Identification of degradation products of indigoids by tandem mass spectrometry

Article abstract of DOI:10.1002/jms.3641

The study concerns identification of photodegradation products of indigotin, indirubin and isoindigo. Experimental methodology consists of degradation of standard solutions of indigoids in a solar box and analysis of samples taken at different aging time by using capillary high-performance liquid chromatography coupled with electrospray ionization tandem mass spectrometric and spectrophotometric detectors. Identification of the formed compounds was based on careful interpretation of the electrospray ionization MS/MS spectra. Apart from the well-known degradation products of indigoids: isatin, isatoic anhydride and anthranilic acid, another seven species were also identified, and their proposed structures were confirmed by high-resolution molecular masses measurements; according to the best knowledge of authors, they have not been reported so far. The obtained results formed the basis for postulating mechanism of the process. Moreover, the MRM (Multiple Reaction Monitoring) method was developed for the identification of natural dyes and their degradation products in textiles of historical value. Apart from such colorants as indigotin and flavonoids, also presence of degradation products of indigoids was confirmed.

Full text of DOI:10.1002/jms.3641

Journal of
MASS
SPECTROMETRY
Research article
Received: 28 May 2015
Revised: 5 August 2015
Accepted: 10 August 2015
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/jms.3641
Identification of degradation products of
indigoids by tandem mass spectrometry
Katarzyna Witkoś, Katarzyna Lech and Maciej Jarosz*
The study concerns identification of photodegradation products of indigotin, indirubin and isoindigo. Experimental methodology
consists of degradation of standard solutions of indigoids in a solar box and analysis of samples taken at different aging time by
using capillary high-performance liquid chromatography coupled with electrospray ionization tandem mass spectrometric and
spectrophotometric detectors. Identification of the formed compounds was based on careful interpretation of the electrospray
ionization MS/MS spectra. Apart from the well-known degradation products of indigoids: isatin, isatoic anhydride and anthranilic
acid, another seven species were also identified, and their proposed structures were confirmed by high-resolution molecular
masses measurements; according to the best knowledge of authors, they have not been reported so far. The obtained results
formed the basis for postulating mechanism of the process. Moreover, the MRM (Multiple Reaction Monitoring) method was
developed for the identification of natural dyes and their degradation products in textiles of historical value. Apart from such
colorants as indigotin and flavonoids, also presence of degradation products of indigoids was confirmed. Copyright © 2015 John
Wiley & Sons, Ltd.
Keywords: degradation of indigoid dyestuffs; photostability of indigoids; high-performance liquid chromatography; tandem electrospray
mass spectrometry
isatin,[13,15,16] isatoic anhydride[13,15,16] and anthranilic acid.[13,16]
Introduction
Less common, 2-benzyl-2-indole,[15] as well as small organic
compounds[17] were also reported. Similar process is observed in
Natural dyes were used for thousands of years for painting and
dyeing of textiles. They had been used until the end of the 19th
century, when in short time they were replaced with cheaper
synthetic dyes, produced in many colors and shades. Colorants
were obtained from various parts of plants or animals.[1] One of
the most widely used natural dyes is indigo.[2] Its most popular
sources were Isatis Tinctoria, Indigofera Tinctoria, Indigofera
Suffruticosa, Indigoferra Arrecta and Polygonum Tinctorium.[3] Plants
contain precursors of color compounds: indican and isatan B,
which can be converted to indigotin by mild oxidation.[4] Apart
from indigotin, isoindigotin, indirubin and isatin are also formed
in this process.[5]
Along a time, works of art containing colorants undergo degra-
dation under an influence of many external factors, e.g. oxygen,
light, humidity and temperature.[6] Exposure to light is a main
factor of fading that resulted from structural changes of color
compounds.[7,8] Understanding of the mechanism of the process
is important for conservators, as it allows to choose the best resto-
ration procedure.[9,10]
the case of indigo carmine (sulfonated derivative of indigotin)
where sulfonic isatin and sulfonic isatoic anhydride are identified
as the main degradation products.[18,19]
Because of lack of standards of formed compounds, for their
identification, mainly mass spectrometry with electrospray
ionization[19–21] or with electron impact ionization[13,16] were
used, also in tandem configuration.[13,18,19] For the separation of
complicated mixture of degradation products, prior to MS detec-
tion, usually, liquid chromatography was recommended.[17,19,21]
Gas chromatography was also used, but with additional derivati-
zation step.[16,17,20,22] More information concerning molecular
structures of the formed species offered tandem mass
spectrometry.[23] The observed main neutral losses were H2O,
CO, CO2 and HCN, and they allowed to identify functional groups
present in degradation products.[24,25]
The aims of the study were to identify photodegradation prod-
ucts of selected indigoids, color constituents of natural indigo
(Fig. 1), as well as to elaborate method for indirect identification
of natural dyestuffs based on identification of their degradation
products.
