Reaction diastereoselectivity of chiral aminoalcohols/[Co(II)NO 3]+ complexes in evaporating ESI nanodroplets: New insights from a joint experimental and computational investigation

Article abstract of DOI:10.1002/jms.1577

Thenatureof theionic species, formedby electrospray ionization (ESI) ofCo(NO₃)₂/CH₃OHsolutionswith a pair of aminoalcohols W and Y, has been investigated by mass spectrometric and computational methods. Collision induced dissociation (CID) of ions, formally corresponding to the [WYCoNO₃]⁺ structures, yields fragmentation patterns which reflect not only the expected [WYCoNO ₃]⁺ connectivity but also that of other isomeric structures. Formation of these latter species is observed only in the presence of a tertiary aminoalcohol, like N-methylpseudoephedrine. The CID patterns are found to be strongly dependent on the chemical form (whether the free aminoalcohol or its hydrochloride), the configuration, and the relative concentration of the WandYaminoalcohols. This variabilityparallels the results of classicalMD(moleculardynamics) simulationsof the[WYCoNO₃] ⁺ adducts which show a drastic alteration of the mechanical-dynamical features of the adducts by simply changing the charge state of W and/or Y, their absolute configuration, or by removing the solvent. The present experimental and computational study confirms the observation of fast stereoselective reactions in ESI nanodroplets before their evaporation andwarns against any automatic correlation between the ESI spectrum of an analyte and its structure in solution. Copyright

Full text of DOI:10.1002/jms.1577

Research Article
Received: 12 January 2009
Accepted: 16 February 2009
Published online in Wiley Interscience: 30 March 2009
(www.interscience.com) DOI 10.1002/jms.1577
Reaction diastereoselectivity of chiral
aminoalcohols/[Co(II)NO3]+ complexes in
evaporating ESI nanodroplets: new insights
from a joint experimental and computational
investigation
Massimiliano Aschi,a Caterina Fraschetti,b Antonello Filippi,b and
Maurizio Speranzab∗
Thenatureoftheionicspecies,formedbyelectrosprayionization(ESI)ofCo(NO3)2/CH3OHsolutionswithapairofaminoalcohols
W and Y, has been investigated by mass spectrometric and computational methods. Collision induced dissociation (CID) of
ions, formally corresponding to the [WYCoNO3]+ structures, yields fragmentation patterns which reflect not only the expected
[WYCoNO3]+ connectivity but also that of other isomeric structures. Formation of these latter species is observed only in the
presence of a tertiary aminoalcohol, like N-methylpseudoephedrine. The CID patterns are found to be strongly dependent on
the chemical form (whether the free aminoalcohol or its hydrochloride), the configuration, and the relative concentration of the
WandYaminoalcohols. ThisvariabilityparallelstheresultsofclassicalMD(moleculardynamics)simulationsofthe[WYCoNO3]+
adducts which show a drastic alteration of the mechanical–dynamical features of the adducts by simply changing the charge
state of W and/or Y, their absolute configuration, or by removing the solvent. The present experimental and computational
study confirms the observation of fast stereoselective reactions in ESI nanodroplets before their evaporation and warns against
c
any automatic correlation between the ESI spectrum of an analyte and its structure in solution. Copyright ꢀ 2009 John Wiley &
Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: diastereoselectivity; Co(II) complexes; chiral aminoalcohols; molecular dynamics; ESI-mass spectrometry
Introduction
its hydrochloride), the configuration, and the concentration of the
ephedrine present in the electrosprayed solution. On the grounds
of these observations, it was concluded that fast stereoselective
reactions may actually occur in the ESI nanodroplets during
evaporation yielding isomeric forms of the m/z 479 ions.
Since ESI-MS has become a routine and widely used analytical
tool, it would be of extreme interest to consolidate this conclusion
once for all.[2–6] Thus, unequivocal evidence must be provided
about the strict correspondence between the observed fragments
and the structure of their precursors and about the reactive
speciesinvolvedintheirformation.Tothesepurposes,ourprevious
investigation[1] hasnowbeenextendedtootherionicspeciesfrom
the same aminoalcohols as well as from different ones (Table 1)
in order to establish unequivocally whether in these further cases
The stereospecific binding of metal ions to biomolecules may
control their conformation as well as their chemical and biological
properties. In this context, we have been interested in the
coordination behaviour of ephedrines [either the (1S,2R)-(+)-
enantiomer ((+)-E) or the (1R,2S)-(−)- one ((−) -E)] and (1S,2S)-(+)-
N-methylpseudoephedrine ((+)-P) towards cobalt ions in the gas
phase, under conditions where the conformational landscape and
the reactivity of these supramolecular systems are not influenced
by solvation and ion pairing effects.[1] In particular, we examined
the collision induced fragmentation (CID) of ions nominally
corresponding to the [((+)-P)2CoNO3]+ aggregates (m/z 479)
generated by electrospray ionization (ESI) of Co(NO3)2/CH3OH
solutions of (+)-P containing variable amounts of either (+)-
E or (−)-E or their hydrochlorides [(+)-E•HCl and (−)-E•HCl,
respectively: Table 1].
∗
Correspondence to: Maurizio Speranza, Dipartimento di Chimica e Tecnologie
We noticed that CID of m/z 479 yields a fragmentation
pattern which reflects not only the expected connectivity of
the [((+)-P)2CoNO3]+ complex (Scheme 1, paths (a)), but also
that of other isomeric structures (Scheme 1, paths (b)). More
importantly, the relative abundances of the ionic fragments from
paths (a) and (b) were found to be strongly dependent on the
presence, the chemical form (whether the free aminoalcohol or
`
del Farmaco, Universita ‘‘La Sapienza’’, P.le Aldo Moro 5-00185 Roma, Italy.
