γ-Glutamyl dipeptides in Petiveria alliacea

Article abstract of DOI:10.1016/j.phytochem.2005.06.029

Three γ-glutamyl dipeptides have been isolated from Petiveria alliacea L. roots. These dipeptides include (S_C2R_C7)- γ-glutamyl-S-benzylcysteine together with two diastereomeric sulfoxides, namely (S_C2R_C7R_S)- and (S_C2R _C7R_S)-γ-glutamyl-S-benzylcysteine S-oxides (γ-glutamyl-petiveriins A and B, respectively). Their structures and absolute configurations have been determined by NMR, MALDI-HRMS, IR and CD spectroscopy, and confirmed by comparison with authentic compounds obtained by synthesis.

Full text of DOI:10.1016/j.phytochem.2005.06.029

PHYTOCHEMISTRY
Phytochemistry 66 (2005) 2494–2497
www.elsevier.com/locate/phytochem
c-Glutamyl dipeptides in Petiveria alliacea
1
*
Roman Kubec , Rabi A. Musah
Department of Chemistry, State University of New York at Albany, 1400 Washington Ave, Albany, NY 12222, USA
Received 8 April 2005; accepted 16 June 2005
Available online 16 September 2005
Abstract
Three c-glutamyl dipeptides have been isolated from Petiveria alliacea L. roots. These dipeptides include (SC2RC7)-c-glutamyl-S-
benzylcysteine together with two diastereomeric sulfoxides, namely (SC2 )- and (SC2 )-c-glutamyl-S-benzylcysteine S-oxi-
R
C7
R
S
C7 S
R R
des (c-glutamyl-petiveriins A and B, respectively). Their structures and absolute configurations have been determined by NMR,
MALDI-HRMS, IR and CD spectroscopy, and confirmed by comparison with authentic compounds obtained by synthesis.
ꢀ
2005 Elsevier Ltd. All rights reserved.
Keywords: Petiveria alliacea; Phytolaccaceae; Glutamyl dipeptide; Petiveriin; S-benzylcysteine sulfoxide; S-benzylcysteine
1
. Introduction
plant as the unique sulfine (Z)-thiobenzaldehyde S-oxide
Kubec et al., 2003). It is likely that the petiveriins serve
(
Petiveria alliacea L. (Phytolaccaceae) is a perennial
as precursors to this sulfine.
shrub indigenous to the Amazon Rainforest, although
it grows abundantly in other areas, including tropical
and Central America, the Caribbean and the southeast-
ern United States. It is a popular folk medicine used to
treat a wide variety of disorders. Preparations from this
plant reportedly exhibit anticancer, antimicrobial and
antiinflammatory effects, among many others.
Typically, S-substituted cysteines and their sulfoxides
are accompanied by the corresponding c-glutamyl
dipeptides in the plant tissue. The biochemical role of
these dipeptides is not completely clear, but it is believed
that they serve as storage compounds of nitrogen and
sulfur, among other functions. They also are intermedi-
ates in the biosynthesis of S-alk(en)ylcysteine sulfoxides
(Kasai and Larsen, 1980; Fenwick and Hanley, 1985;
Lancaster and Shaw, 1989, 1991).
We recently reported isolation of six novel sulfur-
containing amino acids from P. alliacea roots, including
(
R R )/(S R )-S-benzylcysteine sulfoxides (petiveriins
This co-existence in many plants of S-substituted cys-
teine derivatives and their dipeptide forms encouraged
us to search for the presence of such dipeptides in P. alli-
acea. The present paper describes our investigations on
isolation and identification of three novel c-glutamyl
dipeptides from the roots of this plant.
S
C
S C
A and B) and (R R )/(S R )-S-(2-hydroxyethyl)cys-
S
C
S C
teine sulfoxides (6-hydroxyethiins A and B) (Kubec
and Musah, 2001; Kubec et al., 2002). We demonstrated
that these cysteine derivatives are enzymatically cleaved
upon tissue disruption in a fashion similar to that ob-
served in Allium plants, yielding four thiosulfinates in to-
tal. We also identified the lachrymatory principle of the
2
. Results and discussion
*
Corresponding author. Tel.: +1 518 4373740; fax: +1 518 4373741.
The roots of P. alliacea were extracted with boiling
E-mail address: musah@albany.edu (R.A. Musah).
