1 Magnetic susceptibility of L-amino acids in solid state at high
magnetic fields
Jaron C. Swift*, Daniel M. Pajerowski, Mark W. Meisel
Department of Physics, University of Florida, Gainesville, FL 32611-8440
07/23/08
Abstract: Magnetic molar susceptibilities (χm) are determined for amino acids in high magnetic
fields. The magnetization of the solid state form of L-alanine, L-glutamic acid, L-histidine,
L-isoleucine, L-tryptophan, and Ltyrosine were measured at 298 K and 310 K using a
commercial SQUID magnetometer. At low magnetic fields, the SQUID magnetometer observes
a curve not engendered by the prior magnetic susceptibility technique called the Gouy Balance.
The magnetic susceptibility was nominally independent of magnetic fields between 1 Tesla to
7 Tesla. Currently, it is murky as to why the magnetic susceptibility demeans the way it does at
low magnetic fields. The sample’s can holder, which provides 2/3 of the magnetization signal, is
one possibility as to why the magnetic susceptibility responds the way it does at low magnetic
fields. This large signal absorbance may explain to why initially the magnetic susceptibility is at
a large curve when in fact the χm should be a relatively horizontal straight line. Future
experiments may include the shift to different materials to minimize the background signal.
*Permanent Address: Department of Chemistry and Physics, Prairie View A & M University, Prairie View, TX 77446
2 Background: Currently we use magnetic fields in many of our hospitals as a biomedical
imaging tools like Magnetic Resonance Imaging (MRI) or healing instruments like Magnetic
Field therapy (MFT) for a non-invasive solutions to diagnose or treat illnesses such as
rheumatoid disease, cancer, stress-related illness, and pain. These magnetic fields may have
effects on our body proteins. The primary structure of proteins is an amino acid. The general
structure of an amino acid is shown in Figure 1. When a material is placed in a magnetic field
(H), a magnetization (M) is induced in the material which is related to H by M= χmH, where χm
is the molar susceptibility.
Figure 1 Amino acid general structure
χm =
M * ΜW
,
m*H
(1)
where (MW) is the molecular weight in grams/mole and mass (m) is in grams. The
magnetization induced by the magnetic field depends on the strength of the magnetic field and
the nature of the substance [3].
Since amino acids are organic compounds, we expect a diamagnetic response at low and
high magnetic fields. Diamagnetism is associated with the tendency of electrical charges
partially to shield the interior of a body from an applied magnetic field [3]. Since all atoms
3 possess electronic charges, they all have a characteristic diamagnetic susceptibility. The
magnitude of diamagnetic susceptibility is of the order of 10-6 CGS units. In general,
diamagnetism is both temperature and field independent. The magnetic susceptibilities in
magnetic fields strengths of up to 7 Tesla were determined in this study through the use of six
natural occurring amino acids. We used four different groups of amino acids: basic, acidic, nonpolar, and polar side chains. Proteins consist exclusively of L- amino acids and Fig. 2 depicts the
samples studied in our experiment.
L-Alanine
L-Glutamic
Acid
L-Isoleucine
L-Histidine
L-Tryptophan
L-Tyrosine
Figure 2 L-Alanine, L-Isoleucine, and L-Tryptophan are non-polar molecules. Histidine is a basic
molecule. Glutamic acid is an acidic molecule and tyrosine is an uncharged polar molecule.
4 Methods: A Superconducting Quantum Interference Device (SQUID) magnetometer is used to
measure the magnetic moment of the sample. Figure 3 shows a rough sketch of the SQUID’s
core, and in Fig. 4 a sketch is presented to show the mechanism of how the sample is being
measured within the SQUID magnetometer. Due to its low magnetic susceptibility, a plastic
straw was used as a sample holder for a can which contained the sample.
Sample
Pickup
Coils
Multifilament
Superconducting
Wire
Composite
Form for
Solenoid
Figure 3 Cut view of a superconducting magnet [6].
5 Figure 4 Sample moving upward through the SQUID pickup coils. Sample is centered when it is at
middle of scan length.