E-mail: maurizio.speranza@uniroma1.it
`
a
DipartimentodiChimica,IngegneriaChimicaeMateriali,Universitadell’Aquila,
67010 Coppito, AQ, Italy
`
b
Dipartimento di Chimica e Tecnologie del Farmaco, Universita ‘‘La Sapienza’’,
00185 Roma, Italy
c
J. Mass. Spectrom. 2009, 44, 1038–1046
Copyright ꢀ 2009 John Wiley & Sons, Ltd.

Diastereoselective reactions in ESI nanodroplets
Table 1. Structures and symbols of the employed aminoalcohols
Formula
a
b
c
d
Molecular weight
Symbol
b
a
N(CH3)2
CH3
CH3
NHCH3
CH3
H
H
OH
OH
H
179.26
165.24
165.24
201.70
201.70
137.18
137.18
213.28
213.28
207.31
193.29
193.29
(+)-P
H
2
(+)-E
1
c
d
NHCH3
CH3
NH2CH3+Cl−
H
OH
H
(−)-E
NH2CH3+Cl−
CH3
OH
H
(+)-E•HCl
(−)-E•HCl
(+)-A
OH
OH
H
NH2
H
NH2
H
OH
OH
H
(−)-A
Ph
NH2
H
(+)-B
NH2
Ph
OH
H
(−)-B
N(C2H5)2
H
CH3
OH
H
(+)-D
(+)-T
NHC(CH3)3
H
OH
H
NHC(CH3)3
OH
(−)-T
b
D
c
a
D
D
2
1
D
D
D, NHC(CH3)3
D, OH
201.34
( )-d8-T
d
D
by using classical molecular dynamics (MD) simulations, under
equilibrium conditions both in ideal-gas phase and methanol
solution. In this respect, the computational part of the present
investigation is not aimed at elucidating the actual fragmenta-
tion mechanism but, rather, our primary goal is to evaluate more
intrinsic physico–chemical differences between the different iso-
mers to be eventually connected to experimental observations.
More precisely our computational efforts are concentrated on the
search of a plausible differential ‘mechanical-fingerprint’ which
might play, if not the leading role, a not irrelevant part in the ESI
patterns.
[(P2Co)-Hn+1]+ +HNO3 + nH2 (n=0,1)
(m/z 416, 414)
[P+H-nH2O]+ + (P-H)CoNO3 + nH2O (n=0,1)
(m/z 180, 162)
(a)
m/z 479
(b)
[((E•P•Co)-nH2]+ + CH2ONO2 + nH2 (n=0,1)
(m/z 403, 401)
[(E•P•Co)-Hn+1]
+ +CH3ONO2 + nH2 (n=0,1)
In the first part of this paper, precious information is provided
on the origin of species like those responsible of paths (b) of
Scheme 1, the requirements for their formation in ESI droplets,
and the strict connection between their structure and their
CID pattern. The second part of the paper, after an outline of
preliminary Quantum–Mechanical (QM) calculations, is aimed
at describing the underlying strategy and the results of MD
simulations.
(m/z 402, 400)
[E+H –nH2O]+
(m/z 166, 148)
+ [(P•CoCHONO2] + nH2O (n=0,1)
Scheme 1. Complete CID fragmentation pattern of the m/z 479 ion from
electrosprayed P/E solutions (Elab = 5–20 eV). Paths (a) are attributed
to the fragmentation of the [P2•CoNO3]+ and/or [(P2-H)•CoNO3H]+
structures. Paths (b) are attributed to the fragmentation of the
[P•E•CH2CoNO3]+ and [(P-H)•E•CH3CoNO3]+ structures.
Results and Discussion
the connectivity of the ionic species from ESI does or does not
correspond to that of their neutral precursors at equilibrium in
solutions. In this respect, theoretical chemistry may, in principle,
represent a powerful tool of investigation. However, in the present
case, the matter seems too complicated for a straightforward
application of the state-of-the-art computational strategies. A
serious theoretical scenario should rely on a proper treatment
of both desolvation and fragmentation processes which, in the
typical mass-spectrometric conditions are conceivably dominated
by dynamical, i.e. non-ergodic, effects. This means that any
attempt, with the trivial exceptions of processes involving very
small-sized systems, would be frustrated by the complexity of the
problem.
Nevertheless, the above described experimental observation
does indeed stimulate additional very intriguing questions. In par-
ticular, the sharp differences emerging from ESI experiments may
bealsoproducedbyintrinsic,i.e.mechanical-thermodynamicaldif-
ferences between the investigated species. Therefore, we decided
to analyze all the possible [EPCoNO3]+ diastereomeric adducts
ESI-MS-CID experiments
We first tackled the problem of how reliable is the use of
the CID pattern of a given ionic species to establish its con-
nectivity. For instance, a sufficiently intense m/z 507 peak is
detected from ESI of P/T/Co(NO3)2/CH3OH mixtures. After its
isolation from accompanying ions in the first quadrupole of
the instrument, the m/z 507 ion was allowed to collide with
N2 molecules in the second RF-only quadrupole and to frag-
ment (CID). The CID fragmentation of the isolated m/z 507
ion {nominally corresponding to the [T2CoNO3]+ complex} in-
ter alia leads to: m/z 444 (loss of HNO3), m/z 431 (formal
loss of CH2ONO2), m/z 194 (T + H+), and m/z 180 (P + H+).