Indigo is one of the most photostable natural dyes. This is due to
absence of a photoinduced trans ➔ cis isomerization (twisting of
the central C = C bond).[11] Nevertheless, after enough time, degra-
dation of indigo is also observed.
Till now, only few studies concerning identification of indigo
degradation products have been published. In model procedure
examined, preparations were aged and formed degradation
products were separated and identified.[12] Several methods of
accelerated aging were proposed; usually exposure to the light
in a solar box,[13] but also enzymatic[14] or ozone treatment.[15]
There are a few well-known degradation products of indigoids:
*
Correspondence to: Maciej Jarosz, Chair of Analytical Chemistry, Faculty of
Chemistry, Warsaw University of Technology, Noakowskiego 3, 00–664 Warsaw,
Poland. E-mail: mj@ch.pw.edu.pl
Chair of Analytical Chemistry, Faculty of Chemistry, Warsaw University of
Technology, Noakowskiego 3, 00-664, Warsaw, Poland
J. Mass Spectrom. 2015, 50, 1245–1251
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Journal of
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SPECTROMETRY
K. Witkoś, K. Lech and M. Jarosz
operations, acquiring and analysis of data were processed by
MassHunter Software (Agilent Technology, USA). Conditions for
separation and detection are shown in the Additional Supporting
Information 1 (ASI 1). Parameters of the developed MRM method
for each reaction are shown in Table 1.
Ultra-performance liquid chromatograph (UPLC) ACQUITY I-Class
(Waters Inc) coupled with Synapt G2-S HDMS (High Definition Mass
Spectrometry) (Waters Inc) mass spectrometer equipped with an
electrospray ion source and Q ToF (Quadrupole/Time of Flight)
type mass analyzer was used. The instrument was controlled, and
Figure 1. Molecular structures of: (a) indigotin, (b) indirubin and (c)
isoindigotin.
Materials and methods
Instrumentation
Capillary high-performance liquid chromatography coupled
with electrospray ionization tandem mass spectrometry (HPLC ESI
MS/MS) analysis were performed with the use of Agilent 1200
series chromatograph (Agilent Technology, Waldbronn, Germany)
equipped with a binary capillary pump, degasser, autosampler,
thermostatically controlled column compartment, diode array
detector with 80 μl flow cell and electrospray ionization triple
quadrupole mass spectrometer (Agilent 6460 Triple Quad
LC/MS, Agilent Technologies, Santa Clara, CA, USA). All the
Figure 2. μ-High-performance liquid chromatography electrospray
ionization MS chromatograms of indigotin degradation products. Samples
taken after (a) 5 h; (b) 15 h; (c) 30 h; (d) 30 h. Black lines – registration in
positive ion mode, gray ones – negative ion mode.
Table 1. Characteristics of Multiple Reaction Monitoring method for the identification of colorants and degradation products by μ-high-performance
liquid chromatography electrospray ionization MS/MS
Tr, min
Compound
Ion mode
Precursor ion, m/z
Fragmentator energy, V
Fragment ions, m/z (CE,V)
122 (6), 92 (13)
10,8
10,8
12,5
16,1
16,6
16,8
16,8
17,8
18,1
19,0
19,3
20,9
21,8
22,2
22,2
22,9
23,0
23,6
23,6
24,8
25,5
26,0
26,6
Degradation product 1
Isatin
À
À
À
À
À
À
À
À
À
À
À
À
À
+
166
146
162
475
164
491
194
447
178
431
491
491
194
235
285
293
313
269
329
239
263
327
255
80
80
118 (7)
Degradation product 3
dcII
80
118 (8)
100
80
431 (14), 341 (23), 311 (24), 282 (41)
136 (5), 120 (10), 92 (20)
447 (15), 357 (24), 327 (25)
150 (8), 136 (10,) 120 (15), 92 (20)
285 (24)
Degradation product 4
Carminic acid
100
90
Degradation product 5
Luteolin-7-O-glucoside
Degradation product 6
Apigenin-7-O-glucoside
dcIV
140
80
136 (5), 134 (5), 92 (15)
268 (31)
140
100
100
60
447 (15), 357 (24), 327 (26), 284 (32)*
447 (15), 357 (24), 327 (26), 284 (32)*
162 (2), 118 (12)
dcVII
Degradation product 7
Degradation product 8
Luteolin
100
120
100
80
217 (25), 206 (40), 190 (40)
241 (11), 135 (12), 133 (20)
265 (10), 146 (25), 90 (30)
269 (5), 257 (17)*
À
+
Degradation product 9
Flavokermesic acid
Apigenin
À
À
À
À
+
100
80
151 (17), 149 (17), 117 (32)
285 (4), 257 (18)
Kermesic acid
Alizarin
140
60
211 (19), 210 (26), 167 (22)
235 (22), 219 (23), 206 (38), 132 (45), 77 (52)
162 (5), 136 (10), 118 (30)
227 (18), 171 (26), 129 (32)
Indigotin
Degradation product 10
Purpurin
À
À
100
140
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Degradation products of indigoids
recorded data were processed using MassLynx V4.1 software
package (Waters Inc).
