed onto the substrate interface and presented a barrier for lipase approach. The surface tensions of lipase solution declined with the increase of NaDC, which indicated that there was competitive adsorption at the air/solution interface. With increasing concentration of NaDC, the symmetric stretch vibration of CH2 groups at 2853 cm−1, the asymmetric stretch vibration of CH3 groups at 2960 cm−1 and amide I vibration at 1655 cm−1, supposedly caused by lipase amino residues, gradually reduced and eventually disappeared. When NaDC concentration reached high (≥CMCNaDC), the SFG spectra became similar with the pure NaDC spectra, which was direct evidence that NaDC had completely replaced the lipase molecules at the air/solution interface.

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