1
MeOH and the extract was subjected to ion-exchange
chromatography. A dipeptide-containing fraction was
Present address: Department of Chemistry, University of South
ˇ
Bohemia, Studentsk a´ 13, 370 05 Cesk e´ Bud eˇ jovice, Czech Republic.
0
031-9422/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.phytochem.2005.06.029

R. Kubec, R.A. Musah / Phytochemistry 66 (2005) 2494–2497
2495
+
obtained by a combination of cation (Amberlite 200, H )
COOH O
NH
and anion-exchange (Dowex 1 · 8, acetate) chromatogra-
phy. HPLC/MS analysis of the dipeptide fraction revealed
S
COOH
NH2
+
the presence of one component having [MH ] of 341, to-
+
gether with two minor ones each exhibiting [MH ] of
3
57. These three compounds were subsequently isolated
1
, (S R )-γ-Glutamyl-S-benzylcysteine
C2 C7
by prep. HPLC as white and very hygroscopic solids.
1
The H NMR spectrum of the first compound indi-
cated the presence of the glutamyl (d 2.14, 2.45 and
O
S
COOH O
NH
3
.76 ppm, 5H) and cysteinyl (ABX system; d 2.83, 2.98
and 4.34 ppm, 3H) moieties along with an unsubstituted
COOH
NH2
1
3
benzyl group (d 7.33–7.42 ppm, 5H). The C NMR
spectrum revealed the presence of 13 magnetically non-
equivalent carbon atoms, with four of them being aro-
matic (d 129.0, 129.3, 129.4 and 130.7 ppm) and three
of them being carbonyl/carboxyl carbons (d 173.9,
2
a, (S R S )-γ-Glutamyl-S-benzylcysteine sulfoxide
C2 C7 S
(
γ- -glutamyl-petiveriin B)
L
1
74.5 and 175.0 ppm). The IR spectrum contained a
ꢀ1
broad band at 3500–2850 cm
showed [MH ] of 341.1169 (C H N O S req.
and MALDI-HRMS
+
O
S
COOH O
NH
1
5
20
2
5
3
41.1166). Based on the spectral data, it was concluded
COOH
NH2
that the compound was a glutamyl dipeptide of S-benzyl-
cysteine. Because both a- and c-glutamyl dipeptides gen-
erally have very similar NMR spectra, the method of
Kasai and Sakamura (1973) was used to determine the
type of CO–NH linkage in the isolated dipeptide. The
method is based on the sensitivity of the glutamyl a-pro-
2b, (S R R )-γ-Glutamyl-S-benzylcysteine sulfoxide
C2 C7
S
(
γ- -glutamyl-petiveriin A)
L
1
Fig. 1. The structures of the c-glutamyl dipeptides isolated from
Petiveria alliacea roots.
ton to pH changes, leading to significant H NMR signal
shift differences. Addition of DC1 caused a downfield
shift (Dd = + 0.31 ppm) and addition of NaOD an
upfield shift (Dd = ꢀ0.58 ppm) of the proton signal,
indicating that the isolated compound was c-glutamyl-
S-benzylcysteine (1). The correct structural assignment
was further confirmed by comparison with authentic
dipeptides were identified as diastereomers of c-glut-
amyl-S-benzylcysteine sulfoxide (2a/2b). This conclu-
sion was further verified by comparison with authentic
compounds obtained by synthesis.
The absolute configurations about the sulfur atoms in
1
(
SC2RC7)-c-glutamyl-S-benzylcysteine obtained by syn-
2a and 2b were determined by CD and H NMR spec-
thesis. Because the synthetic compound had identical
spectral characteristics in all respects with the isolated
dipeptide, we could unambiguously determine the abso-
troscopy together with enzymatic hydrolysis. The CD
spectra of the dipeptides were almost mirror images,
both exhibiting a maximum around 224 nm. The CD
spectrum of the first eluting isomer (2a) showed a nega-
tive sign of the Cotton effect (Demax = ꢀ2.7; 225 nm),
whereas the second one (2b) gave a CD spectrum with
a positive sign of the Cotton effect (Demax = + 3.0;
224 nm). As we showed in a previous contribution (Ku-
bec and Musah, 2001), the negative sign of the Cotton
effect correlates with the S configuration about the sul-
fur atom. Employing this rule, we determined the abso-
lute configuration of 1 to be S R (Fig. 1).