Accelerated aging of the dyestuffs was performed with the use of
a solar box SunTest CPS+ (Atlas, Chicago, USA) equipped in a xenon
lamp (200–900 nm; 500 W/m2) with an outdoor filter. Solutions for
analysis were sampled every 5 h, up to 60 hs of aging.
Standard stock solutions (100 μg/ml) of indigotin and isoindigotin
were prepared by dissolving 1 mg of each preparation in 10 ml of di-
methyl sulfoxide (DMSO), and indirubin – in methanol. The obtained
solutions were filtered over a 0.45 μm syringe filter (Supelco, Sigma-
Aldrich, Bellefonte, PA, USA).
Historical samples
Chemicals
The examined fibers were taken from tapestry ‘Scenes from the
Opera’ – collection of the Museum of King John III’s Palace at
Wilanów (Poland). They were extracted with 50μl of DMSO and
formic acid (97:3, v/v). The obtained solutions were kept in an
ultrasonic bath for 5 min and in a water bath at 80 °C for 5 min.
Then, they were filtered over a 0.22μm PET (polyester) syringe filter
and diluted with 50μl of methanol and water (2:3, v/v) followed
by analysis.
Indigotin was purchased from Fluka. Indirubin and isoindigotin
have been synthesized according to Puchalska.[26] Formic acid of
LC/MS purity from Fisher Scientific (Fair Lawn, NJ, USA) was
obtained. Methanol (LC-MS grade) and dimethyl sulfoxide (HPLC
grade) were purchased from POCH (Gliwice, Poland). Demineralized
water from Milli-Q system Model Milipore Elix 3 (France) was used
throughout.
Figure 3. Electrospray ionization tandem mass spectrometry (MS/MS) spectra of small degradation products of indigotin. Numbers describing spectra reflect
elution order of identified compounds (cf. Fig. 2).
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K. Witkoś, K. Lech and M. Jarosz
Molecules of compounds 1–7 are relatively small and, according
to nitrogen rule, contain one nitrogen atom (Fig. 3); they are formed
by cleavage of the double bond between indol moieties occurring
in indigotin structure. Careful evaluation of their fragmentation
patterns creates the basis for drawing hypotheses concerning
molecular structures. Typical observed losses were CO2 and CO
from carboxyl and carbonyl groups. Lost more characteristic
fragments: formaldehyde (compound 1), oxalic aldehyde (5),
ethylenone (6) as well as methanol (7), allowed to propose more
complicated structures. Identification of compounds 2 and 3 was
confirmed by analysis of their standards, as they are well-known
degradation products of indigotin: isatin (2) and isatoic anhydride
(3). Moreover, in order to confirm identity of compound 7, synthesis
Results and discussion
Products of indigotin degradation – MS/MS study
Samples taken after different aging time were analyzed by using
capillary μ-HPLC UV Vis ESI QqQ MS. On the obtained chromato-
grams, ten peaks were observed (Fig. 2); signals of compounds
1–7 and 10 could be registered when ESI MS detection was
performed in negative ion mode and 8 and 9 – exclusively in
positive ions mode. Only protonated ions were observed in
positive ion mode. It has to be mentioned, that compounds 6
and 8 were not observed, when UV detection was used.
The most abundant MS signals were attributed to the potential
quasi-molecular ions and analyzed in product ion mode. The
obtained MS/MS spectra of separated degradation products
allowed identification of lost fragments and, in consequence,
propose their structures.
