MALDI TOF mass spectrometry for the characterization of phosphorus-containing dendrimers. Scope and limitations

Article abstract of DOI:10.1021/ac0003854

Neutral phosphorus-containing dendrimers with aldehyde groups at the periphery have been analyzed using matrixassisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOFMS) up to generation four. Although the expected quasi-molecular

Full text of DOI:10.1021/ac0003854

Anal. Chem. 2000, 72, 5097-5105
MALDI TOF Ma s s S p e c t ro m e t ry fo r t h e
Ch a ra c t e riza t io n o f P h o s p h o ru s -Co n t a in in g
De n d rim e rs . S c o p e a n d Lim it a t io n s
J e a n-Cla ude Bla is ,*,† C e´ dric -Olivie r Turrin, Anne -Ma rie Ca m ina de , a nd J e a n-Pie rre Ma jora l
‡
‡
‡
Laboratoire de Chimie Structurale Organique et Biologique, CNRS (UMR 7613), Universit e´ Pierre et Marie Curie,
4
Place Jussieu, 75252 Paris Cedex 05, France, and Laboratoire de Chimie de Coordination, CNRS, 205 route de
Narbonne, 31077 Toulouse Cedex 04, France
Neutral phosphorus-containing dendrimers with aldehyde
groups at the periphery have been analyzed using matrix-
assisted laser desorption ionization time-of-flight mass
spectrometry (MALDI-TOFMS) up to generation four.
Although the expected quasi-molecular ion is generally
observed, the mass spectral pattern, presence of frag-
ments and adducts related to the original skeleton, is
highly relevant to the sample preparation (nature of the
matrix: 2 -5 -dihydroxybenzoic acid (2 .5 -DHB), 1 ,8 -dihy-
droxy-9 [1 0 H ]-anthracenone (dithranol), 6 -azathiothy-
mine, 2 ,4 ,6 -trihydroxyacetophenon, 7 -hydroxycoumarin
or 2 -anthramine, and addition of alkali metal salts). The
dithranol matrix with addition of LiI offers milder condi-
tions; however, abundant fragments are still observed for
the higher generation dendrimers. Investigation of these
effects in connection with SEC, NMR, and MALDI-TOFMS
studies of UV preirradiated dendrimers allows the as-
sumption to be made that fragmentation occurs in MALDI
due to the relatively strong absorption of the dendrimers
at 3 3 7 nm. Fragmentations and formation of adducts
involve nitrogen-nitrogen bond cleavage, imine meta-
thesis, and reaction of aldehyde groups with internal
imino groups.
(however not useful for higher generations), size exclusion
chromatography (SEC), laser-light scattering, osmometry and
mass spectrometry using mainly electrospray ionization (ESI),1
0-15
and matrix-assisted laser desorption ionization (MALDI).1
6-23
A
few structures, concerning only small dendrimers (up to genera-
tion two), have been determined by X-ray diffraction.
In the case of phosphorus-containing dendrimers, the main
characterization was done by 31P NMR. Indeed, 31P NMR spec-
troscopy has been shown to be an extraordinary tool for following
the growth of these macromolecules and for controlling the
progress of all reactions performed either in the internal layers
or on the surface.2
4-37
Fast-atom bombardment (FAB) mass
(
6) Chow, H. F.; Mong, T. K. K.; Nongrum, M. F.; Wan, C. W. Tetrahedron
1 9 9 8 , 54, 8543.
(
(
(
7) Fischer, M.; V o¨ gtle, F. Angew. Chem., Int. Ed. 1 9 9 9 , 38, 884.
8) Majoral, J. P.; Caminade, A. M. Chem. Rev. 1 9 9 9 , 99, 845.
9) Bosman, A. W.; Janssen, H. M.; Meijer, E. W. Chem. Rev. 1 9 9 9 , 99, 1665.
(10) Dvornic, P. R.; Tomalia, D. A. Macromol. Symp. 1 9 9 5 , 98, 403.
(
11) Hummelen, J. C.; Van Dongen, J. L. J.; Meijer, E. W. Chem.sEur. J. 1 9 9 7 ,
, 1489.
