pH-Switchable vitamin B9 gels for stoichiometry-controlled spherical co-crystallization

Article abstract of DOI:10.1039/c6cc07701c

Vitamin B₉ gels were found to be pH-switchable by adding triethylamine or acetic acid. Such recyclable gels can be used as novel co-crystallization media, resulting in four stoichiometric vitamin C co-crystals. Under highly supersaturated conditions, uniform microspheres of (VC)·(NA) with different particle sizes were obtained through a spherical crystallization process.

Full text of DOI:10.1039/c6cc07701c

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DOI: 10.1039/C6CC07701C
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pH-switchable vitamin B9 gels for stoichiometry-controlled
spherical co-crystallization†
Received 00th January 20xx,
Accepted 00th January 20xx
Jian-Rong Wang, Junjie Bao, Xiaowu Fan, Wenjuan Dai and Xuefeng Mei *
DOI: 10.1039/x0xx00000x
www.rsc.org/
Low-molecular-weight gelators (LMWGs) have received
substantial attention owing to their self-assembly properties,
which can be reversible under mild conditions through the use
of competing supramolecular interactions such as anion
binding.9-11 One of the most important applications of such
reversible gels is in the area of pharmaceutical crystallization,
where the LMWGs are used as crystallization media for
pharmaceutical solid form control.12-14 The potential
advantages of this supramolecular organogel crystallization
approach include diversity in solvent choice and facility in
recovering of the grown single crystals.15, 16 However, the
potential of this well-established technique for pharmaceutical
co-crystallization is still an unexplored subject. Co-
crystallization from organogel can be absence of the
reversible nature and solubility effects of co-crystals in
solution,17 hence has the possibility of stoichiometry-
controlled co-crystals.
Vitamin B9 (VB9, folic acid) exhibits rich self-assembly
behavior via Hoogsteen H-bonding to form discrete
supramolecular nanostructures gels.18-20 This natural chiral
molecule (Fig. 1) with carboxylic acid residues on the gels
has the potential to interact strongly with the H-bonding
functionality of API or co-former, effectively locally
immobilizing the drug substances on the surface of the gel
fiber and providing a nucleation surface. In the present work,
we report the pH-switchable property of VB9 gels by adding
Et3N or AcOH. Such recyclable gels can be used as co-
crystallization media to selectively grow stoichiometric co-
crystals by controlling the ratio of the feed components and to
simply recover the grown crystals by filtration.
Vitamin B9 gels were found to be pH-switchable by adding
triethylamine or acetic acid. Such recyclable gels can be used as
novel co-crystallization media, resulting in four stoichiometric
vitamin C co-crystals. Under highly supersaturated condition,
uniform microspheres of (VC).(NA) with different particle size
were obtained through spherical crystallization process.
Co-crystallization of active pharmaceutical ingredients
(APIs) offer a tremendous opportunity for the development of
new drug products with superior physical and
pharmacological properties such as solubility, stability,
2
hydroscopicity, dissolution rates and bioavailability.1, In
many co-crystallization systems, co-crystals present multiple
stoichiometries with the same co-former, which increase the
number of solid forms and hence provides more opportunities
to engineer properties of a given molecule. This abundance of
structures helps elucidate the nature of co-crystallization,
much like the availability of many polymorphs aids in the
study of structure-property relations. However, in some cases
co-crystallization lead to more than one stoichiometric
product and directly affect the phase purity and fail the
product specification. The design and synthesis of
stoichiometric variations were rarely reported.1 Grinding
approach by altering the ratio of the reactant mixture exhibits
a higher degree of control over stoichiometric variations than
co-crystallization from solution.1, 3-7 Solvent free continuous
co-crystallization (SFCC) technology based on the application
of melt extrusion is an effective process for mixing whilst
maintaining the stoichiometric ratio of the feed components.6,
8
However, both reported methods sometimes lead to low
crystalline or even amorphous (especially for hard-to-nucleate
samples) and are also not suitable for preparation of single
crystals. Thus, there remains a demand for novel technologies
to control stoichiometric composition of co-crystals.
Pharmaceutical Analytical & Solid-State Chemistry Research Center, Shanghai
Institute of Materia Medica, Chinese Academy of Sciences, Shanghai 201203,
China. E-mail: xuefengmei@simm.ac.cn
†
Electronic Supplementary Information (ESI) available: Experimental,
Fig. 1 (a) Chemical structure of vitamin B9, (b) pH-switchable vitamin B9 gels.
spectroscopic, and crystallographic details. CCDC 1505517-1505522. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/x0xx00000x
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Gelation of VB9 was retested in a range of organic solvents
(Fig. S1, ESI†). VB9 in methanol or DMSO / nitromethane reversible VB9 gels were applied as co-crystallization media.
