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line observed at 676 cm−1. The corresponding infrared mode is
observed at 675 cm−1. The S–S stretching vibration is assigned
to the strongest feature in the Raman spectrum observed near
500 cm−1. As expected, this mode is observed as a weak feature
at 499 cm−1 in the infrared.
The C–S stretching vibrations of both cysteine-S-
thiosulfonate and cysteine-S-sulfonate are observed at
660 cm−1. They appear as weak shoulders in the infrared and
as medium intense lines in the Raman. While both cysteine-
S-thiosulfonate and cysteine-S-sulfonate exhibit strong S–SO3
lines near 408 cm−1, the S-thiosulfonate also exhibits a mod-
erate line at 509 cm−1. This latter line has been tentatively
assigned to the S–S stretching vibration in accordance with the
corresponding vibration observed in the spectrum of cystine.
It is interesting to note that Lecomte et al., in their study of
polythionates [17], did not observe the S–S stretching vibration
(495 and 504 cm−1) until the chain length reached five sulfur
atoms.
The S–S–O deformations of the cysteine-S-thiosulfonate are
assigned to the weak and moderate Raman lines observed at 327
and 309 cm−1. The corresponding vibrations of the S-sulfonate
are observed at 314 and 292 cm−1. These assignments are con-
sistent with those for the corresponding vibrations of similar
molecules presented in the literature [16,17]. The observed
frequencies and assignments presented above for cysteine-S-
sulfonate, cysteine-S-thiosulfonate and cystine are summarized
in Table 1. In comparing the spectra obtained from the model
compounds, the most distinctive differences are in the Raman
lines associated with the S–S and S–SO3 stretching modes. From
vibrations can be used to distinguish between cysteine-S-
sulfonate and cysteine-S-thiosulfonate groups present in the
reaction mixture.
According to Eqs. (1)–(3), sodium sulfite and sodium thio-
sulfate can be present in the reaction mixtures. The fundamental
vibrations of these ions are very similar to those expected for
cysteine-S-thiosulfonate. The ability to identify these species
in the spectra obtained from the reaction mixtures is thus very
important. While the vibrational assignments of the sulfite and
thiosulfate ions are well established [16,18,20], the observed fre-
quencies have not been presented in detail for the solid phase. In
order to obtain accurate solid state frequencies for use in com-
parison to the spectra obtained from our reaction mixtures, we
have investigated the infrared and Raman spectra of sodium sul-
fite and sodium thiosulfate pentahydrate in the solid state. As
free ions, both thiosulfate and sulfite have C3v symmetry and
9 (3A1 + 3E) and 6 (2A1 + 2E) vibrational modes are expected,
respectively.
The observed vibrational frequencies and assignments for
solid sodium thiosulfate pentahydrate and sodium sulfite are
presented in Table 2. The assignments given are consistent with
those reported in the literature [18,20]. We have observed how-
ever that most of the thiosulfate bands appear to exhibit multiple
splitting, four bands instead of two for the E modes and two
bands instead of one for the A1 modes. The splitting can be
attributed to a reduced site and crystal symmetry (P21/c-C25h)
Fig. 2. Infrared spectra (1450–400 cm−1) of (A) cystine, (B) cysteine-S-
thiosulfonate and (C) cysteine-S-sulfonate.
The assignments of the SO3 stretching modes are fairly
straightforward [15–18]. The anti-symmetric modes for the
cysteine-S-thiosulfonate and cysteine-S-sulfonate groups are
assigned to moderate Raman lines observed at 1215 and
1214 cm−1, respectively. In the infrared this mode is very strong
and split into several components. For the cysteine-S-sulfonate,
these components are observed at 1237 and 1218 cm−1 while
for the cysteine-S-thiosulfonate they appear at 1234, 1216 and
1202 cm−1. The additional component at 1202 cm−1 is not con-
sistent with what is observed for the cysteine-S-sulfonate and
none of the other degenerate modes exhibit more than two com-
ponents.
The symmetric SO3 stretching modes of the cysteine-S-
thiosulfonate and cysteine-S-sulfonate groups are assigned to
very strong Raman lines observed at 1044 and 1048 cm−1
,
respectively. In the infrared these modes have been assigned to
the very strong sharp symmetric features observed at 1038 and
1032 cm−1. From these assignments it is clear that the frequen-
cies of the SO3 stretching modes of cysteine-S-thiosulfonate
and cysteine-S-sulfonate are very similar. In actual protein
molecules, where more conformational and environmental
diversity is expected, the SO3 stretching vibrations would be
broadened thus limiting the ability to distinguish between the
two species.
The SO3 deformation modes can also be confidently assigned
based on the literature [16–18]. The symmetric deformations
of cysteine-S-thiosulfonate and cysteine-S-sulfonate can be
assigned to the moderately intense infrared bands observed at
645 and 639 cm−1, respectively. These modes appear as weak
shoulders at 650 cm−1 in the Raman. The anti-symmetric SO3
deformations of cysteine-S-thiosulfonate are assigned to strong
infraredbandat547 cm−1 andtheweakRamanlineat544 cm−1
The corresponding vibrations of cysteine-S-sulfonate are both
observed at 567 cm−1
.
.
ments of the skeletal modes are expected to be quite complex. Of
these modes, the C–S and S–S stretching vibrations are expected
to be the most useful in terms of qualitative analysis. The assign-
ments of the C–S [15,16,19] and S–S [15,17–19] stretching
vibrational modes of cysteine are well established. The C–S
stretching mode is assigned to the moderately intense Raman