No significant effect of the T configuration on the relevant
CID patterns is noticed. Although the loss of HNO3 witnesses
the expected occurrence of [T2CoNO3]+ {and its protomer
[(T2-H)CoNO3H]+}, the formal CH2ONO2 loss can be attributed
to at least two isomeric structures, i.e. [(P • T•CoCH2ONO2]+
c
J. Mass. Spectrom. 2009, 44, 1038–1046
Copyright ꢀ 2009 John Wiley & Sons, Ltd.
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M. Aschi et al.
(m/z 507), and [(S • T•CoCH2ONO2]+ (m/z 507,
S
=
α-
400, 387, and 386, respectively, whereas fragment ions 4 and 5 by
their dehydrated forms, i.e. m/z 162 and 148, respectively. The pair
of fragment ions differing for the loss of a hydrogen or a water
molecule will be henceforth denoted by writing in italic the mass
oftheparentfragmention, e.g. m/z 402 form/z 402+m/z 400, and
m/z 180 for m/z 180+m/z 162. The neutral species accompanying
their formation will be denoted in italic as well [e.g. HNO3 for the
loss of HNO3 (to give m/z 402) and of HNO3 + H2 (to give m/z 400
from m/z 465)].
isopropylaminomethyl)benzyl alcohol).
To discriminate between these two structures, the experiments
were repeated by using d8-T, instead of T. In this case, we de-
tected two signals at m/z 523 and m/z 515, corresponding to
[(d8-T)2CoNO3]+ (anditsprotomer), and[(d8-T • P•CoCH2ONO2]+
complexes, respectively. CID of m/z 523 does not produce any
detectable amounts of m/z 447, which would be formed by for-
mal elimination of CH2ONO2 from [(d8-S • d8-T • CoCH2ONO2]+.
Rather, it yields, besides m/z 202 (d8-T + H+), two other signals
at m/z 460 and at m/z 457 which suggest the fragmentation se-
quence of Scheme 2. The loss of the HNO3 fragment necessarily
involves an intracomplex H shift, whereas the accompanying
loss of HD requires the oxidation of the –CDOH–moiety of
d8-T to –CO–. The CID fragmentation of m/z 515 ions gen-
erates several fragment ions, including m/z 439, m/z 436, m/z
202 (d8-T + H+), and m/z 180 (P + H+). Detection of these frag-
mentionsconfirmstheoccurrenceofthe[d8-T • P•CoCH2ONO2]+
structure and its fragmentation sequence (Scheme 3). The ex-
clusive loss of HD further confirms what is mentioned earlier
about the oxidation of the –CDOH–moiety of d8-T to yield
–CO–.
Having established the strict relationship between the CID
patterns of isomeric binary complexes and their structures, we can
nowproceedtotheanalysisofthemechanismoftheirformationin
ESI droplets. Besides the m/z 479 ion, nominally corresponding to
the [((+)-P)2CoNO3]+ structure, ESI of Co(NO3)2/CH3OH solutions
containing (+)-P and E [i.e (+)-E, (−)-E, (+)-E• HCl, or (+)-
E•HCl] leads to the formation of sufficiently intense m/z 465 peaks
nominally corresponding to the [E • (+)-P•CoNO3]+ adducts. As
shown in Scheme 4, an ion fragmentation pattern analogous to
that of the m/z 479 companion (Scheme 1) was observed for the
m/z 465 species, characterized by: (1) loss of HNO3 (m/z 402);
(2) formal loss of CH2ONO2 (m/z 389); (3) formal loss of CH3ONO2
(m/z 388); (4) formation of P + H+ (m/z 180); and (5) formation of
E + H+ (m/z 166). As shown earlier for similar structures, fragment
ions 1–3 are accompanied by their dehydrogenated forms, i.e. m/z
The combined relative abundances of each pair of fragment
ions are given in Figs 1 and 2 as a function of the collision
energy Elab (Fig. 1) and of the composition of the relevant mixtures
at Elab = 15 eV (Fig. 2). As for m/z 479, CID of m/z 465 yields
a fragmentation pattern which reflects not only the expected
connectivity of the [E • P•CoNO3]+ complex (Scheme 4, paths (a)),
but also that of other isomeric structures (Scheme 4, paths (b)).
Their formation requires the transfer of a CHx (x = 2, 3) group
to the nitrate moiety of the complex and the conversion of the
P ligand to ephedrine or pseudoephedrine and/or that of the E
ligand to norephedrine or norpseudoephedrine (N in Scheme 4).
At this point, another question arises: what is the CHx (x = 2, 3)
group donor? Certainly, the P molecule. Indeed, a m/z 493 ion
is formed in the ESI of the Co(NO3)2/CH3OH solutions with
only P, whose CID pattern is dominated by the formal loss of
CH2ONO2 (to give m/z 417) and CH3ONO2 (to give m/z 416).[1] This
implies that both the [P2CoCH2ONO2]+ and [(P2-H)CoCH3ONO2]+
isomers contribute to the m/z 493 ion. In contrast, no m/z 465
ion is observable in the ESI of the Co(NO3)2/CH3OH solutions with
only E in any chemical form and configuration. This means that
both E and E•HCl, as well as the solvent MeOH, are unable to
transfer the CHx (x = 2, 3) group to the nitrate moiety of the
complex to give the [E2CoCH2ONO2]+ and [(E2-H)CoCH3ONO2]+
structures. Rather, the only binary complex formed in the ESI
of E/Co(NO3)2/CH3OH solutions corresponds to m/z 451, whose
CID pattern is only consistent with the expected [E2CoNO3]+ {or
[(E2-H)CoNO3H]+} structures.