The sample must be centered in the SQUID to ensure that all four coils sense the
magnetic moment of the sample. If the sample is not centered, the coils read only part of the
magnetic moment. The Labview program used to acquire the data was MPMS Multivu.
Functionally, the MPMS operates by measuring the magnetic moment of a sample by reading the
output of the SQUID detector while the sample moves upward, from the initial position, through
the SQUID pickup coils. We used the MPMS to run partial DC centering measurements. The
scan length is 4 cm, which is digitized into 48 discrete steps where data points are acquired. The
data points are individual voltage readings that plot a response curve in centering the scan data
file, shown in Fig. 5. Finally, Origin 5.0 was used to organize the acquired data from the MPMS
Multivu program.
6 a
b
c
Voltage (v) Z
Scan (cm) Figure 5 (a) Represents the sample as it travels upward through the coils. (b) The SQUID response
against the scan. (c) The output indicates that the sample is too high (the vertical line is not in the middle
of the period) and the sample must be lowered to get accurate magnetic moment.
Results: A Gouy Balance was used previously to measure the magnetic susceptibilities of the
reported amino acids [2]. When we reproduce the experiment using the SQUID we establish
different magnetic susceptibilities. In Fig. 6 we can see that the magnetization for the can holder
is 2/3 of the total signal. Fig. 7 depicts the total measurement of L- Tryptophan from 0-70 Gauss,
in which a mysterious curve appears in the low magnetic field region. As we closely study the
amino acids at high fields ranging above 1 Tesla, we begin to elucidate what we hypothesized
the samples to do even at low magnetic fields.
7 0.002
Magnetization (G emu)
0.000
Tryptophan
-0.002
-0.004
-0.006
Can
-0.008
-0.010
-0.012
Can and Tryptophan
-0.014
-0.016
-10
0
10
20
30
40
50
60
70
80
Magnetic Field (kG)
Figure 6 Magnetization vs. magnetic field for L-Tryptophan at T=298K. The magnetization for the can
was fitted to a line by using a polynomial 1 fit, Mcan = 2.92*10-5 + (-1.70657*10-7)H, which (H) represents
the magnetic field.
Fig. 8 and Fig. 9 depicts a relatively constant magnetic susceptibility above 1 Tesla once a Linear
fit line was place through the data points, when
χm = M0 + mH, m=0.
(2.)
8 0
-200
-1
χm(cm * mol )
-400
-3
-600
Trytophan at 298 K
-800
-1000
-1200
-10
0
10
20
30
40
50
60
70
80
Field (kG)
Figure 7 Molar susceptibility (χm) vs magnetic field for L-Tryptophan at T= 298K from 0-70 Gauss.
-118
-1
-124
χm (cm mol )
-122
3
-120
Tryptophan at 298 K
-126
-128
-130
10
20
30
40
50
Magnetic Field (kG)
60
70
Figure 8 Molar susceptibility (χm) vs magnetic field for L-Tryptophan at T= 298K from 10-70 Gauss. The
red line represents a linear fit to the data constrained to have a zero slope, yielding
χm = (-125.4 ± 0.8) x10-6 cm3 mol-1.
9 -118
-120
3
-1
χm (cm mol )
Tryptophan at 310 K
-122
-124
-126
-128
10
20
30
40
50
60
70
Magnetic Field (kG)
Figure 9 Molar susceptibility (χm) vs Field for L-Tryptophan at T= 310 K from 10-70 Gauss. The red line
represents a linear fit to the data constrained to have a zero slope, yielding
χm = (-125.2 ± 0.7) x10-6 cm3 mol-1.
To first approximation, the magnetic molar susceptibility (χm) is a scalar vector that is
temperature independent at room temperature. The error bars are derived from the error of
propagation calculations. The balancer used to measure the mass (m) of the sample has an
uncertainty to the order of 0.02 mg. We also have an uncertainty surrounding the magnetization
(M) of the sample. The magnetization deviation is an output given by the MPMS and varies
depending on the magnetic field. In Eq. 3 and Eq. 4, the magnetic susceptibility’s error of
⎛
⎞
propagation is derived. In Eqs. 3 and 4, the ⎜ σ
⎟ represents the standard deviation of the molar
⎝ χm ⎠
susceptibility, the sample’s magnetization is (Ms) and the sample’s mass is (ms).