12) Kemmitt, T.; Henderson, W. J. Chem. Soc., Perkin Trans. 1 1 9 9 7 , 729.
3
(
(13) Kriesel, J. W.; Konig, S.; Freitas, M. A.; Marshall, A. G.; Leary, J. A.; Tilley,
T. D. J. Am. Chem. Soc. 1 9 9 8 , 120, 12207.
(
14) Moucheron, C.; Kirsch DeMesmaeker, A.; Dupont Gervais, A.; Leize, E.;
Van Dorsselaer, A. J. Am. Chem. Soc. 1 9 9 6 , 118, 12834.
(15) Van der Wal, S.; Mengerink, Y.; Brackman, J. C.; DeBrabander, E. M. M.;
Jeronimus Stratingh, C. M.; Bruins, A. P. J. Chromatogr., A. 1 9 9 8 , 825,
1
35.
The design of monodisperse polyfunctionalized macromol-
ecules as dendrimers is a field of considerable growing interest,
these macromolecules possessing a number of specific properties
(
(
(
(
(
16) Chessa, G.; Scrivanti, A.; Seraglia, R.; Traldi, P. Rapid Commun. Mass
Spectrom. 1 9 9 8 , 12, 1533.
17) Derrick, P. J.; Haddelton, D. M.; Lloyd, P.; Sahota, H.; Taylor, P. C.; Yeates,
S. G. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1 9 9 4 , 35, 826.
18) Gooden, J. K.; Gross, M. L.; Mueller, A.; Stefanescu, A. D.; Wooley, K. L. J.
Am. Chem. Soc. 1 9 9 8 , 120, 10180.
19) Mowat, I. A.; Donovan, R. J.; Bruce, M.; Feast, W. J.; Stainton, N. M. Eur.
Mass Spectrom. 1 9 9 8 , 4, 451.
20) Puapaiboon, U.; Taylor, R. T. Rapid Commun. Mass Spectrom. 1 9 9 9 , 13,
508.
(21) Sahota, H. S.; Lloyd, P. M.; Yeates, S. G.; Derrick, P. J.; Taylor, P. C.;
Haddleton, D. M. J. Chem. Soc., Chem. Commun. 1 9 9 4 , 2445.
(22) Wu, Z. C.; Biemann, K. Int. J. Mass Spectrom. Ion Processes 1 9 9 7 , 165, 349.
(23) Yu, D.; Vladimirov, N.; Frechet, J. M. J. Macromolecules 1 9 9 9 , 32, 5186.
(24) Launay, N.; Caminade, A. M.; Majoral, J. P. J. Am. Chem. Soc. 1 9 9 5 , 117,
3282.
(25) Slany, M.; Bardaj ´ı , M.; Casanove, M. J.; Caminade, A. M.; Majoral, J. P.;
Chaudret, B. J. Am. Chem. Soc. 1 9 9 5 , 117, 9764.
(26) Galliot, C.; Pr e´ vot e´ , D.; Caminade, A. M.; Majoral, J. P. J. Am. Chem. Soc.
1 9 9 5 , 117, 5470.
(
solubility, viscosity, thermal stability...) different from those of
more classical polymers. These properties are generally due to
the presence of a large number of functional groups on the surface
and of internal cavities and to the fact that dendrimers have a
very narrow molecular weight distribution.1 Full characterization
-9
of dendrimers includes a variety of techniques: 1H, C, Si NMR
13
29
†
Laboratoire de Chimie Structurale Organique et Biologique, CNRS (UMR
7
613), Universit e´ Pierre et Marie Curie.
‡
Laboratoire de Chimie de Coordination, CNRS.
(
(
1) Tomalia, D. A.; Naylor, A. M.; Goddard, W. A. Angew. Chem., Int. Ed. Engl.
9 9 0 , 29, 138.
2) Tomalia, D. A.; Durst, H. D. In Topics in Current Chemistry, vol. 165;
Supramolecular Chemistry I: Weber, E. Ed.; Springer-Verlag: Berlin, 1993;
pp 193-313.
1
(
3) Newkome, G. R.; Moorefield, C. N.; V o¨ gtle, F. Dendritic Molecules; VCH:
Weinheim, Germany, 1996.
