Mechanistic study of the collision-induced dissociation of sodium-cationized
polylactide oligomers : A joint experimental and theoretical investigation
Julien De Winter,a Vincent Lemaur,b Philippe Marsal,b,d Olivier Coulembier,c Jérôme Cornil,b Philippe Dubois,c Pascal Gerbaux a
a Mass Spectrometry Research Group, Interdisciplinary Center of Mass Spectrometry (CISMa),
b Laboratory for Chemistry of Novel Materials,
c Center of Innovation and Research in Materials and Polymers (CIRMAP), Laboratory of Polymeric and Composite Materials,
University of Mons, 20 Place du Parc, B-7000 Mons
Julien.Dewinter@umons.ac.be
d CINaM-CNRS, Université Aix-Marseille 2 & 3, Campus de Luminy, Case 913 13288 Marseille Cedex 9, France
Introduction
Amongst the biodegradable polyesters, polylactic acid (PLA) is nowadays a product of great interest and technological developments needed for mass production of high
molecular weight PLA at low cost are thus awaited. Ring-opening polymerization of lactide, a cyclic dimer of lactic acid, is the most important method of synthesizing
biodegradable and biocompatible PLA. In the case of macromolecular engineering, the study of PLA end groups is necessary. In this optic, it is suggested to analyze sodiumcationized polylactide oligomers by tandem mass spectrometry (MS/MS). Indeed, the collision induced dissociation spectrum is often considered as characteristic of the
structure of the investigated ions and so should be oligomers’end group specific. In the present report, the tandem mass spectrometry characteristics of different sodiumcationized PLA oligomers were investigated.
Materials
The polylactide oligomers were prepared following a state of the art polymerization procedure that relies on the use of
a non-organometallic catalysis. The SEC and NMR characterized oligomers were then subjected to Electrospray
Ionization. The obtained sodium-cationized oligomers were mass selected and their low-kinetic energy CID mass
spectra were recorded. All the experiments were performed on a QToF instrument. Since the systems are too large to
be fully optimized at a quantum-chemical level, the most stable conformation of each system was first determined by
using molecular mechanics and molecular dynamics calculations based on the Universal Force Field. DFT calculations
(B3LYP/6-31 g(d,p)) have also been performed to evaluate the total energy of the different systems without further
geometry optimizations.
O
CH3
CH 3-PLA-OH
O
Acetylation
O
O
CH3
OH
O
O
O
CH 3-PLA-acetyl
O
O
O
n
1H-NMR
n
α-methyl, ω-hydroxy polylactide
α-methyl, ω-acetyl polylactide
Does the 72u loss involve the H from the hydroxyl end-group?
Results and discussion
A Experiments
McLafferty-Like fragmentation
1° CID spectrum of Na+ cationized CH 3-PLA-OH
H
O
O
O
CH3O
H
11
2° CID spectrum of Na+ cationized linear CH3-PLA-acetyl
Na

Na
O
O
O
O
O
H
7
?
O
HO
OH
H
O
8
CH3O
m/z 761
MW = 86 u
OH
CH3-PLA-acetyl
O
O
CH3O
A
O
O
O
O
ion series B
ion series A

Na+
CH3O
spectra before and after acetylation
B
O
O
O
O
CH3O
O
O
O
O
O
O
O
O
6
Na
m/z 847
Na
O
O
O
CH3
O
m/z 889
O
O
O
O
H
- 86 u
OH
- 60 u
OH
8
m/z 757
MW = 90 u
The 2 end-group specific fragments
No!
3° Proposed mechanism for the unspecific fragment
To get more information and to prove
that the loss of 72 u from the middle of
the chain is competitive with the endgroups specific fragmentations, some
calculations were performed
O
O
O CH3
O CH3
O
HO
H
O
O
O
O
O
Na
HO
O
Collisionnal
O
activation
O
O
O CH3
O
Na
HO
O
Na
H
O
O
C
O
CH3
O
O
O
C CH3
O
O
O
C
CH3
O
B Theoretical Calculations
The formation of all the
complexes is thermodynamically
competitive
Molecular Dynamics to prove that
the elimination of a neutral is
kinetically competitive
Conclusion
Investigations of end-groups modified PLA reveal that, upon collisional activation, PLA.Na+ systematically suffers from end-group specific dissociations. In addition to those
structurally -indicative dissociations, we demonstrated that collisionally-excited PLA.Na+ competitively suffers from two consecutive backbone cleavages leading to the loss
of an unspecific fragment (72u). The experimental observations, performed on a hydrid Q-ToF instrument, were totally corroborated by a theoretical study involving DFT
calculations, molecular mechanics and molecular dynamics simulations.
Acknowledgments
The MS laboratory acknowledges the “Fonds de la Recherche Scientifique (FRS-FNRS)” for its contribution to the acquisition of the QToF2 Mass Spectrometer. J.C., P.G.,
O.C., and J.D.W. are FNRS research fellows. This work was partially supported by the Belgian Federal Government Office of Science Policy (SSTC-PAI 6/27).
In Press DOI: 10.1016/j.jasms.2010.03.026