[27]
of N-methoxycarbonyl-anthranilic acid according to Erdmann
was performed. Its retention time (tr = 32.1min) registered on
chromatogram with UV detection at 254 nm, as well as MS/MS
spectrum, were identical with those of the compound 7. Addition
Figure 4. Electrospray ionization tandem mass spectrometry (MS/MS) spectra of compounds eluted in peaks 8 (a) and 9 (b) in chromatogram of indigotin
degradation products (cf. Fig. 2).
Figure 5. Fragmentation pathway of the ion at m/z 327 (compound eluted in peak 10 in the chromatogram of indigotin degradation products).
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Degradation products of indigoids
of the synthesized acid to the sample of photodegraded indigo
resulted in the increase of the peak 7, as well.
Mechanism of indigotin degradation
There are many theories concerning photodegradation of organic
compounds as well as the role of oxygen in this process. They gen-
erally agree that the first step of degradation involves photoexcita-
tion of species present in examined solution (e.g. analyte or solvent
molecules),[8] As a result of further energy transfers, different
reactive species may be formed,[8,29–31] among others OH• free
radicals. Under experimental conditions, their active role seems to
be the most probable. However, also, another degradation pathway
cannot be excluded, as in the present study, only species
detectable with the use of UV-Vis and ESI MS were investigated.
Assuming, that OH• free radicals (or another oxygen species
activated by light) attack an aromatic ring or unsaturated bond or
cause abstraction of a hydrogen atom,[32] allows to propose
probable mechanism of indigotin degradation (Fig. 6).
In contrast to what was previously discussed, molecules of
compounds 8–10 are bigger and, according to nitrogen rule,
contain two nitrogen atoms. MS/MS spectrum of compound 8 is
shown in Fig. 4(a). The registered fragment ions are the same as
those observed in spectrum of indigotin.[28] Moreover, ion at m/z
235 (parent for compound 8) was identified in spectrum of indigo-
tin as fragment obtained by loss of carbon oxide. Hence, we would
like to propose that compound 8 most probably is decarbonylated
indigotin.
Molecular masses of compounds 9 and 10 are higher than mass
of indigotin and it creates basis for the hypothesis, that they are
products of photooxidation of the latter. In MS/MS spectrum of
compound 9, the precursor ion at 293 m/z is observed (Fig. 4(b)).
The ion registered at m/z 146 is formed by loss of the fragment of
m/z 147, what suggests cleavage of the bond between rings within
molecule of symmetrical structure. Formation of the ion at m/z 90
by further losses of two carbon oxide molecules confirm presence
of two oxygen atoms in each ‘half molecule’.
Compounds identified in solutions containing indigotin degrada-
tion products (cf. Figs 3–5) were probably formed as the result of
cleavage of the following bonds (numbering of atoms in indigotin
molecule is shown on Fig. 1):
MS/MS spectrum of compound 10 is much more complicated. Its
careful interpretation allowed to propose fragmentation pathway
of the precursor ion at 327 m/z shown in Fig. 5. In the scheme,
two directions are distinguished: loss small molecules, e.g. carbon
mono or dioxide or water (ions at m/z 309, 255, 211, 193), and cleav-
age of the bond between rings within molecules (ions at m/z 162,
146, 136). Further fragmentation of obtained ions by loss of small
molecules was also observed.
The above hypothesis concerning structures of quasi-molecular
and fragment ions of degradation products of indigotin was
confirmed by their high-resolution spectra registered by using
UHPLC-ESI Q TOF MS. Accurate molecular masses were measured
and elemental compositions of the separated species were settled
as well (Additional Supporting Information 2, ASI 2).
-
-
-
C1–C1′ and C1-C9: compounds 1 and 7;
C1–C1′: compounds 2 and 3;
C1–C9, C1′–C9′ and C1′–C2′: compound 6.
Primary degradation products of indigotin may undergo further
transformations. Hence:
-
Cleavage of bond C1–N2 in compound 2 causes formation of
compound 4;
Oxidation of compound 6 (attack of hydroxyl radical on C
-
atom in amide group) leads to compound 5.
More complicated seem to be the mechanisms of the formation
of compounds 8, 9 and 10, as here no cleavage of the bond C1–C1′
Figure 6. Proposed mechanism of degradation of indigotin.
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K. Witkoś, K. Lech and M. Jarosz
Table 2. Colorants and degradation products identified in historical
samples
hydroxyl radical. Nevertheless, its spectrum obtained with use of
Q ToF MS confirms such structure.
Color of fiber
Dark brown
Colorants
Degradation
products
3–1
Carminic acid
3
5
Other indigoids – indirubin and isoindigotin
dcII
dcVII
10
Indigotin and indirubin after 5 h of aging are completely degraded;
in the case of isoindigotin, even after 15 h of exposition to light, it is
still present in examined sample among its degradation products.