(27) Lartigue, M. L.; Slany, M.; Caminade, A. M.; Majoral, J. P. Chem.sEur. J.
1 9 9 6 , 2, 1417.
(
(
4) Zeng, F.; Zimmerman, S. C. Chem. Rev. 1 9 9 7 , 97, 1681.
5) Archut, A.; V o¨ gtle, F. Chem. Soc. Rev. 1 9 9 8 , 27, 233.
(28) Lartigue, M. L.; Donnadieu, B.; Galliot, C.; Caminade, A. M.; Majoral, J. P.;
Fayet, J. P. Macromolecules 1 9 9 7 , 30, 7335.
1
0.1021/ac0003854 CCC: $19.00 © 2000 American Chemical Society
Analytical Chemistry, Vol. 72, No. 20, October 15, 2000 5097
Published on Web 09/19/2000

S c h e m e 1 . S yn t h e s is o f De n d rim e rs G′1 -G′4
spectrometry (limited to lower generations) and size-exclusion
chromatography were the other techniques used for this special
class of dendrimers. Therefore, it is clear that there is a lack of
knowledge and information concerning the characterization and
physicochemical properties of these dendrimers.
In this paper, we report efforts to characterize phosphorus-
containing dendrimers using MALDI-TOFMS. In particular, pos-
sible fragmentation occurring during the MALDI processes has
been investigated.
MALDI-TOF Mass Spectrometry. MALDI mass spectra were
recorded with a PerSeptive Biosystems Voyager Elite (Framing-
ham, MA) time-of-flight mass spectrometer. This instrument is
equipped with a nitrogen laser (337 nm), a delayed extraction,
and a reflector. It was operated at an accelerating potential of 20
kV in both linear and reflection modes. The mass spectra shown
represent an average over 256 consecutive laser shots (3 Hz
repetition rate). Peptides were used to calibrate the mass scale
using the two points calibration software 3.07.1 from PerSeptive
Biosystems. Mentioned m/ z values correspond to monoisotopic
EXPERIMENTAL SECTION
masses except for dendrimers G′3 and G′4 where average masses
are given. The dendrimer solutions (10-3 M) were prepared in
Dendrimer Synthesis. Phosphorus-containing dendrimers of
generations one to four were prepared by the conventional
divergent approach38 (Scheme 1).
tetrahydrofuran (THF). Matrix compounds were from Sigma
(
France) and used without further purification. The matrixes, 2-5-
Size Exclusion Chromatography (SEC). SEC was performed
using a Waters (Milford, MA) 515 pump, a Waters 410 differential
refractometer working at room temperature with tetrahydrofuran
dihydroxybenzoic acid (2.5-DHB), 1,8-dihydroxy-9[10H]-anthra-
cenone (dithranol), 6-azathiothymine, 2,4,6-trihydroxyacetophe-
non, 7-hydroxycoumarin, or 2-anthramine, were also dissolved in
-
1
(
THF) eluent at a flow rate of 5 mL min and equipped with a
-1
THF (10 g L ). One microliter of dendrimer solution was mixed
3
4
set of 100, 500, 10 , and 10 Å Ultrastyragel columns (19 × 300
mm). Molecular weights were derived from a calibration curve
based on polystyrene standards.
with 50 µL of matrix solution. Ten microliters of alkali iodide (LiI,
-1
NaI) solution (5 g L in THF) were added in some experiments
to induce cationization. One microliter of the final solution was
deposited onto the sample stage and allowed to dry in air.
(
(
(
(
29) Galliot, C.; Larr e´ , C.; Caminade, A. M.; Majoral, J. P. Science (Washington,
D.C.) 1 9 9 7 , 277, 1981.
30) Larr e´ , C.; Caminade, A. M.; Majoral, J. P. Angew. Chem., Int. Ed. Engl. 1997,
3
Irradiation Experiments. CDCl solutions of dendrimers G′1
-
1
to G′4 (20 g L ) were irradiated at 350 nm for 12 h using the
3
6, 596.
31) Larr e´ , C.; Donnadieu, B.; Caminade, A. M.; Majoral, J. P. J. Am. Chem. Soc.
9 9 8 , 120, 4029.
Rayonet photochemical reactor RPR 100.