Onthesegrounds,itisconcludedthatthecomplexes,generated
in the ESI of E/P/Co(NO3)2/CH3OH solutions, can undergo the
CHx (x = 2, 3) group transfer only from P and not from E (or E•HCl)
or the solvent MeOH. Furthermore, the observation that the ion
patterns of both m/z 465 and m/z 479[1] dramatically depend on
the configuration and the chemical form of ephedrine E in the
E/P/Co(NO3)2/CH3OH mixtures lends strong support to the view
of the CHx (x = 2, 3) transfer as taking place inside higher order
complexes, e.g. [( )-E• ((+)-P)2 • CoNO3]+, from a P ligand to the
nitrate moiety. This view is supported by the observation that the
ionic patterns from CID of m/z 465 are strongly sensitive to: (1) the
specific form of ephedrine, whether as a neutral molecule or as
the hydrochloride salt (cfr. e.g. (+)-P/(+)-E and (+)-P/(+)-E•HCl:
Fig. 1); (2) the [P]/[E] concentration ratio (sol.1–sol.3 in Fig. 2); and
(3) the specific configuration of ephedrine (cfr. e.g. (+)-P/(+)-E
and (+)-P/(−)-E: Fig. 1).
[(d8-T)2CoNO3]+
[((d8-T)2-H)CoNO3H]+
(m/z 523)
(m/z 523)
CID
-HNO3
- HD
[((d8-T)2-H)Co]+
[((d8-T)2-H2D)Co]+
(m/z 460)
(m/z 457)
Scheme 2. Fragmentation sequence of the m/z 523 ion suggesting the
occurrence of the [(d8-T)2 • CoNO3]+ structure.
[d8-T•P•CoCH2ONO2]+
Having identified the species involved in fast stereoselective
reactions in the ESI nanodroplets from E/P/Co(NO3)2/CH3OH
mixtures, wearenowinterestedincheckingwhethertheefficiency
of these reactions depends on the structure and the electronic
properties of the aminoalcoholic ligands. To this purpose, we
investigated the CID of the binary complexes formed in the
ESI of Co(NO3)2/CH3OH mixtures with the other aminoalcohols
listed in Table 1 (generically denoted as W and Y in Table 2).
‘No’ in Table 2 means that the CID pattern of the relevant binary
complex is exclusively consistent with the [WYCoNO3]+ structure
(m/z 515)
-CH2ONO2 CID
- HD
[d8-T•P•Co]+
[(d8-T-HD)•P•Co]+
(m/z 439)
(m/z 436)
Scheme 3. Fragmentation sequence of the m/z 515 ion suggesting the
occurrence of the [d8-T • P•CH2CoNO3]+ structure.
c
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Copyright ꢀ 2009 John Wiley & Sons, Ltd.
J. Mass. Spectrom. 2009, 44, 1038–1046

Diastereoselective reactions in ESI nanodroplets
{orits[(WY-H)CoNO3H]+ protomer}. Incontrast, ‘yes’indicatesthat
formation of the [WYCoNO3]+ structure is always accompanied by
the [(WY-(x-2)H)CoCH2ONO2]+ (x = 2, 3) ones.
[Dx • P(2−x) • CoCH2ONO2]+ (x = 1, 2)}. Besides, CID of a signal
at m/z 507 is consistent with the [D • P•CoNO3]+ connectivity,
and not with the [P2CoC2H4ONO2]+ one. Analogously, CID of a
signal at m/z 535 is consistent with the [D2CoNO3]+ structure,
and not with the [D • P•CoC2H4ONO2]+ one. Finally, no peaks
at m/z 563 {nominally corresponding to [D2CoC2H4ONO2]+} are
detected.
For instance, as said before, CID of m/z 507 from
Co(NO3)2/CH3OH mixtures containing (+)-P and either (+)-T
or (−)-T are consistent with both the [(T2-H)CoNO3H]+ (m/z 444
by formal loss of HNO3) and the [P•(T-H)•CoCH2ONO2]+ (m/z
431 by formal loss of CH2ONO2). In contrast, no signals corre-
sponding to m/z 431 were detected in the CID spectra of the
m/z 507 ion, when electrosprayed from mixtures containing only
T, thus indicating the exclusive occurrence of the [T2CoNO3]+
complex {or its [(T2-H)CoNO3H]+ protomer}. The intracomplex
CHx (x = 2, 3) group transfer is not only observed in electro-
sprayed W/P/Co(NO3)2/CH3OH solutions with W = E, P, or T,
but it can be detected in solution containing primary aminoal-
cohols, like W = A or B. Instead, the same reaction does not
take place in the complexes from ESI of P/Co(NO3)2/CH3OH mix-
tures with another tertiary aminoalcohol, i.e. D (Table 2). The
full scan of the ESI-MS of D/P/Co(NO3)2/CH3OH mixture is char-
acterized by the presence of moderate amounts of m/z 493
(nominally corresponding to [P2CoCH2ONO2]+) and by the com-
plete absence of the m/z 521 and m/z 549 {corresponding to
All these observations can be summarized as follows:
(1) An intracomplex CHx (x = 2, 3) group transfer takes place
in ESI nanodroplets which is promoted by ionization of
the ligand tertiary amino group of P. If actually accessible,
ionization of secondary (in the E and T ligands) or primary
amino groups (in the A and B ligands) tend to transfer the
proton rather than the CHx (x = 2, 3) group. Therefore, to
observe an intracomplex CHx (x = 2, 3) group transfer, the
presence of a tertiary aminoalcohol is essential.