On chromatograms of solutions containing aging products of
idigotin, indirubin or isoindigo, all 1–7 compounds are registered.
However, they can be easily differentiated, as among degradation
products of indirubin, compounds 8 and 9 are not observed and
among these of isoindigo – compound 10 (characteristics of degra-
dation products of indigoids is presented in Additional Supporting
Information 2).
Kermesic acid
Flavokermesic acid
Indigotin
Isatin
Luteolin-7-O-glucoside
Carminic acid
Alizarin
3–4
Dark brown
3
4
Purpurin
5
Indigotin
7
Apigenin
10
Luteolin
Apigenin-7-O-glucoside
Luteolin-7-O-glucoside
Kermesic acid
Flavokermesic acid
dcII
Historical samples
The examined fibers taken from the tapestry ‘Scenes from the
Opera’ were extracted and analyzed by MRM method by using
μ-HPLC ESI MS/MS. The obtained results are shown in Table 2.
On the chromatogram of the extract from green fiber (Fig. 7),
there are nine peaks observed. Major peaks correspond to luteolin,
apigenin, their glucosides and indigotin. In this case, green color of
the fiber was obtained by mixing blue and yellow dyestuffs. Indigo-
tin could originate from either woad (Isatis tinctoria L.) or indigo
(Indigofera tinctoria L.). The identified flavonoids indicate that the
used yellow dyestuff was weld (Reseda luteola L.). In the chromato-
gram, there are also peaks of degradation products of indigoids 3, 5
and 10.
dcIV
dcVII
Isatin
3–10
Green
Luteolin-7-O-glucoside
Luteolin
3
5
Indigotin
7
Apigenin-7-O-glucoside
Apigenin
10
Major colorants.
Minor colorants.
Trace colorants.
Extract of analyzed brown fiber contained indigotin and red col-
orants – carminic acid and kermesic acid. It indicated that this
textile was dyed with cochineal (Dactylopius coccus Costa) and
indigo (or woad). Another fiber contained also anthraquinones: aliz-
arin and purpurin. In this case, probably, madder (Rubia tinctorum)
was also a component of the dyeing mixture.
All extracts from the examined fibers contained degradation
products of indigoids, mainly compounds 3 and 10, but also 4, 5
and 7 were identified. It proved that the developed methodology
(MRM method) can be a useful tool for identification of natural dyes
based on their degradation products, also in the case when
examined work of art is totally discolored.
is observed. Compound 9 is probably formed as the result of the
attack of hydroxyl radical on C1–C1′ bond, but without its
cleavage and formation of six-member rings after incorporation of
oxygen atoms (similar mechanism of degradation of Indigo
Carmine – indigotindisulfonate – has already been proposed).
Further oxidation of 9 leads to structure 10.
[18]
Formation of compound 8 is rather unexpected and cannot be
explained by simple red-ox mechanism involving participation of
Figure 7. μ-High-performance liquid chromatography electrospray ionization tandem mass spectrometry (MS/MS) chromatogram of the extract from the
green fiber (MRM detection).
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Degradation products of indigoids
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Conclusions
Capillary HPLC coupled with ESI MS/MS, first time (according to the
best knowledge of the authors) used for identification of natural
colorants and products of their degradation obtained during artifi-
cial aging, was proven to be an efficient tool in this field, allowing to
analyze samples as small as of volume 0.1μl.
The developed methodology allows to differentiate indigotin,
indirubin and isoindigo; as a result of photodegradation of each
compound, different number of products is formed. Moreover,
the MRM method can be used for the examination of even
discolored textiles, as it allows to identify indigoid colorants, thanks
to the identification of products of their degradation.
[18] A. P. F. M. de Urzedo, C. C. Nascentes, M. E. R. Diniz, R. R. Catharino,
M. N. Eberlin, R. Augusti. Indigo Carmine degradation by hypochlorite
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Acknowledgement
The authors are indebted to Prof. Witold Danikiewicz for fruitful
discussions and affording possibilities for UPLC ESI Q TOF MS
measurements. This work has been partially supported by the
Ministry of Science and Higher Education grant no. N N204 247040.
[21] M. Ramya, B. Anusha, S. Kalavathy. Decolorization and biodegradation
of Indigo carmine by
a textile soil isolate Paenibacillus larva.
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materials for photocatalytic degradation of indigo carmine - synthesis,
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J. Mass Spectrom. 2015, 50, 1245–1251
Copyright © 2015 John Wiley & Sons, Ltd.
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