1
32) Larr e´ , C.; Bressolles, D.; Turrin, C.; Donnadieu, B.; Caminade, A. M.; Majoral,
J. P. J. Am. Chem. Soc. 1 9 9 8 , 120, 13070.
RESULTS AND DISCUSSION
These compounds were first characterized by size exclusion
chromatography (SEC) which shows a narrow distribution of
molecular weight (Figure 1). The width of the signals obtained
by SEC confirms the good purity of these dendritic systems.
However, it is possible to detect for the dendrimers of generations
1 (G′1) and 2 (G′2) a second signal corresponding to higher
molecular species. In addition, a tailing toward the higher retention
times is observed for G′3 and G′4, possibly indicating presence
of structure defects. A plot of the SEC elution time versus the
(
(
33) Majoral, J. P.; Caminade, A. M. Top. Curr. Chem. 1 9 9 8 , 197, 79.
34) Caminade, A. M.; Laurent, R.; Chaudret, B.; Majoral, J. P. Coord. Chem.
Rev. 1 9 9 8 , 178-180, 793.
(
(
(
(
35) Larr e´ , C.; Donnadieu, B.; Caminade, A. M.; Majoral, J. P. Chem.sEur. J.
1
9 9 8 , 4, 2031.
36) Loup, C.; Zanta, M. A.; Caminade, A. M.; Majoral, J. P.; Meunier, B. Chem.s
Eur. J. 1 9 9 9 , 5, 3644.
37) Majoral, J. P.; Larr e´ , C.; Laurent, R.; Caminade, A. M. Coord. Chem. Rev.
1
9 9 9 , 190-192,
38) Launay, N.; Caminade, A. M.; Lahana, R.; Majoral, J. P. Angew. Chem., Int.
Ed. Engl. 1 9 9 4 , 33, 1589.
5098 Analytical Chemistry, Vol. 72, No. 20, October 15, 2000

Fig u re 1 . Size exclusion chromatography (SEC) of dendrimers G′1-G′4.
Fig u re 2 . Elution time of dendrimers G′1-G′4 versus logarithm of the molecular weight.
1
logarithm of the molecular weight of G′1-G′4 gives a straight
g mol- for the "G′2 dimer" (theoretical molecular weight 6834 g
line (Figure 2); this is in agreement with what was observed for
mol-1) and of 3197 g mol-1 for the "G′1 dimer" (theoretical
molecular weight 2846 g mol ). However, characterization by SEC
6
-1
different other dendrimers. The value of the elution time observed
for the minor signals detected for G′1 (20.38 min) or G′2 (19.22
min) can be attributed to species resulting from self-assembly of
two dendrimers of generation 1 or generation 2, respectively. This
can be shown in Figure 2 where values of the elution time of the
minor signals allowed us to determine a molecular weight of 6786
is generally of limited value since the dendrimers themselves are
less polydisperse than the polymer standards used to calibrate
the columns (polydispersity of polystyrene used in this work: 1.03)
and it does not provide information on the structure of the species.
Therefore there was a need to characterize more carefully our
Analytical Chemistry, Vol. 72, No. 20, October 15, 2000 5099

Fig u re 3 . Positive-ion MALDI mass spectra of G′1: 2,5-DHB matrix (a) and dithranol matrix (b).
Fig u re 4 . Positive-ion MALDI mass spectra of G′2: 2,5-DHB matrix (a) and dithranol matrix (b).
dendrimers by the MALDI-TOFMS technique which should be
the best mass spectrometry method for this category of neutral
dendrimers.
