Supplementary Material
Mapping the surface charge distribution of amyloid fibril
Gyudo Lee,1,2 Wonseok Lee,2 Hyungbeen Lee,2 Sang Woo Lee,2 Dae Sung Yoon,2
Kilho Eom,1,2 and Taeyun Kwon1,2,a
1
2
Institute for Molecular Sciences, Seoul 120-749, Republic of Korea
Department of Biomedical Engineering, Yonsei University, Wonju 220-710, Republic of
Korea
S1. Kelvin probe force microscopy (KPFM) experimental details
The surface potential of an assembled fibril on Si substrate was measured using lift-mode KPFM
(FIG. 1). In a theoretical model,1 the surface potential (i.e. work function, ΔΦ) is determined as
minimizing the force felt by the vibrating capacitor plate (AFM stylus apex; radius ~20 nm). In the
proximity of resonant frequency of AFM cantilever, the tip-sample force is dependent on DC bias
voltage1,2:
Ftip  sample  F0 
a
dC
VDCVAC sin 0t 
dZ
(1)
Author to whom the correspondence should be addressed. Electronic mail: tkwon@yonsei.ac.kr
1
where C is capacitance, Z is distance, ΔVDC and VAC are the D.C. and A.C. voltage between AFM tip
apex and sample surface, and ω0 is the resonant frequency of the conductive cantilever (SCM-PIT,
Veeco). Here, an applied D.C. voltage (VDC,App) is adjusted until the cantilever amplitude should be
zero (Fω0 = 0),
VDC  VDC , App   sample
(2)
All KPFM imaging is performed using a same conductive AFM cantilever (ω0 = 57.3 kHz) under
identical condition such as tip bias voltage (VDC,App) of +4 V, scan rate of 0.5 Hz, and lift height of 5
nm, respectively. In KPFM experiment, the tip-sample distance (Z) is critical because the measured
surface charge (σ) is almost obeyed to coulomb’s law (σ ∝ Z-2).3,4 For imaging the surface potential
of amyloid fibrils (whose diameter is given as d ~ 1.2 - 3.3 nm), the lift height of 5 nm is very
sufficient to sensitively detect the surface charge of amyloid fibril.
S2. UV-vis-absorbance data
Fig. S2 shows the UV-vis absorbance spectra of filtered supernatant of β-lactoglobulin solution as
well as heated supernatant, respectively. A maximum absorbance for supernatant was recorded at
wavelength of 278 nm, while a maximum absorbance of heated supernatant was observed at
wavelength of 290 nm, which is consistent with the result of previous studies.5,6
S3. pH-dependent surface potential distribution of amyloid oligomer
The β-lactoglobulin amyloid fibrils and oligomers prepared in buffer solution solution (1 wt%) at
pH 2 were diluted into another solution (0.02 wt%, pH 3-7) in order to measure the pH-dependent
surface charge distribution of amyloid fibrils. The surface charge distribution of β-lactoglobulin
2
amyloid oligomers is shown in Fig. S3.
Fig. S1. Transmission election microscope (JEM-2100F, JEOL, Japan) image of βlactoglobulin amyloid protofilaments and oligomers. The scale bar is 500 nm.
3
Fig. S2. Absorbance spectra of the supernatant of filtered β-lactoglobulin (without heating)
and heated supernatant, respectively.
4
Fig. S3. Surface charge distribution of β-lactoglobulin oligomers adjusted at different pH
solution to determine pI value, which is compared with that of β-lactoglobulin fibrils (Fig. 4).
A relationship between surface charge distribution and pH is fitted to Boltzmann sigmoidal
curve to determine isoelectric point (pIoligomer ~ 4.7).
5
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3
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K. Eom, and T. Kwon, ACS Nano 5, 6981 (2011).
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