C2 C7
1
13
The H and C NMR spectra (incl. 2D NMR) of the
two minor compounds were very similar. The proton
spectra showed the presence of the glutamyl moiety,
an unsubstituted benzyl group (d 7.39–7.49 ppm, 5H)
and two pairs of heterosteric methylene protons. The
1
3
C NMR spectra revealed the presence of 13 magneti-
cally non-equivalent carbon atoms, with four being aro-
matic and three being carbonyl/carboxyl carbons. As
with 1, addition of DC1 and NaOD caused a significant
downfield (Dd = + 0.19 ppm) and upfield (Dd =
lute configuration of 2a and 2b as SC2
, respectively (Fig. 1).
The correct assignment of the absolute configurations
R S and
C7 S
S R R
C2 C7 S
1
ꢀ
0.55 ppm) shift, respectively. Along with a broad band
was further confirmed by H NMR spectroscopy.
ꢀ
1
13
between 3500 and 2850 cm , both IR spectra contained
Although the C NMR spectra of 2a and 2b were nearly
ꢀ
1
1
a strong absorption band near 1015 cm , indicating the
presence of a sulfoxide group. The MALDI-HRMS data
showed m/z [MH] of 357.1115 and 357.1116, respec-
identical, the H spectra displayed significantly different
signal patterns for the S(O)CH CH(NH ) methylene
2 2
+
protons. As discussed previously (Kubec and Musah,
2001), this typical feature of all S-substituted cysteine
sulfoxides and their glutamyl dipeptides allows
convenient determination of the absolute configuration
tively, corresponding to a molecular formula of
C H N O S for both compounds (req. 357.1115). In
1
5
20
2
6
accordance with the above spectral data, the two minor

2496
R. Kubec, R.A. Musah / Phytochemistry 66 (2005) 2494–2497
about the sulfur. Whereas the methylene proton signals
in 2a appeared as two distinctive doublets of doublets (d
ment of Horticulture, University of the West Indies at
Cave Hill, Barbados) in April 2004. They had been
stored at 4 ꢁC for 3 weeks prior to being analyzed. A
voucher specimen is deposited at the National Herbar-
ium of Barbados, University of the West Indies, Cave
Hill Campus, St. Michael, Barbados (Accession No.
JC37).
3
.11 and 3.49 ppm; JAX = 7.8 Hz and JBX = 6.0 Hz),
those in 2b were represented by a poorly resolved multi-
plet (d 3.28 ppm). These splitting patterns showed that
the sulfoxide oxygen and the amino group of 2a are
on the same face of the molecule (i.e., S R S ).
C2 C7
S
Furthermore, the absolute configurations were deter-
mined by enzymatic hydrolysis of 2a and 2b by c-glutam-
yltranspeptidase (EC 2.3.2.2). Indeed, 2a was cleaved
into glutamic acid and (R S )-S-benzylcysteine sulfoxide
3.2. General experimental procedures
HPLC separations were performed on a Dynamax
SD-200 binary pump system, employing a Varian
PDA 330 detector and a C-18 reverse phase column
C
S
(
petiveriin B). Analogously, 2b yielded glutamic acid
along with (R R )-S-benzylcysteine sulfoxide (petiveriin
C
S
˚
A). Petiveriins A and B thus show a reverse elution order
on a C-18 column to that of their c-glutamyl dipeptides
(Rainin Microsorb-MV 100 A, 250 · 4.6 mm, 5 lm).
Alternatively, a preparative C-18 reverse phase column
˚
(
2b and 2a, respectively) (Kubec and Musah, 2001).
(Rainin Dynamax-100 A, 250 · 21.4 mm, 8 lm) was
We also searched for the presence of the analogous
used. 10 mM of NH OAc buffer (solv. A, pH 5.6) and
4
dipeptide forms of the minor cysteine derivatives present
in P. alliacea, i.e., c-glutamyl-S-(2-hydroxyethyl)cys-
teine and its sulfoxides. We were able to confirm that
MeCN (solv. B) were used as the mobile phase, with a
ꢀ
1
ꢀ1
flow rate of 0.9 mL min (18 mL min with the prep.
column). The gradient was as follows: A/B 97/3
(0 min), 90/10 (in 15 min), 65/35 (in 25 min) and 97/3
(in 30 min). Other instrumentation used was identical
to that described previously (Kubec and Musah, 2001).