(2) The results obtained in the D/P/Co(NO3)2/CH3OH mixtures
indicate that the intracomplex CHx (x = 2, 3) group transfer
from P is inhibited by the presence of the D ligand in the
complex. This effect can be explained by considering that
the N(C2H5)2 of D is more easily ionizable than the N(CH3)2
[EPCoNO3]+ (m/z 465)
(+)-E/(+)-P
(-)-E/(+)-P
(+)-E•HCl/(+)-P
(-)-E•HCl/(+)-P
100%
80%
60%
40%
20%
0%
5
10 15 20
(402+400)
5
10 15 20
5
10 15 20
5
10 15 20
(166+148)
collision energy (eV; Elab
)
465,0
(389+387)
(388+386)
(180+162)
Figure 1. Relative abundance of the fragment ions from CID of m/z 465 from [P]/[E] = 1 as a function of the collision energy (Elab).
[EPCoNO3]+ (m/z 465)
(+)-E/(+)P
(-)-E/(+)P
(+)-E•HCl/(+)P
(-)-E•HCl/(+)P
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
sol.1 sol.2 sol.3
sol.1 sol.2 sol.3
(389+387)
sol.1 sol.2 sol.3
sol.1 sol.2 sol.3
(166+148)
465
(402+400)
(388+386)
(180+162)
Figure 2. Relative abundance of the fragment ions from CID (Elab = 15 eV) of m/z 465 as a function of the relative concentrations of E and P in the
electrosprayed solution {[P]/[E] = 0.5 (sol. 1); 1.0 (sol. 2); 2.0 (sol. 3)}.
c
J. Mass. Spectrom. 2009, 44, 1038–1046
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M. Aschi et al.
[(E•P•Co)-Hn+1]+ +HNO3 + nH2 (n=0,1)
(m/z 402, 400)
[P+H-nH2O]+ + (E-H)CoNO3 + nH2O (n=0,1)
(m/z 180, 162)
[E+H –nH2O]+
+ [(P•CoNO3)+CH] + nH2O (n=0,1)
(m/z 166, 148)
(a)
m/z 465
(b)
[(E2Co)-nH2]+ + CH2ONO2 + nH2 (n=0,1)
(m/z 389, 387
[(P•N•Co)-nH2]+ + CH2ONO2 + nH2 (n=0,1)
(m/z 389, 387)
and/or
[(E2Co)-Hn+1]+ +CH3ONO2 + nH2 (n=0,1)
(m/z 388, 386)
[(P•N•Co)-Hn+1]+ +CH3ONO2 + nH2 (n=0,1)
(m/z 388, 386)
and/or
Scheme 4. Complete CID fragmentation pattern of the m/z 465 ion from electrosprayed P/E solutions (Elab = 5–20 eV). Paths (a) are attributed to the
fragmentation of the [P•E•CoNO3]+ and/or [(P-H)•E•CoNO3H]+ structures. Paths (b) are attributed to the fragmentation of the [E2•CH2CoNO3]+ and
[(E2-H)•CH3CoNO3]+ structures.
transfer from a conceivably ionized P would be preceded by
D
D
OH
NHtBu
a fast proton transfer from the NH2CH3+ group.
D
D
D
D
In conclusion, CID spectra of the binary ions at m/z 479 and m/z
465, formed by ESI of Co(NO3)2/CH3OH solutions with either pure
(+)-P or its mixtures with (+)-E or (−)-E, provide indirect evidence
of stereoselective CHx migration within high-order aggregates
during solvent evaporation of the ESI nanodroplets. The CID
spectra of d8-T/P/Co(NO3)2 solutions allow us to discriminate
between [(d8-T)2CoNO3]+ and [P • d8-T • CoCH2ONO2]+, thus
confirming the strict correspondence of the CID patterns with
the complex connectivities. Furthermore, the experimental results
identifyP as the CHx-donor, indicating that a tertiary aminoalcohol
is essential. An activating ionization of the donor is necessary,
although its protonation can favour the faster proton transfer.
This view is confirmed by the observation that, with the more
easily ionizable D ligand, any intracomplex alkyl group transfer is
inhibited by a faster ethylene elimination, leading to the loss of
C2H4 and HNO3.
D
D
D
D
D
OH
NHtBu
D
D
D
D
D
α
β
Scheme 5. Structures of the α and β regioisomers formed in the synthesis
of ( )-d8-T.[11]
.
Table 2. Formation of [(WY-(x-2)H)CoCHxNO3]+ (x = 2, 3) structures
from ESI of W/Y/Co(NO3)2/CH3OH solutions ([W]/[Y] = 1)
Aminoalcohol W
Aminoalcohol Y
E
P
T
A
B
D
E
No
Yes
No
–
Yes
Yes
Yes
Yes
Yes
No
No
Yes
No
–
Yes
–
–
–
No
–
Theoretical calculations
P
T
Yes
No
Classical MD simulations do represent a very powerful tool of
investigation provided a reliable force field is available. This point
has been tackled in the present study by using Density Functional
Theory(DFT)calculations(seeExperimentalSectionforthedetails).