Since experimental conditions (matrix, metal salt addition,
fluence, etc.) can strongly influence mass spectral characteristics,16
they have been optimized using generation 1 (G′1) and 2 (G′2)
dendrimers. MALDI mass spectra of G′1 using 2,5-DHB and
dithranol matrixes are shown in Figure 3a,b. Besides the peaks
at m/ z 1423.1 and 1445.1 attributed to protonated and sodium
cationized molecules, respectively, fragment ions at m/ z 1090.1
with this matrix. In addition, examination of the isotopic cluster
at m/ z 757.1 shows that it does not fit with a single species and
that a second species corresponding to a loss of 332 u from m/ z
1090.1 is present. This phenomenon is more pronounced for G′2
as shown in Figure 4a,b. With the dithranol matrix, in addition to
loss of 333 u (m/ z 3085) from the protonated molecule at m/ z
3418.2, further losses of 332 u are also observed (m/ z 2752.8 and
2420.5, Figure 4b). With the 2,5-DHB matrix, these fragmentations
are considerably reinforced and the protonated molecule is not
the dominant peak of the mass spectrum (Figure 4a). Thus, a
milder desorption is observed with dithranol compared with that
with 2,5-DHB. In addition, adducts (+332 u) are also observed in
the case of dithranol (Figure 4b). Experiments have been carried
(
2,5-DHB and dithranol) and m/ z 757.1 (2,5-DHB) are observed.
Fragmentation is enhanced with the 2,5-DHB matrix. Loss of 333
u from the protonated molecule and further loss of 333 u occurs
5100 Analytical Chemistry, Vol. 72, No. 20, October 15, 2000

Fig u re 5 . Positive-ion MALDI mass spectra of G′1 (a) and G′2 (b) in the presence of LiI (dithranol matrix).
4
out with other matrixes (in particular, less acidic matrixes such
as 6-azathiothymine, 2,4,6-trihydroxyacetophenon, 7-hydroxycou-
marin, or 2-anthramine). The highest quasi-molecular ion yields
and the lowest fragmentation were observed with dithranol, and
modification of the laser fluence does not change this behavior.
As alkali ion cationization is known to reduce fragmentation, the
same experiments have been carried out with addition of alkali
metal salts (LiI and NaI) to the sample to induce cationization. In
the presence of LiI, protonation is completely suppressed, which
is not the case with NaI. Thus, LiI addition has been preferred.
Under such experimental conditions, the fragmentation is dramati-
cally reduced as shown in Figure 5a,b. The G′1 mass spectrum
exhibits essentially the Li+ cationized molecule at m/ z 1429.2,
while for G′2, fragments at m/ z 3090.9 (loss of 333 u from the
lithiated molecule at m/ z 3424.2) and 2758.8 (loss of 332 u from
m/ z 3090.9) are of low abundance. It can be noted that G′2 was
also characterized by FAB mass spectrometry.38
be detected by 31P NMR without any doubt. However, presence
of side products or chemical degradation products cannot be
excluded. Moreover it is interesting to note that mass spectrom-
etry does not reveal the presence of chlorine atoms, thus allowing
2 6 4
us to reject the presence of P(S)Cl or P(S)(Cl)(OC H CHO)
groups either on the surface or within the cascade structure of
the dendrimers G′1-G′4.
Therefore, one can assume that degradation occurs during
MALDI analysis, either in the course of sample preparation or
during and/ or after UV laser irradiation. Decomposition during
sample preparation is not likely since it is very fast (less than 30
2
s) and an extended stay (4 h) of the dendrimer in a THF/ H O
(20:1 v/ v) solution containing the matrix and LiI does not modify
the mass spectrum. On the other hand, degradation consecutive
to laser irradiation is more likely. UV spectra of dendrimers G′1-
G′4 in THF show a very broad absorption band between 210 and
360 nm, with a red shift for the highest generations (Figure 8).
At 337 nm, the extinction coefficient, ꢀ, of G′1 has been evaluated
to 1400 L mol- cm-1. Indeed, irradiation of G′1-G′4 in chloroform
for 12 h at 350 nm provokes serious damages as can be seen by
31P and 1H NMR, SEC, and MALDI-TOFMS. NMR spectra
recorded after irradiation exhibit broader signals than before and
new peaks corresponding to the degradation of the dendrimer.
Such a dramatic effect can be also visualized by SEC, which shows
an important broadening of the distribution of mass for each
generation, indicating the formation of decomposition products
and also adducts. MALDI-TOF mass spectra of the irradiated
samples corroborate such a decomposition. Two main sets of
peaks can be observed for G′1, corresponding to successive losses
of 332 u from the quasi-molecular ion [M+Li]+ and losses of 94
u from these species. Adducts (+332 u) are also observed. For
dendrimers G′2, G′3, G′4, signals are only observed in the low
mass range (m/ z < 3000). It must be emphasized that a
nonsoluble residue was present in the irradiated solutions of these
higher generation dendrimers.