synthetically
obtained
c-glutamyl-S-(2-hydroxy-
ethyl)cysteine and its stereoisomeric sulfoxides yield
+
the expected [MH ] of 294 and 310, respectively, on
HPLC/MS analysis. However, no compounds showing
+
the corresponding [MH ] of 294 or 310 were detected
3.3. Extraction and purification
in the dipeptide-containing fraction by HPLC/MS. Gi-
ven the fact that the content of c-glutamyl dipeptides
in plants is known to vary significantly throughout their
growth cycle, being very low or even non-detectable at
times (Kasai and Larsen, 1980; Lancaster and Shaw,
Several portions of fresh P. alliacea roots (755 g in to-
tal) were carefully cleaned and homogenized in MeOH
(3 L) using a blender. The slurries obtained were re-
fluxed for 10 min and filtered. The individual extracts
were combined, concentrated to ca. 400 mL by vacuum
evaporation (40 ꢁC) and extracted with hexane
(2 · 400 mL) to remove lipidic compounds. The polar
fraction was adjusted to 600 mL by addition of 3%
HCl. The precipitate that appeared on acidification
was filtered off and the filtrate was passed through a cat-
1
991; Matsuura et al., 1996), it is reasonable to expect
a similar seasonal variation in the c-glutamyl dipeptide
content in P. alliacea. Thus, we believe that our current
observation is a consequence of the temporary absence
or very low content of these compounds as a result of
the particular growth stage of the sample we analyzed.
In Allium and Brassica plants, c-glutamyl dipeptides
have been shown to be biochemical precursors of the
corresponding S-alk(en)ylcysteines and their sulfoxides
+
ion-exchange column (20 · 2.1 cm, Amberlite 200, H
form, 20–50 mesh). After washing the column with 3%
HCl (300 mL) and H O (300 mL), the dipeptide fraction
2
(
Kasai and Larsen, 1980; Fenwick and Hanley, 1985;
was eluted with 500 mL of 1 M NH OH. The yellowish
4
Lancaster and Shaw, 1989, 1991). During sprouting,
the dipeptide forms are cleaved by c-glutamyltranspep-
tidases to yield S-alk(en)ylcysteines and S-alk(en)ylcys-
teine sulfoxides. Our observation of 1 and 2a/2b
suggests that analogous biochemical pathways are oper-
ative in the biosynthesis of S-benzylcysteine sulfoxides
eluent obtained was concentrated to ca. 150 mL, the pH
was adjusted to 7.0 by addition of 10% HOAc, and the
resulting solution was applied to an anion-exchange col-
umn (15 · 2.1 cm, Dowex 1 · 8, acetate form, 200–400
mesh). The column was washed with 500 mL of H O
2
(discarded) and the dipeptide fraction was eluted with
10% HOAc (500 mL). The eluate was evaporated to dry-
ness and the yellowish residue redissolved in 15 mL of 10
(
petiveriins A/B) in P. alliacea. However, the origin of
the benzyl moiety in petiveriins A/B remains unknown.
mM NH OAc buffer (pH 5.6) and subjected to prep.
4
HPLC. The fractions eluting at 16.8, 8.1 and 8.6 min
were repeatedly collected, concentrated to ca. 3 mL,
3
. Experimental
and again subjected to prep. HPLC using H O instead
2
3
.1. Plant material
of NH OAC (to remove the buffer salt). The eluted
4
fractions were evaporated to dryness to yield 19, 3 and
4 mg of 1, 2a and 2b, respectively, as white hygroscopic
solids.
