Solubilization of aromatic and hydrophobic moieties by
arginine in aqueous solutions
Jianguo Li1, Manju Garg1, Dhawal Shah1, Raj Rajagopalan1,2,
1Chemical
and Pharmaceutical Engineering Program, Singapore-MIT Alliance
and
2Department
of Chemical and Biomolecular Engineering
National University of Singapore, Singapore 117576
Corresponding author: Raj Rajagopalan, Email: raj@nus.edu.sg
Amber force field was employed in the simulations. The carbon atoms in fullerene C20 were treated as
aromatic type (“CA” atom type), with LJ parameters being   0.34 nm and   0.36 kJ/mol . In order
to create a true hydrophobe (denoted as hC20), the energy parameter  in LJ potential for CC
interaction between hC20 particles and COW interaction between hC20 and water were weakened
as:  C C  0.208 kJ/mol ;  C OW  0.276 kJ/mol .
1
Table S1 shows the RESP charges obtained from Gaussian quantum calculations using Gaussian
package.
Figure S1 shows that if one only reduces the vdW interaction between the C20 particles, the vdW
attraction between water and the carbon makes the C20 structure soluble in water. Therefore we suppress
both vdW interactions to create a fullerene-like hydrophobes.
Figure S2 shows that hydrophobic association between two hC20 also gets suppressed at low pH (i.e.,
with arginine ARGp), but to a lesser extent than in the presence of ARGz.
Figure S3 shows the coordination number between the hydrophobe and the three chemical groups of
arginine in the case of both normal pH (ARGz) and low pH (ARGp). Results observed at low pH (ARGp)
are similar to those at normal pH. However, in the case of low pH, the coordination number between
hC
20
and the guanidinium group is lower than that at normal pH because of the reduced number of
arginine molecules in the vicinity of the hydrophobe due to the repulsion between the polar groups of
the surface-bound ARGp molecules and the arginine molecules in the solution. In the case of normal pH,
the polar groups of surface-bound ARGz in the solvation shell of hC20 form strong hydrogen bonds with
the guanidinium groups of the arginine molecules in the solution. Additionally, polar ends of arginine
molecules also form hydrogen bonds with the guanidinium groups in the vicinity of the hydrophobe. The
resulting cage-like structure around the hydrophobe further contributes to the suppression of
hydrophobic associations.
Figure S4 shows the coordination number between fullerene C20 and the polar, aliphatic, and
guanidinium groups of arginine at pH = 1 (ARGz) at an arginine concentration of 1000 mM. The results
are similar to those observed for hydrophobe hC20.
2
Table S1. RESP charges for arginine at normal pH (ARGz) and low pH (ARGp) developed by using
Gaussian quantum mechanics package. The table shows that the charges of guanidinium at both normal
pH and low pH are essentially the same, although the charges of the carboxyl groups of the two states of
arginine differ from each other, as expected.
ARGz (normal pH)
hydrophilic
group
aliphatic
group
guanidinium
group
ARGz
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
O1
C2
O2
C1
N1
H13
H14
H15
H1
C3
H2
H3
C4
H4
H5
C5
H6
H7
N2
H8
C6
N4
H11
H12
N3
H9
H10
0.83728
0.24326
0.3814
0.3814
0.3814
0.02716
0.06338
0.06338
0.10953
0.02543
0.02543
0.1356
0.03714
0.03714
0.37571
0.9378
0.50722
0.50722
0.50722
0.50722
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
ARGp (low pH)
O1
C2
0.67018
O2
H16
0.49345
C1
0.1948
N1
H13
0.41674
H14
0.41674
H15
0.41674
H1
0.12187
C3
H2
0.13603
H3
0.13603
C4
0.1088
H4
0.0352
H5
0.0352
C5
0.15187
H6
0.04593
H7
0.04593
N2
H8
0.36714
C6
0.94088
N4
H11
0.49363
H12
0.49363
N3
H9
0.49363
H10
0.49363
ARGp
3
160
Normal parameter
RDF
120
Both C-C and C-water
interactions weakened
80
Only C-C interaction
in C20 weakened
40
0
0.4
0.6
0.8
1.0
1.2
1.4
1.6
r (nm)
Figure S1: Radial distribution function between the hC20 molecules with the Amber force field
parameters, with weakened CC interaction, and with both CC and Cwater interaction weakened. The
figure shows that the modified C20 becomes soluble when only the C-C interactions are weakened.
4
120
Low pH
A R G p (m M )
100
0
500
RDF
80
1000
1500
60
40
20
0
0 .4
0 .6
0 .8
1 .0
1 .2
1 .4
1 .6
1 .2
1 .4
1 .6
r (n m )
C o o rd in a tio n N u m b e r
12
10
Low pH
A R G p (m M )
0
500
8
1000
1500
6
4
2
0
0 .4
0 .6
0 .8
1 .0
r (n m )
Figure S2: Radial distribution function between the hydrophobe hC20 at various ARGp concentrations.
5
Coordination Number
8
ARGz (Normal pH ) 1000 mM
6
h
C20  Guanidinium
h
C20  Aliphatic
h
C20  Polar
4
2
0
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0.7
0.8
r (nm)
0.9
1.0
r (nm)
Coordination Number
5
ARGp (low pH ) 1000 mM
4
3
h
C20  Guanidinium
h
C20  Aliphatic
h
C20  Polar
2
1
0
0.4
Figure S3:
0.5
0.6
Coordination number between the model hydrophobe hC20 and the different chemical
groups (polar, aliphatic, and guanidinium) of arginine at both normal pH (ARGz) and low pH (ARGp) at
an concentration of 1000 mM.
6
Coordination Number
6
ARGz (Normal pH )
5
4
1000 mM
C20Guanidinium
C20Aliphatic
C20Polar
3
2
1
0
0.4
0.5
0.6
0.7
0.8
0.9
1.0
r (nm)
Figure S4:
Coordination numbers between C20 and different chemical groups (polar, aliphatic, and
guanidinium) of arginine (ARGz) at an arginine concentration of 1000 mM for at normal pH.
7