(2:8, v/v) resulted in the formation of a faint yellow organogel In a typical procedure, the mixture sample of VC and co-
at around 0.5% by weight. It is well-known that the 1D fibers former (1:1, 1:2, or 1:3) was dissolved in 1 mL MeOH. Then
mainly lie on H-bonding and pi-pi interactions of pterin (2- 4 mg (0.5 wt%) VB9 was added in the solution and heated up
aminopteridin-4-ol) moiety, but carboxylic acid moiety plays to give a clear solution. After slowly cooling down to room
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In order to improve the stoichiometric selectivity, the
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DOI: 10.1039/C6CC07701C
a vital role in formation of VB9 gels. The protonation or
deprotonation can strengthen and weaken the driving forces
of self-assembly respectively. As expected, by adding Et3N
directly onto the gel surface, a rapid gel-to-solution phase
transition was observed. It is proposed that salt was formed
and affects the solubility of VB9, which disturbs the H-bonds
of fibers between carboxylic acids. As a consequence, only a
small amount of base could turn gels into the solutions.
Interestingly, by adding equimolar quantity of AcOH as that
of Et3N, carboxylates are protonated, resulting in the
supramolecular gels reformed. This process can be repeated at
least 5 cycles. In a further attempt to confirm the reversibility
related to the carboxylic acid moiety, four diester derivatives
(dimethyl, diethyl, dipropyl and dibutyl esters) were
synthesized (Fig. S3, ESI†). As expected, these compounds
could also form gels in methanol, but without the reversible
property as that of VB9. Although it was reported that the
addition of NaOH or HCl disassociates the self-assembled
structures of VB9,19 the controlled reversibility of the sol-gel
process by pH change is reported here for the first time. This
system as well as the method provides an efficient alternative
for fabricating recyclable organogels and for recovering the
grown crystals.
temperature gel formation occurred, and then kept in 4 C.
Crystals grew within a few days. The gels are then dissolved
by addition of 10 L Et3N. The crystals can be readily
recovered by filtration or manual extraction and mounted for
an experiment such as single-crystal X-ray crystallography.
Different stiochiometric variations exhibit different color
(orange needles for (VC).(INA), pale blue plates for
(VC).(INA)2, colorless columns (VC).(NA), and yellow
blocks for (VC).(NA)3, Fig. 2) albeit both the API and the co-
former are colorless. The significant color difference between
the four co-crystals may be attributed to the different
arrangement of chromophores, which caused by the different
H-bonding interactions in their co-crystal structures.
As a part of our continuous study on improving
physicochemical properties of vitamins via co-
crystallization,21-23 we have recently conducted the co-crystal
screening of vitamin C (VC, ascorbic acid). VC crystals are
unsuitable for direct tableting due to their poorly compactible
properties.24 Moreover, VC is unstable in solution and
undergoes oxidative decomposition to various products.25 Co-
crystallization of VC with isonicotinamide (INA) is
particularly difficult as VC and INA have very different
solubility (0.6 mM for VC and 1.5 mM for INA in methanol
at room temperature), and always mixtures of two different
co-crystals (orange needles and pale blue plates) were
obtained by solution methods. The mixture was also
determined to be two sets of XRPD patterns (Fig. S4, ESI†),
which indicates the lack of selectivity in solution co-
crystallization. To circumvent this problem, we turned to an
alternative based on liquid-assisted grinding (LAG),1, 26, 27 in
which VC and INA are combined in the presence of a trace of
methanol. Characterization by powder X-ray diffraction
confirmed that two LAG-produced solids, (VC).(INA) and
(VC).(INA)2, were new crystalline phases. Although these
solids have no signals of VC or co-former, there are found to
be contained a certain amorphous state (Fig. S3, ESI†). These
materials were in turn used to seed the growth of
stoichiometric variations from a solution containing the
precursors VC and co-formers. Surprisingly, mixtures of
(VC).(INA) and (VC).(INA)2 were still obtained. Similar to
that of VC/INA, co-crystallization of VC with nicotinamide
(NA) led to mixtures of (VC).(NA) and (VC).(NA)3 in
solution.
Fig. 2 (a) Chemical structures of VC, INA, and NA. (b) Single crystals from VB9 gels and
crystal structures of co-crystals.