Fresh roots of P. alliacea were collected in Hillaby,
St. Andrew (Barbados) by Mr. Jeff Chandler (Depart-

R. Kubec, R.A. Musah / Phytochemistry 66 (2005) 2494–2497
2497
3
.4. Reference compounds
R)-S-Benzylcysteine and both diastereomers of (R )-
3.4.3. (S R R )-c-Glutamyl-S-benzylcysteine
C2 C7 S
sulfoxide (2b, c-L-glutamyl-petiveriin A)
2
D
2
(
A white hygroscopic solid. M.p. 126–129 ꢁC; ½aꢁ :
C
S-benzylcysteine sulfoxide (petiveriins A and B) were
obtained as described previously (Kubec and Musah,
2
+3.2ꢁ (H O; c 0.06); CD De
(H O; c 0.06; 22 ꢁC):
2
max
2
ꢀ1
+3.0 (224 nm); IR (KBr) m
cm : 3500–2850 (s, br),
max
1
001). (S R )-c-Glutamyl-S-benzylcysteine (1) was
1610 (s, br), 1012 (s); H NMR (300 MHz; D O): d
C2 C7
2
synthesized according to the procedure of Orlowski
2.15 (2H, m, CH CH CH), 2.51 (2H, t, J = 7.5 Hz,
2
2
and Meister (1971). The diastereomers of (S R )-c-
CH CH CH), 3.28 (2H, m, SOCH CH-a, b), 3.81 (1H,
2 2 2
C2 C7
glutamyl-S-benzylcysteine sulfoxide (2a and 2b) were
obtained by oxidation of 1 with H O and subsequent
t, J = 6.2 Hz, Glu-CH). 4.17 (1H, d, J = 13.2 Hz.
CH SO-a), 4.33 (1H, d, J = 13.2 Hz, CH SO-b), 4.64
2
2
2
2
separation by prep. C-18 HPLC. N-Phthaloyl-L-glu-
tamic anhydride was prepared from N-phthaloyl-L-glu-
tamic acid (Aldrich) and acetic anhydride (King and
Kidd, 1949). c-Glutamyltranspeptidase (GTP) (EC
(1H, dd, J = 5.0, 9.2 Hz, Cys-CH), 7.39–7.49 (5H, m,
1
3
Harom);
C
NMR (75 MHz; D O):
d
26.4
2
(CH CH CH), 31.7 (CH CH CH), 49.6 (Cys-CH),
2
2
2
2
53.0 (SOCH CH), 54.3 (Glu-CH), 56.8 (CH SOCH ),
2
2
2
2
.3.2.2) from equine kidney (11 U/mg) was purchased
129.0 (Carom,para), 129.3 (Carom,meta), 129.4 (Carom,q),
from Aldrich.
130.7 (Carom,ortho), 173.9 (Glu-COOH), 174.5
(
NHCOCH ), 175.0 (Cys-COOH). MALDI-HRMS
2
+
3
.4.1. (S R )-c-Glutamyl-S-benzylcysteine (1)
[MH ] 357.1116 (C H N O S req. 357.1115).
15 20 2 6
C2 C7
2
D
2
A white hygroscopic solid. M.p. 152–155 ꢁC; ½aꢁ :
ꢀ
14.7ꢁ (H O; c 0.03); CD De
(H O; c 0.03; 22 ꢁC):
2
max
2
ꢀ1
Acknowledgments
+
0.51 (220 nm); IR (KBr) mmax cm : 3500–2850 (s,
1
br), 1736 (m), 1226 (m); H NMR (300 MHz; D O): d
2
We thank Kenneth Bousman (SUNY) for his help
with recording the CD spectra and Jeff Chandler for col-
lecting the roots. We are also grateful to Dr. N.S.K. Rao
2
2
.14 (2H, m, CH CH CH), 2.45 (2H, m, CH CH CH),
2 2 2 2
.83 (1H, dd, J = 8.3, 13.9 Hz, SCH CH-a), 2.98 (1H,
2
dd, J = 4.5, 13.9 Hz, SCH CH-b), 3.76 (1H, m, Glu-
CH), 3.81 (2H, s, CH SCH ), 4.34 (1H, dd, J = 4.5,
2
(SUNY) for his assistance in the acquisition of 2D
2
2
1
3
NMR spectra. Support for this work was provided by
the National Science Foundation (Grant 0239755).
8
.0 Hz, Cys-CH), 7.33–7.42 (5H, m, Harom); C NMR
(
(
(
(
(
75 MHz; D O): 26.8 (CH CH CH), 31.7
CH CH CH), 33.6 (SCH CH), 35.8 (CH SCH ), 54.5
Cys-CH), 54.6 (Glu-CH), 127.6 (Carom,para), 129.0
Carom,meta), 129.1 (Carom,ortho), 138.7 (Carom,q), 174.3
d
2
2
2
2
2
2
2
2
References
Fenwick, G.R., Hanley, A.B., 1985. The genus Allium – part 2. Crit.
Rev. Food Sci. Nutr. 22, 273–377.