We focussed our attention on the [(−)-E•(+)-P•CoNO3]+ adducts
considering both the doublet and quartet electron state and all
the conceivable covalent frameworks, namely neutral, mono-, and
bis-zwitterionic, schematically shown in Fig. 3. Then, the [(−)-
E•(+)-P•CoNO3]+ force field parameters have been exported to
all [(+)-E•(+)-P•CoNO3]+ adducts.
A
B
D
No
–
–
–
No
–
No
–
–
–
–
–
of P (For instance, ionization potentials IP(eV molecule−1
)
(N(C2H5)3 = 7.53 < (CH3)2NC2H5) = 7.74 eV); proton
affinity PA(kcal mol−1) (N(C2H5)3 = 234.7 > (CH3)2NC2H5) =
229.5)[7]. This minimizes any conceivable CH2 group transfer
from P in favour of the elimination of ethylene from the
ionized N(C2H5)2 moiety induced by proton interaction with
the NO3 moiety.
Results, briefly collected in Table 3, clearly indicate that,
irrespectiveofthecovalentframework, the[(−)-E•(+)-P•CoNO3]+
adducts preferentially adopt a quartet state in the gas phase. The
very large energy differences with doublet state also suggest that
quartet species should represent the most abundant, if not the
unique, magnetic state also in solution. This finding prompted us
to use, as force field for the present study, the atomic charges
(3) The general decrease in reactivity with EH+Cl−, instead of
+
the free base E, is attributed to the presence of the NH2CH3
group of E in the complexes. Indeed, the CHx (x = 2, 3) group
c
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Copyright ꢀ 2009 John Wiley & Sons, Ltd.
J. Mass. Spectrom. 2009, 44, 1038–1046

Diastereoselective reactions in ESI nanodroplets
Me
Me
Me
N
H
H
H
N
H
Me
Me
Me
Ph
+
neutral
bis-zwitterionic
-
O
O
H
+
Co
N
+
H
Ph
H
Ph
Co
N
H
O
O
O
O
O
H
-
O
O
Ph
Me
O
Me
[(-)-E•(+)-P•CoNO3]+
[(-)-E’•(+)-P’•CoNO3]+
H
+
N
N
H
H
H
Me
Me
H
H
Me
Me
Me
H
H
+
N
Me
Ph
N
Me
Ph
Me
H
H
O
O
H
H
mono-zwitterionic
+
+
Co
N
Co
N
O
O
O
O
O
O
H
H
[(-)-E•(+)-P’•CoNO3]+
[(-)-E’•(+)-P•CoNO3]+
-
-
O
O
Ph
Me
Ph
Me
H
Me
+
N
N
Me
H
H
H
H
Figure 3. Schematic view of the covalent frameworks utilized for MD simulations.
calculations, rather poorly representative of the system. From this
preliminary observation it immediately emerges the importance
of a MD-based approach for better addressing the problem at
hand. Moreover all the species turn out to be characterized by a
very similar mechanical fingerprint in which the CoNO3 moiety
appears very rigid with all the fluctuation pattern concentrated
withinEandPgroups. InparticularEandProughlyshowtwokinds
of motions: (1) a ‘rigid-body’ rotating motion with respect CoNO3
moiety and (2) internal fluctuations, e.g. mutual rotation of phenyl
and amino groups. Essential dynamics[9] was employed to obtain
a more quantitative and systematic comparison of the overall
mechanical properties of the investigated systems. This method
is based on the construction of the covariance matrix, C, of the
positional deviations of the atoms along the trajectories (note that
in our simulations the roto-translations have been removed and
all the structures are least-squared fitted). The symmetric matrix C,
when diagonalized, provides a set of eigenvectors (η) representing
the directions in the configurational space along which the system
fluctuates. The sum of the eigenvalues, i.e. the trace associated
to each eigenvector provides the extent of the fluctuation along
the above directions, i.e. the flexibility of the systems reported in
Table 4.
Table 3. B3LYP/BS (BS: LANL2DZ on Co, 6-31G∗ on others) relative
energies (at 0 K) of the species reported in Figure 3
Species
ꢀE (kcal/mol)
[(−)-E•(+)-P•CoNO3]+
doublet
quartet
20.5a
8.5
[(−)-Eꢁ • (+)-P•CoNO3]+
[(−)-E•(+)-Pꢁ•CoNO3]+
[(−)-Eꢁ • (+)-Pꢁ•CoNO3]+
doublet
quartet
14.4
3.9a
doublet
quartet
12.7
1.5
doublet
quartet
12.7
0.0
a Not fully optimized
of the quartet species with all the rest of the parameters, i.e.
Lennard-Jones, stretching, bending, and torsional, taken from
the Gromos96 force field.[8] The above parameters were finally
optimized by reproducing the DFT gas phase quartet energy
differences of Table 3.