Single crystal X-ray analysis revealed that co-crystals
between VC and INA are formed in 1:1 and 1:2 ratio, while
co-crystals between VC and NA are formed in 1:1 and 1:3
ratio (Fig. 2). Four co-crystals are sustained by OH…Narom
interactions. The H-bonding connections are quite different
from all previously reported five VC co-crystals (refcode:
SERASC10, RVWFEV, RVWFAR, RVWDUJ and
ERAVAU), which formed through R22 (9) synthon (Fig. S5,
ESI†) between the carboxylate moiety of co-formers and the
vicinal hydroxyl moieties on the five-membered ring of
VC.28-30 Overall, H-bonding of stoichiometric variations
generate significantly different 3D architectures as presented
in Fig. S6-S9 (ESI†). In the case of (VC).(NA), colorless
plates formed within the gels upon standing at 4 C. One of
these crystals was soon mounted for single crystal X-ray
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diffraction at 100 K and proved to be a 1 : 1 MeOH solvate. measured patterns from bulk powder are closely matched with
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While other crystals were identified as (VC).(NA) under those in the simulated patterns generated from single crystal
DOI: 10.1039/C6CC07701C
ambient condition for a few minutes, which indicates the diffraction data, confirming the formation of co-crystals with
single-crystal to single-crystal transformation. The crystal essentially stoichiometric selectivity and high phase purity.
data show great resemblance in the unit cell parameters of Therefore, VB9 gels can be used to synthesize a desired
(VC).(NA).MeOH and (VC).(NA), with only c axis differing specific VC co-crystal by simply controlling the ratio of the
by 2.6 Å (Table S1, ESI†). The MeOH elimination from feed components and further recycled by adding AcOH.
MeOH-VC H-bonding unit in (VC).(NA).MeOH resulted in
the structural transformation from 3D H-bonding network
architecture to 2D interactions in (VC).(NA) (Fig. S10, ESI†).
The presence of (VC).(NA) solvate, which it does not
crystallize in the absence of the gel, indicates that the gel has
a remarkable influence on the crystallization behavior of this
16
co-crystal.14,
Mechanism of stoichiometry-controlled co-
crystallization in gels was proposed in Fig. 3. VC and co-
former were stoichiometrically locked in an independent
space of 3D VB9 nano-fibers in the process of gelation. The
local ordering of gel fibers potentially offers an active surface
on which nucleation of co-crystal can occur. Gels slow down
the diffusion of both VC and co-former, eliminating
convection, and providing homogeneous crystal growth
medium.11 Single crystals grow in gels slowly and are
generally better formed than in solvent.
Fig. 4 Recycable VB9 gels for stoichiometry-controlled VC co-crystals.
Under highly supersaturated condition, the homogeneous
crystal growth of (VC).(INA) is transferred to a process of
heterogeneous secondary nucleation, resulting in spherically
agglomerated crystals. For example, 2.0, 1.8 or 1.5 mmol VC-
INA (1:1) was dissolved in 1 mL methanol by heating,
respectively. The addition of 4 mg of vitamin B9 and cooling
down to room temperature resulted in the immediate
formation of gels. Surprisingly, the uniform microspheres
with different size were produced 1-2 days after gelation (Fig.
5), where the size of microsphere were increasing with
decreasing the solute concentration. The samples of
microsphere were determined to be co-crystal (VC).(INA) by
XRPD (Fig. S14, ESI†).
Fig. 3 Mechanism of stoichiometry-controlled co-crystallization in gels.
VC is very unstable in solution, and easily decompose to
threonic acid and oxalic acid.25 The impurities can further
influence the co-crystal formation. Long time of co-
crystallization process in solution, no VC co-crystals will be
obtained but co-crystals of decomposition product. Co-
crystallization of VC with NA by slow evaporation in
methanol at 4 C leads to a new NA oxalate (2:1) (Fig. S11,
ESI†) only but not VC co-crystal. Therefore the mother liquid
is impossible to be reused by solution method. The chemical
stability of VC in gels was determined by HPLC and
compared with that in same methanol solution. The assay
value of were found to be 93% at 24 h in solution, while no
obvious degradation was observed for VC in gels (Fig. S12,
ESI†). Thus, in addition to easily recovering co-crystals by a
simple filtration, the VB9 gels make it possible to be recycled
(Fig. 4) for VC co-crystallization. Furthermore, the yield was
about 30% in the first cycle due to its high solubility in
methanol system, while the recovery was improved to more
than 90% in second or third cycles. The PXRD was used to
check the phase purity of four co-crystals (Fig. S13, ESI†).
Fig. 5 (a) Particle size of microsphere vs concentration of (VC).(INA) from VB9 gels,
The results showed that all the peaks displayed in the SEM images for evolution of spherical crystallization. (b) gel induced nucleation phase,
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SYNOPSIS: The recyclable VB9 gels can be applied to control co-crystal stoichiometry, resulting in
isolation of four stoichiometric vitamin C co-crystals.
TOC:
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