Kasai, T., Larsen, P.O., 1980. Chemistry and biochemistry of c-
glutamyl derivatives from plants including mushrooms (Basidio-
mycetes). Prog. Chem. Nat. Prod. 39, 173–285.
Glu-COOH), 174.5 (NHCOCH ), 177.1 (Cys-COOH).
2
+
MALDI-HRMS [MH ] 341.1169 (C H N O S req.
1
5
20
2
5
3
41.1166).
3
(
.4.2. (S R S )-c-Glutamyl-S-benzylcysteine sulfoxide
Kasai, T., Sakamura, S., 1973. NMR spectra of glutamic acid-
containing dipeptides in relation to sequence determination. Agri.
Biol. Chem. 37, 2155–2157.
King, F.E., Kidd, D.A.A., 1949. A new synthesis of glutamine and of
c-dipeptides of glutamic acid from phthalylated intermediates. J.
Chem. Soc, 3315–3319.
Kubec, R., Musah, R.A., 2001. Cysteine sulfoxide derivatives in
Petiveria alliacea. Phytochemistry 58, 981–985.
Kubec, R., Kim, S., Musah, R.A., 2002. S-Substituted cysteine
derivatives and thiosulfinate formation in Petiveria alliacea – part
II. Phytochemistry 61, 675–680.
C2 C7
S
2a, c-L-glutamyl-petiveriin B)
2
D
2
A white hygroscopic solid. M.p. 138–140 ꢁC; ½aꢁ :
ꢀ
ꢀ
26.2ꢁ (H O; c 0.06); CD De
(H O; c 0.06; 22 ꢁC):
cm : 3500–2850 (s, br),
2
max
2
ꢀ1
2.7 (225 nm); IR (KBr) v
max
1
1
2
610 (s, br), 1015 (s); H NMR (300 MHz; D O): d
2
.15 (2 H, m, CH CH CH), 2.48 (2H, t, J = 7.6 Hz,
2
2
CH CH CH), 3.11 (1H, dd, J = 8.3, 13.6 Hz,
SOCH CH-a), 3.49 (1H, dd, J = 5.5, 13.6 Hz,
2
2
2
SCH CH-b), 3.80 (1H, t, J = 6.2 Hz, Glu-CH), 4.14
2
Kubec, R., Kim, S., Musah, R.A., 2003. The lachrymatory principle of
Petiveria alliacea. Phytochemistry 63, 37–40.
(
1H, d, J = 13.2 Hz, CH SO-a), 4.35 (1H, d,
2
Lancaster, J.E., Shaw, M.L., 1989. c-Glutamyl peptides in the
biosynthesis of S-alk(en)yl-L-cysteine sulphoxides (flavour precur-
sors) in Allium. Phytochemistry 28, 455–460.
Lancaster, J.E., Shaw, M.L., 1991. Metabolism of c-glutamyl peptides
during development, storage and sprouting of onion bulbs.
Phytochemistry 30, 2857–2859.
J = 13.2 Hz, CH SO-b), 4.66 (1H, dd, J = 5.5, 8.3 Hz,
2
1
3
Cys-CH), 7.39–7.49 (5H, m, Harom);
C
NMR
1
3
(
75 MHz; D O): C NMR (75 MHz; D O; DSS): d
2
2
2
6.3 (CH CH CH), 31.7 (CH CH CH), 50.2 (Cys-
2 2 2 2
CH), 53.3 (SOCH CH), 54.3 (Glu-CH), 56.9
2
Matsuura, H., Inagaki, M., Maeshige, K., Ide, N., Kajimura, Y.,
Itakura, Y., 1996. Changes in contents of c-glutamyl peptides and
fructan during growth of Allium sativum. Planta Med. 62, 70–71.
Orlowski, M., Meister, A., 1971. Partial reactions catalyzed by c-
glutamylcysteine synthetase and evidence for an activated gluta-
mate intermediate. J. Biol. Chem. 246, 7095–7105.
(
1
1
CH SOCH ), 129.0 (Carom,para), 129.3 (Carom,meta),
2 2
29.5 (Carom,q), 130.7 (Carom,ortho), 173.9 (Glu-COOH),
74.3 (NHCOCH ), 174.7 (Cys-COOH). MALDI-
2
+
HRMS
57.1115).
[MH ]
357.1115
(C H N 0 S
req.
1
5
20 2 6
3