The results indicate that the adducts undergo a change
of flexibility upon variation of the covalent framework and
systematically become more confined, i.e. more rigid, when
passing from gas phase to methanol solution. At the same
time, all the investigated systems show a rather steep spectrum
of eigenvalues showing only two values significantly different
from zero which, therefore, are sufficient to describe the overall
mechanical–dynamical properties of the adducts. This finding
allowed us to better characterize the MD results by projecting
all the trajectories onto the plane described such eigenvectors
offering a geometrically simple representation. For the sake of
clarity we show, in Fig. 4, the result of the above procedure for
MD simulations were carried out at 300 K both in gas phase and
in methanol solution at the typical density of 792 kg/m3 at 1 bar of
pressure(seeExperimentalsectionforthedetails).Foreachadduct,
we simulated the neutral {[(−)-E•(+)-P•CoNO3]+, [(+)-E•(+)-
P•CoNO3]+}, the bis-zwitterionic {[(−)-Eꢁ•(+)-Pꢁ•CoNO3]+, [(+)-
Eꢁ•(+)-Pꢁ•CoNO3]+} and the two mono-zwitterionic {[(−)-Eꢁ•(+)-
P•CoNO3]+, [(+)-Eꢁ • (+)-P•CoNO3]+, [(−)-E•(+)-Pꢁ•CoNO3]+,
[(+)-E(+)-Pꢁ•CoNO3]+} species for a total of eight simulations
in gas phase and eight in methanol solution (the zwitterionic
forms of E and P are respectively denoted as Eꢁ and Pꢁ).
At a first sight, all the simulated adducts show a rather
high flexibility making the above structures, obtained from QM
c
J. Mass. Spectrom. 2009, 44, 1038–1046
Copyright ꢀ 2009 John Wiley & Sons, Ltd.
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M. Aschi et al.
in Fig. 4, we calculated and compared the extent of the overlap
between the corresponding eigenvectors η by using the Root
Mean Square Inner Product (RMSIP) formula:[10]
Table 4. Trace of the diagonalized all-atom covariance matrix for all
of the investigated species
Species
Solvent
Trace (nm2)

1/2
N
N
[(−)-E•(+)-P•CoNO3]+
None
1.0
0.5
ꢀ ꢀ
(ηiA • ηjB)2












Methanol
i=1 j=1
[(+)-E•(+)-P•CoNO3]+
[(−)-Eꢁ • (+)-Pꢁ • CoNO3]+
[(+)-Eꢁ • (+)-Pꢁ • CoNO3]+
[(−)-Eꢁ • (+)-P•CoNO3]+
[(+)-Eꢁ • (+)-P•CoNO3]+
[(−)-E•(+)-Pꢁ • CoNO3]+
[(+)-E•(+)-Pꢁ • CoNO3]+
None
0.7
0.6
RMSIP =
N
Methanol
None
1.2
0.4
Methanol
where A and B indicate two generic systems and N the dimension
of the covariance matrix (i.e. equal to 3N if all the N atoms are
considered). The obtained value equal to 0.19, much lower than
unity, unequivocally points out the poor overlap between the two
conformational spaces, i.e. very different mechanical–dynamical
features sharply indicating that the solvent produces not only a
geometrical but also a mechanical–dynamical differential effect.
In order to extend the latter findings also to the other adducts,
we calculated the above overlap between the eigenvectors of
gaseous and methanolic [(−)-E•(+)-P•CoNO3]+ with the ones of
all the other species. The results, collected in Table 5, confirm the
previous trend. Note that the same analysis carried out for all the
species provide the same qualitative scenario, i.e. RMSIP never
exceeding 0.4.
None
1.1
0.5
Methanol
None
1.3
0.4
Methanol
None
1.1
0.6
Methanol
None
1.0
0.5
Methanol
None
1.2
0.6
Methanol
one adduct (with both (−)-E and (+)-P in neutral form, with all the
others reported in the Supporting Information) in gas phase and
in methanol.
Therefore, a drastic alteration of the mechanical–dynamical
features of the adduct is observed by simply changing the
charge state of E and/or P or the absolute configuration of E
or by removing the solvent. This somewhat surprising result,
although not explaining, may partially justify the differences
experimentallydetectedintheESI-MS-CID fragmentationpatterns
of [E • P•CoNO3]+. As a matter of fact, even neglecting the
important dynamical effects of non-equilibrium conditions, the
sharp differences emerged at the equilibrium can somehow play
an important role in the fragmentation dynamics. Unfortunately,
the complexity of the investigated systems does not allow, at the
moment, a more direct structure–activity relationship. Finally. it
must be also stressed that, at room temperature, both in the gas
The spots appearing in the figure, corresponding to a non-
zero probability to find the molecule around a given geometry,
indicate that the [(−)-E•(+)-P•CoNO3]+ adduct is characterized
by different conformers, separated by relatively high barriers (lack
of spots) in gas phase and in rapid mutual equilibrium (lack of
discontinuity) in methanol. It follows that the solvent, beyond
the previously mentioned mechanical hindrance, also leads the
system to a different conformational sampling. To quantitatively
evaluate the mechanical–dynamical differences between the two
simulations, i.e. gas phase versus methanol for the case reported
1.0
2.0
1.5
0.5
0.0
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
-0.5
-1.0
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
Figure 4. Projection of the MD simulations onto the plane formed by the first two essential eigenvectors for [(−)-E•(+)-P•CoNO3]+ adduct in methanol
(left side panel) and gas phase (right side panel) (units are in nanometre).
c
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Copyright ꢀ 2009 John Wiley & Sons, Ltd.
J. Mass. Spectrom. 2009, 44, 1038–1046

Diastereoselective reactions in ESI nanodroplets
34 psi; nebulizer gas = 15 psi; focusing rod offset (IS) = +10 V;
orifice plate = +35 V; capillary temperature = 210 ◦C. Methanolic
solutionsareinfusedviaasyringepumpataflowrateof10 µl/ min.
CIDexperimentsareperformedinthefollowingway:afterisolation
in the first quadrupole, precursor ions are allowed to go through
the collision region, where their CID takes place. The survivor
precursor and its product ions were accumulated in the LIT of the
instrument (fill time of the trap = 20 ms; scan rate = 1000 amu/s)
to improve the signal-to-noise ratio and eventually detected.
