Human islet amyloid polypeptide at the air-aqueous interface: a Langmuir monolayer approach

Abstract

Human islet amyloid polypeptide (hIAPP) is the source of the major component of the amyloid deposits found in the islets of Langerhans of around 95 per cent type 2 diabetic patients. The formation of aggregates and mature fibrils is thought to be responsible for the dysfunction and death of the insulin-producing pancreatic β-cells. Investigation on the conformation, orientation and self-assembly of the hIAPP at time zero could be beneficial for our understanding of its stability and aggregation process. To obtain these insights, the hIAPP at time zero was studied at the air-aqueous interface using the Langmuir monolayer technique. The properties of the hIAPP Langmuir monolayer at the air-aqueous interface on a NaCl subphase with pH 2.0, 5.6 and 9.0 were examined by surface pressure- and potential-area isotherms, UV-Vis absorption, fluorescence spectroscopy and Brewster angle microscopy. The conformational and orientational changes of the hIAPP Langmuir monolayer under different surface pressures were characterized by p-polarized infrared-reflection absorption spectroscopy, and the results did not show any prominent changes of conformation or orientation. The predominant secondary structure of the hIAPP at the air-aqueous interface was α-helix conformation, with a parallel orientation to the interface during compression. These results showed that the hIAPP Langmuir monolayer at the air-aqueous interface was stable, and no aggregate or domain of the hIAPP at the air-aqueous interface was observed during the time of experiments.

Figure 1.

Surface pressure-area isotherm (a) and surface potential-area isotherm (b) of the hIAPP Langmuir monolayer on pure water and subphase solution with 0.1, 0.5, 1.0 and 2.0 M NaCl. (Online version in colour.)

Figure 2.

Surface pressure-area isotherms of the hIAPP Langmuir monolayer on 1.0 M NaCl subphase solution with pH 2.0, 5.6 and 9.0. (Online version in colour.)

Figure 3.

Compression–decompression cycles of the hIAPP Langmuir monolayer on 1.0 M NaCl subphase with pH 5.6 at surface pressure (a) 15 and (b) 5 mN m⁻¹.

Figure 4.

Stability measurement of the hIAPP Langmuir monolayer at surface pressure 10 mN m⁻¹ on 1.0 M NaCl subphase with pH 5.6. Surface pressure, red; mean molecular area, black. (Online version in colour.)

Figure 5.

UV–Vis absorption spectra of the hIAPP Langmuir monolayer on 1.0 M NaCl subphase at pH 5.6. Inset: absorbance at 202 nm versus surface pressure. (Online version in colour.)

Figure 6.

p-Polarized IRRAS of the hIAPP Langmuir monolayer on 1.0 M NaCl subphase with pH 5.6 at surface pressure 10 mN m⁻¹ at various incident angles. The spectra were shifted vertically for better visibility. (Online version in colour.)

Figure 7.

p-Polarized IRRAS of the hIAPP Langmuir monolayer on 1.0 M NaCl subphase with pH 5.6 at various surface pressures at the incident angle 65°. The spectra were shifted vertically for better visibility. (Online version in colour.)

Figure 8.

BAM image of phase domains of arachidic acid monolayer in the two-phase coexistence region of the isotherm with pH 12 at (a) 25°C. BAM image of the hIAPP Langmuir monolayer on 1.0 M NaCl subphase with pH 5.6 at (b) 20°C before compression and at (c) 10 mN m⁻¹. Size: 768 × 572 μm².