The dithranol matrix, offering milder desorption/ ionization
conditions than other tested matrixes for G′1 and G′2 analysis,
has been used to analyze higher generation dendrimers. For G′3,
in the absence or presence of LiI, the quasi-molecular ion
1
(
[M+H]+ at m/ z 7406 or [M+Li]+ at m/ z 7411.9) is again the
dominant peak in the mass spectrum but, even in the presence
of LiI offering milder conditions, two sets of abundant fragments
(
losses of 333 and/ or 332 u and 664 u) and adducts (+332 u) are
detected (Figure 6a,b). Within this mass range, due to a lack of
resolution, it is not possible to distinguish loss of 333 u from loss
of 332 u. The situation starts to be much more complicated for
G′4: peaks are observed along the 2000-18 000 m/ z range with
+
the expected quasi-molecular ion [MLi] at m/ z 15 383 and sets
of fragment ions (-332 and -664 u) and adducts (+332 u) (Figure
7
).
It is clear that these fragments, and of course these adducts,
appear not to correspond as a whole to the existence of a number
of structure defects. Such an abundance of structure defects would
Analytical Chemistry, Vol. 72, No. 20, October 15, 2000 5101

Fig u re 6 . Positive-ion MALDI mass spectra of G′3 in the absence (a) or presence (b) of LiI (dithranol matrix).
Fig u re 7 . Positive-ion MALDI mass spectra of G′4 in the presence of LiI (dithranol matrix).
Although irradiation conditions are quite different in MALDI
analysis (solid phase and presence of an absorbing matrix, short
laser pulse), it is evident that UV irradiation at 337 nm (the
extinction coefficient being higher for the dendrimer at 337 nm
than at 350 nm) can induce decompositions. Indeed, a laser
desorption ionization (LDI) mass spectrum (Figure 9) of the neat
G′2 (absence of matrix) shows abundant fragments (losses of 333
and 332 u). The presence of the matrix would not completely
protect the dendrimer from degradation during MALDI analysis.
Now a question arises: how to explain such fragmentations as
losses of 333, 332, and 664 u and also the formation of adducts
(Scheme 2) can be attributed to charge-induced gas-phase
fragmentation of protonated or lithiated species. Such a fragmen-
tation pathway has been observed in post-source-decay (PSD)40
experiments with G′1 (results not shown). Considering the PA of
matrixes, the fragmentation should not be significantly increased
with dithranol (PA ) 209 kcal mol 1)41 compared with 2,5-DHB
-
-1 41
(PA ) 204 kcal mol ) since the exothermicity of the protonation
reaction with both matrixes is very close. Thus, other parameters
(39) Hawker, C. J.; Fr e´ chet, J. M. J. J. Am. Chem Soc. 1 9 9 0 , 112, 7638.
(
40) Spengler, B.; Kirsch, B.; Kaufmann, R. Rapid Commun. Mass Spectrom.
9 9 1 , 5, 198.
1
+332 and +664 u, these phenomena being enhanced when
(
41) Burton, R. D.; Watson, C. H.; Eyler, J. R.; Lang, G. L.; Powel, D. H.; Avery,
moving from generation one to generation four? Losses of 333 u
M. Y. Rapid Commun. Mass Spectrom. 1 9 9 7 , 11, 443.
5102 Analytical Chemistry, Vol. 72, No. 20, October 15, 2000

Fig u re 8 . UV spectra of dendrimers G′1-G′4 in THF.
Fig u re 9 . Laser desorption ionization mass spectrum of neat G′2.
should be taken into consideration. For instance, higher laser
fluence is used with 2,5-DHB compared with dithranol due to
lower extinction coefficients at 337 nm (ꢀdithranol ) 7300 L mol-
cm- and ꢀDHB ) 4400 L mol-1 cm-1 in solution).42 The higher
fluence used with 2,5-DHB could increase the internal energy of
the desorbed dendrimer and thus the fragmentation yields. For
more packed structures, proton migration is favored, which
possibly leads to consecutive losses. It must be pointed out that
this enhancement of fragmentation with 2,5-DHB has also been
observed for dendritic polypyridine.16
1
1
(42) Guittard, J. Ph.D. Thesis, Paris, 1997.