Operating conditions in CID experiments are: nominal pressure of
nitrogen in collision chamber, 1, 4 × 10−5 torr; the CID collision
energy (in eV; lab frame) is calculated from the potential difference
between IS and collision cell rod offset. The relative abundance
of fragment ions results from peak integration from the spectra
acquired in profile mode. In each acquisition the final spectra are
the average of about 70 scans, each consisting of two microscans.
The standard deviation of the relative ion abundances is about
10%.
Table 5. RMSIP between neutral [(−)-E(+)-PCoNO3]+ adducts in gas
(g) and methanol (m) phase and all the other investigated species
[(−)-E(+)-PCoNO3]+
Species
Gas (g)
Methanol (m)
[(+)-E•(+)-P•CoNO3]+
[(+)-E•(+)-P•CoNO3]+
[(−)-Eꢁ • (+)-Pꢁ•CoNO3]+
[(−)-Eꢁ • (+)-Pꢁ•CoNO3]+
[(+)-Eꢁ • (+)-Pꢁ•CoNO3]+
[(+)-Eꢁ • (+)-Pꢁ•CoNO3]+
[(−)-Eꢁ • (+)-P•CoNO3]+
[(−)-Eꢁ • (+)-P•CoNO3]+
[(+)-Eꢁ • (+)-P•CoNO3]+
[(+)-Eꢁ • (+)-P•CoNO3]+
[(−)-E•(+)-Pꢁ•CoNO3]+
[(−)-E•(+)-Pꢁ•CoNO3]+
[(+)-E•(+)-Pꢁ•CoNO3]+
[(+)-E•(+)-Pꢁ•CoNO3]+
g
0.31
0.19
0.20
0.25
0.31
0.13
0.10
0.15
0.25
0.12
0.24
0.25
0.14
0.31
0.25
0.32
0.27
0.16
0.19
0.34
0.21
0.19
0.10
0.14
0.23
0.25
0.32
0.14
m
g
m
g
m
g
m
g
m
g
m
g
m
Computational details
QMcalculations(geometryoptimizationandatomiccharges)were
performed using the Gaussian 03 suite of programmes[12] installed
on dual processor Opteron workstations. The calculations were
carried out at the B3LYP[13] level of theory using the 6-31G∗ basis
set for H, C, N, O, and the LANL2DZ[14] one for Co.
phase and in solution, the systems may exist in a large number of
conformations in mutual equilibrium. This obvious consideration
poses a severe question as to whether it makes sense to use
static QM calculations, without thermal or dynamical effects,
to provide semi-quantitative explanations to the unimolecular
reactivity under mass spectrometric conditions.
A modified version of Gromacs package[14] was used for
producing all the MD trajectories in the number pf particles at
constant volume and temperature (NVT) ensemble.[15] All the
systems, after an initial mechanical relaxation and (if present)
solvent equilibration, were slowly heated using short trajectories
of 50 ps length from 50 to 300 K. After a few nanosecond
of equilibration 20 ns of simulation were produced at 300 K.
A time step of 2 fs was adopted and the rototranslational
motion was removed,[16] the temperature was kept fixed at 300 K
by the isokinetic temperature coupling[17] and the long-range
electrostatics was treated by means of the Particle Mesh Ewald
method.[18] All the subsequent analyses were carried out using
standard Gromacs tools or using home-made routines.
Experimental section
Materials
Enantiomerically
methylpseudoephedrine
pure
compounds
((+)-P),
(1S,2S)-(+)-N-
(1S,2R)-(+)-ephedrine
hydrochloride ((+)-E•HCl), (1R,2S)-(−)-ephedrine hydrochloride
((−)-E•HCl), (1R,2S)-(−)-ephedrine ((−)-E), (S)-(+)-2-amino-
1-phenylethanol
((−)-A),
((+)-A),
(R)-(−)-2-amino-1-phenylethanol
(1S,2R)-(+)-2-amino-1,2-diphenylethanol
((+)-B),
(1R,2S)-(−)-2-amino-1,2-diphenylethanol ((−)-B), (1S,2S)-(+)-
diethylnorpseudoephedrine ((+)-D), (S)-(+)-2-tert-butylamino-
1-phenylethanol ((+)-T), and (R)-(−)-2-tert-butylamino-1-
phenylethanol ((−)-T) were purchased from a commercial source
and used without further purification. (+)-E was obtained
by treatment of corresponding hydrochloride with NaHCO3
saturated aqueous solution.
Acknowledgements
Financial supports by Universita` ‘Sapienza’, Roma, Italy (Funds
for selected research topics 2008-2010) and PRIN grant n.
2007H9S8SW 002 (MIUR) are acknowledged.
A simple and safe procedure was used to prepare the racemic
mixture of ( )-d8-T:[11] 3 mmol of tert-butylamine was added
dropwise to a stirred solution of commercially available d8-styrene
epoxide (2.5 mmol) in 1 ml of water. The mixture was stirred for
about14 hatroomtemperature.Afteradditionof1 mlofwater,the
organic materials were extracted with diethyl ether (2 × 5 ml) and
dried over anhydrous Na2SO4. The crude product was analyzed by
1H-NMR (yield, 80%; ratio α : β regioisomers, 10 : 90 (Scheme 5)).
Supporting information
Supporting information may be found in the online version of this
article.
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J. Mass. Spectrom. 2009, 44, 1038–1046