⎛ σ χm
⎜
⎜χ
⎝ m
⎞
(M s ± δM s )
⎟ =
⎟
ms ± δms
⎠
2
(3)
10 σχ
m
⎛ σ Ms
= ⎜⎜
⎝ Ms
2
2
⎞ ⎛ σ ms ⎞
⎟⎟ + ⎜⎜
⎟⎟ * χ m
⎠ ⎝ ms ⎠
(4)
When L-Tryptophan is measured in the SQUID, the magnetic molar susceptibility in cgs
units (x10-6 cm3 mol-1), at 298 K is -125.4 ± 0.8 and at 310 K is -125.2 ± 0.7, shown in Table 1.
There are slight differences between the two values, but they are quantitatively close since, in
principle, χm is temperature independent. Table 2 includes values taken from others, who used a
Gouy balancer to develop the reported data.
Table 1 χm values obtained with the SQUID magnetometer.
χm (x10-6 cm3 mol-1)
L-Amino Acid
Mass
Purity (%)
(mg)
Molecular
Weight
(g/mol)
SQUID
SQUID
310K
298K
10 kG – 70 kG
10 kG – 70 kG
Alanine 99%
190.45
89.09
-48.7 ± 0.1
-49. 0± 0.1
Glutamic Acid
99%
76.89
147.13
-75.2 ± 0.4
-75.2 ± 0.4
Histidine 98%
117.48
155.16
-92.8 ± 0.3
-93.1 ± 0.3
Isoleucine 99%
173.44
131.17
-80.9 ± 0.1
-81.9 ± 0.1
Tryptophan 99%
55.59
204.23
-125.2 ± 0.7
-125.4 ± 0.8
Tyrosine 99%
57.43
181.19
-117.8 ± 0.5
-118 ± 0.5
11 Table 2 Comparison of χm values obtained with the SQUID magnetometer, this work, and those obtained
using Gouy Balance [2].
χm (x10-6 cm3 mol-1)
Amino Acid
Alanine
SQUID
SQUID
Ref. [2]
Ref. [2]
Ref [5]
Ref [4]
Measured
Measured
Measured
Calculations
Measured
Measured
10 kG – 70 kG
10 kG – 70 kG
~1 kG
298K
310K
298K
-49. 0± 0.1
-48.7 ± 0.1
-51.37
Reference 163
298K
298K
-50.78
-50.5
N/A
-75.36
N/A
-78.5
-83.86
N/A
N/A
-86.36
-85
N/A
-118.04
-132
-132
-103.11
-105.3
-105.3
Very pure
Glutamic
Acid
-75.2 ± 0.4
Histidine
-93.1 ± 0.3
-75.2 ± 0.4
-75.10
Very pure
-92.8 ± 0.3
-83.29
Pure
Isoleucine
-81.9 ± 0.1
-80.9 ± 0.1
-82.99
Very pure
Tryptophan
-125.4 ± 0.8
-125.2 ± 0.7
-121.64
Pure
Tyrosine
-118 ± 0.5
-117.8 ± 0.5
-100.67
Pure
12 Summary & Discussions
Based on the above results, it we propose that the differences between our magnetic
susceptibilities and others may come from the background of the sample’s can holder and not
calibrating the SQUID magnetometer before taking data. In Figure 7, a large signal from the can
is shown, 2/3 of the total signal, which means a small signal is taken from the sample itself. This
phenomenon is mysterious because the diamagnetic susceptibility, in principle, should not
depend on the magnetic field, however, the data show otherwise. After calibrating the SQUID’s
system for the last amino acid, isoleucine, we inferred a ± 5% deviation for the magnetic
susceptibility. This small deviation does not represent the significant spread between our results
for the tyrosine and histidine when compared to others with lower results. The future direction
for our project may include the use of latex gloves as our sample holder. Latex gloves may prove
to be a better source to mitigate the background signal, which should increase the signal output
for the sample itself.
Acknowledgments:
This work was supported by the NSF through the University of Florida Department of
Physics REU program and by DMR-0701400.
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