CO2_2016 - page 15

13
Chimica Oggi - Chemistry Today
- vol. 34(2) March/April 2016
of 2-hexenoic acid from fragment ion m/z 395. However, the
fragment ion at m/z 309 (HB
2
HH
1
) may be formed from fragment
ion m/z 395 due to the loss of crotonic acid or from the
fragment ion m/z 423 by the expulsion of 2-hexenoic acid. Thus,
the MS
2
fragment ion spectrum indicates the random distribution
of HB and HH units within copolymer chains.
Sequence distribution in microbial PHA reviled by ESI-MS
n
was
also assessed by NMR measurements and both techniques
gave results that were in good agreement for all investigated
samples (6). Recently, this relatively rapid ESI-MS
n
approach,
developed at our laboratories, has been adopted for the
routine analysis of PHA copolymers by other authors (7).
SEQUENCE ANALYSIS OF SYNTHETIC BIODEGRADABLE
COPOLYESTERS WITH ESI-MS
n
The ESI-MS
n
was successfully applied for the sequencing of
copolyester oligomers with a composition ranging from 34–77
mol% of (R,S)-3-HB units, obtained
via
polytransesterification
reactions conducted in bulk between racemic (R,S)-
β
-
butyrolactone and 6-hydroxyhexanoic acid or (R,S)-2-
hydroxyhexanoic acid, respectively as presentedoin Scheme 2 (8).
The arrangements of co-monomer structural units along the
oligocopolyester chains were determined by sequencing
with the aid of ESI MS
n
and investigation of the respective
fragmentation pathways. The complete structural and
SEQUENCE ANALYSIS OF NATURAL BIODEGRADABLE
COPOLYESTERS WITH ESI-MS
n
Among biodegradable polymers polyhydroxyalkanoates
(PHA) offer a range of processing and performance
improvements since they exhibit thermal and mechanical
properties that are similar to petroleum-based plastics.
Being bio-based and biodegradable PHA are commercially-
valuable materials with many potential industrial applications.
PHA properties may be adjusted by varying molecular
structure, average co-monomer composition and co-
monomer composition distribution along with molecular mass
distribution.
The electrorpray ionisation-mass spectrometry (ESI-MS)
method has been originally applied for the determination
of the co-monomer unit composition and composition
distribution in bacterial PHA copolymers based upon
the analysis of their oligomers obtained by partial
alkalinendepolymerisation, as presentedoin Scheme 1 (4).
In order to reconstruct the sequence of the un-degraded
copolymer from the examination of a partially degraded
sample the ESI-MS
n
technique was subsequently used for the
analyses of such oligomers,tThe structural characteristics of
mass-selected macromolecular ions have been determined,
providing information about the structure of PHA copolymers
at the molecular level. For example, in order to verifd the
structure of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)
biopolyester (PHBH), oligomers obtained by partial
alkaline depolymerisation of this natural copolymer that
contained 13–14 mol% of hydroxyhexanoate (HH) units were
investigated, and its microstructure was assessed up to the
level of 28 repeat units (5). The subtle structural details of the
PHBH were evaluated based upon sequencing of individual
quasimacromolecular ions thus showin, that both HH and
hydroxybutyrate (HB) units of the PHBH copolymer were
randomly distributed (Figure 1).
Some of the possible combinations of the co-monomer
distribution for the sodium adduct ion of HB
3
HH
2
oligomer are
presentedoin Figure 1. The MS
2
experiment conducted for this
ion indicated two sets of fragment ions with a 28 Da spacing.
The fragment ion at m/z 423 corresponds to the HB
2
HH
2
oligomer
formed by the loss of crotonic acid (86 Da), and the ion at m/z
395 corresponds to the HB
3
HH
1
oligomer formed by the expulsion
of 2-hexenoic acid (114 Da) from the HB
3
HH
2
sodium adduct ion.
The fragment ion at m/z 281 (HB
3
) is formed due to the expulsion
Scheme 1.
Figure 1.
MS
2
fragment ion spectrum obtained for the molecular
sodium adduct ion of HB
3
HH
2
at m/z 509, selected from the
5-mer cluster of PHBH oligomers.
Scheme 2.
1...,5,6,7,8,9,10,11,12,13,14 16,17,18,19,20,21,22,23,24,25,...68
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