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Supporting Information
Fig. 1: Chemical structure of the used thiols: (1) OEG(6)-Thiol, (2) biotinthiol and (3)
mercaptoundecan-1-ol
QCM
Q577C
-20
Frequency
Streptavidin hGBP1-mutant
-40 Q577C/K485C
0
-6
-60
-80
-12
Dissipation
-100
-120
Q577C
-140 Q577C/K485C
0
50
-18
100
150
dissipation change
frequency change [Hz]
0
200
time [min]
Fig. 2: Frequency and dissipation change during adsorption of streptavidin and the
hGBP1 mutants for the overtones 5 (black and green) and 7 (red and blue).
The used crystal with a constant (C) of 17.7 ng cm-2 was covered with a 100 nm gold layer,
which had a mean-square roughness less than 3 nm. The main principle of the method is the
excitation of the quartz crystal at its resonant frequency, so that the crystal starts oscillating.
By applying a RF voltage across the electrodes near the resonant frequency a layer, which is
adsorbed to the gold covered crystal, can be detected by a decreasing frequency proportional
to the mass of the adsorbed material.
In air or vacuum the Sauerbrey equation is applicable for rigid materials. In this case there is a
direct relation between Δf , the overtone numbers and Δm:
m  
 q l q f
f0
n
n = overtone numbers, f0 = fundamental frequency, ρq= specific density oft the quartz crystal
lq = thickness of quartz crystal
For deposited films that are soft or viscous the dissipation of the system has to be considered.
The dissipation factor is defined by
D 
Ed
2E s
where Ed is the energy that expands during one oscillation and Es the stored energy of the
system. Structural information about the attached film, its compactness and stability can be
derived by fitting the obtained data. The Kanazawa-Gordon relation allows a quantitative
description of the frequency response:
f  n 0,5 f 01,5 (nl  l / q  q ) 0,5
f0 = fundamental frequency, n = harmonic number, ρq / μq= density/ shear modulus of quartz
crystal
Additionally the dissipation is given by
D  2( f 0 / n) 0,5 (l l / q  q ) 0,5
and in combination with the frequency response and in consideration of the density of the
protein solution quantitative studies of interfacial phenomena accompanied with mass and
viscoelastic changes are possible.