The interaction between
cysteine with Li
+
and LiF in
the microcosmic water environment was investigated to elucidate how
ions interact with amino acids and the cation–anion correlation
effect involved. The structures of Cys·Li
+
(H
2
O)
n
and Cys·LiF(H
2
O)
n
(
n
= 0–6) were characterized
using ab initio calculations. Our studies show that the water preferentially
interacts with Li
+
/LiF. In Cys·Li
+
(H
2
O)
0–6
, Li
+
interacts with amino
nitrogen, carbonyl oxygen, and hydrophobic sulfur of Cys to form a
tridentate mode, whereas in Cys·LiF(H
2
O)
n
, Li
+
and F
–
work in
cooperation and interact with carbonyl oxygen and hydroxyl hydrogen
of Cys to form a bidentate type. The neutral and zwitterionic forms
are essentially isoenergetic when the water number reaches three in
the presence of Li
+
, whereas this occurs at four water
molecules in the presence of LiF. Further research revealed that the
interaction between Li
+
/LiF and Cys was mainly electrostatic,
followed by dispersion, and the weakest interaction occurs at the
transition from the neutral form to zwitterionic form. Natural population
analysis charge analyses show that for Cys·Li
+
(H
2
O)
n
, the positive charge is mostly
concentrated on Li
+
except for the system containing three
water molecules. For Cys·LiF(H
2
O)
n
, the positive charge is centered on the LiF unit in the range
n
= 0–6, and at
n
= 5, electron
transfer from Cys to water occurs. Our study shows that the contribution
of anions in zwitterionic state stabilization should be addressed
more generally along with cations.
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