Analytical Chemistry, Vol. 72, No. 20, October 15, 2000 5103

S c h e m e 2 . P ro p o s e d Me c h a n is m o f Ch a rg e -Orie n t e d Ga s -P h a s e Fra g m e n t a t io n o f P ro t o n a t e d o r
Lit h ia t e d S p e c ie s
S c h e m e 3 . Fra g m e n t a t io n In vo lvin g Im in e Me t a t h e s is
On the other hand, losses of 332 u/ 664 u and formation of
adducts probably originate from a fragmentation mechanism
involving a rearrangement. Two types of rearrangement can be
postulated: imine metathesis or reaction of terminal aldehyde
groups with internal imino groups. Metathesis of imines requires
generally more drastic conditions than olefin metathesis. It has
been known for a long time that such a process can be acid-
catalyzed,43 and recently it was found that such a reaction can be
5104 Analytical Chemistry, Vol. 72, No. 20, October 15, 2000

S c h e m e 4 . Fra g m e n t a t io n In vo lvin g Re a c t io n o f Te rm in a l Ald e h yd e Gro u p s w it h In t e rn a l Im in o Gro u p s
catalyzed by imido molybdenium complexes.44 In the present case,
it can be assumed that under irradiation such an imine metathesis,
or more precisely such an hydrazone metathesis, takes place
between two dendrimers of a given generation with, as a
consequence, the formation of “unperfect” dendritic species (see
Scheme 3) with concomitant loss of 664 u for one macromolecule
and gain of 664 u for the other. The second mechanism of
degradation consists of the reaction of internal hydrazone groups
with aldehyde groups located at the outer shell. Such an exchange
can afford fragments and adducts with loss and gain, respectively,
of 332 and 664 u (or multiples of 332 u) (see Scheme 4). To check
this last assumption, a dendrimer of generation three with 24
phenoxy groups, instead of 24 aldehyde groups, located on the
outer shell was prepared. As a result, losses and adducts of 304
u (corresponding to 332 u for the terminal aldehyde group)
disappear, and only losses and gains of 608 u (corresponding to
dendrimers. Interactions should be reduced in the presence of
the matrix; however, we have not experimentally observed any
dilution effect (molar ratio of matrix/ dendrimer varying from 150
to 15 000) on the fragmentation yields. The stability decrease
observed for higher generation dendrimers could also be related
to a higher steric strain energy.
CONCLUSION
Phosphorus-containing dendrimers constituted by the repeti-
6 4 3
tion of POC H CHdNN(CH ) fragments appeared very sensitive
to UV MALDI conditions. Even if molecular peaks are detected
for dendrimers of generations one to four, it is clear that
fragmentations occur due to the relatively strong UV absorption
of these dendrimers at 337 nm. These fragmentations involve
nitrogen-nitrogen bond cleavage, imine metathesis, or reaction
of terminal aldehyde groups with internal imino groups.
As a consequence, such a technique cannot be used to fully
characterize structure defects in the case of these phosphorus-
containing dendrimers. However, the presence of chlorine atoms
was never detected, allowing us to reject structure defects
involving remaining P-Cl bonds. These defects, if any, might only
come from incomplete Schiff reactions between terminal aldehyde
groups and methylhydrazinodichlorophosphine sulfide.
664 u for the terminal aldehyde group) were observed; this clearly
indicates that reaction of terminal hydrazone groups with aldehyde
groups is the cause of the fragmentations observed. Intramolecular
rearrangements can also occur for the higher generations. A
possible mechanism could be the decomposition of neutral species
induced by UV irradiation followed by gas-phase ionization of
neutrals in the plume. Such mechanisms involve inter- or intramo-
lecular interactions which could be favorized for higher generation
Received for review April 3, 2000. Accepted July 5, 2000.
AC0003854
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(
43) Toth, G.; Pinter, I.; Messmer, A. Tetrahedron Lett. 1 9 7 4 , 9, 735.
44) B u¨ hl, M. Chem.sEur. J. 1 9 9 9 , 5, 3514.
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