Substituted 1,3,5-triazaadamantanes: Biocompatible and degradable building blocks

Article abstract of DOI:10.1002/anie.200802222

(Chemical Equation Presented) Basic groups from acid hydrolysis: 1,3,5-Triazaadamantanes (TAAs) are shown to degrade in acidic conditions to produce basic byproducts (see scheme). The rate of hydrolysis can be tuned by changing the substituents present on

Full text of DOI:10.1002/anie.200802222

Communications
DOI: 10.1002/anie.200802222
Tuneable Degradation
Substituted 1,3,5-Triazaadamantanes: Biocompatible and Degradable
Building Blocks**
Amy M. Balija, Richie E. Kohman, and Steven C. Zimmerman*
Degradable polymers are important constituents of environ-
mentally benign products and are used in applications that
range from tissue engineering[1] to gene[2] and drug deliv-
ery.[1,3] Hydrolytically labile polymers, such as polyesters, are
particularly useful because of the ubiquitous presence of
water in the environment and in living organisms. However,
polyesters produce acidic byproducts upon degradation, a
limitation for a number of applications.[4] Polyphosphazenes[5]
are promising alternatives, but there is a need for new
monomers that degrade to form benign byproducts. Herein
we report the synthesis and study of 1,3,5-triazaadamantane
(TAA) as a water-soluble unit for controlled degradation and
aldehyde release. Formed from the condensation of a
tris(aminomethyl)methane unit and three aromatic alde-
hydes,[6] the TAA unit hydrolyzes under physiological con-
ditions reverting to its basic precursors. We demonstrate how
Figure 1. a) Synthesis of benzaldehyde-derived 1,3,5-triazaadamantane
(TAA) 2. b) Difference NOEs detected for 2a in deuterated chloroform.
c) ORTEP representation (from X-ray analysis) of 2a. Thermal ellip-
soids are set at 50% probability. See supporting information for
the degradation rate of the TAA unit can be tuned with
substituents, and further synthesize a hydrophilic TAA-based
dendrimer that is capable of binding solvatochromic probes.
The preparation of TAA 2 by the condensation of
tris(aminomethyl)ethanol (1) and benzaldehyde (Figure 1a)
was reported by Woulfe and co-workers.[6a] In our group, this
procedure produced 2a and 2b in a 9:1 ratio (determined by
1H NMR spectroscopy). Difference nuclear Overhauser
effect (NOE) studies were performed on 2a to establish the
relative spatial orientation of the methine protons (Fig-
ure 1b). To confirm further its identity, an X-ray analysis was
performed on crystals of 2a grown by the slow evaporation of
an acetonitrile solution (Figure 1c). Submitting the minor
isomer 2b to the reaction conditions afforded a 9:1 ratio of 2a
to 2b, indicating the reaction to be thermodynamically
controlled.
additional details.
were synthesized[7] and condensed with 1 to produce 4a–c
1
(Scheme 1). H NMR spectroscopic analysis confirmed that
these model TAAs degraded to their monomer units upon
exposure to aqueous acidic conditions.
Although the TAA unit has been used as a protecting
group for the tris(aminomethyl)methane unit, little work has
focused on controlling its rate of decomposition.[6] We
therefore examined the effect of aromatic ring substitutents
on the TAA hydrolysis rate. Water-soluble aldehydes 3a–c
Scheme 1. Synthesis of substituted TAAs 4a–c.
[*] A. M. Balija,[+] R. E. Kohman, Prof. S. C. Zimmerman
Department of Chemistry
University of Illinois at Urbana-Champaign
600 South Mathews Avenue, Urbana, IL 61801 (USA)
Fax: (+1)217-244-9919
Rate constants for TAA hydrolysis at various pH values
were measured using UV spectroscopy. A red shift in the
absorbance at lmax was observed upon exposure of the TAA
monomer to acidic conditions. The half-life for hydrolysis was
calculated by plotting the change of absorbance over time. As
seen in Figure 2, 4a–c rapidly hydrolyzed at pH < 5. Under
basic conditions, the TAA units degraded at different rates,
such that 4a hydrolyzed the fastest and 4b degraded the
slowest. Based on the s-values used in Hammett analyses for
corresponding substituents, 4c might reasonably be expected
E-mail: sczimmer@uiuc.edu
[+] Current Address: Department of Chemistry, Fordham University
Bronx, NY 10458 (USA)
[**] The authors thank the National Institutes of Health for financial
support. In addition we would like to thank Dave Drake, M. Laird
Forrest, and Prof. Dan Pack for performing the XTT assays.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200802222.
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Figure 2. Half-lives of TAAs 4a–c at 22.58C.
to be least stable. The mechanism of hydrolysis is not known
and may change as a function of the substituent. For many
applications, 4b may have the most desirable degradation
profile by being stable under neutral conditions but hydro-
lyzing rapidly upon acidification.
Although multiple uses for the TAA unit can be envi-
sioned, its AB3 functionality suggested use as a building block
for dendrimers. Thus, dendrimer 10 was synthesized and its
ability tested to encapsulate small molecules in aqueous
environments. Core 6 was prepared from tribromide 5[8] and 2
(Scheme 2). Compound 7 was oxidized, deprotected, and
alkylated in situ with tri(ethylene glycol) tosylate 8, to afford
aldehyde 9.[9,10] Condensation of 6 with ten equivalents of 9
afforded dendrimer 10 in 49% yield. The product was
estimated to be greater than 90% pure determined by
analytical size exclusion chromatography (SEC), MALDI-
MS, and 1H NMR spectroscopy. As a control, dendron 11 was
prepared from the condensation of 1 with substituted
benzaldehyde 9 (Scheme 3).
Binding studies were performed using solvatochromic
dyes to assess the binding of small molecules by dendrimer
10.[11] Titration of 10 into a phosphate-buffered saline (PBS)
solution of Rose Bengal[12] resulted in a red shift of 19 nm in
the lmax of the dyesꢀs absorption spectrum.[7] A single
isosbestic point was detected, consistent with free and
complexed dye species in solution.[13] Based on a 1:1 binding
isotherm for 10 with Rose Bengal, an apparent association
Scheme 2. Synthesis of dendrimer 10.
constant,
K
assoc = 3.1 Æ 0.5 ” 105 mÀ1
, was determined. No
appreciable red shift occurred on using 4b in place of 10.
However, adding three equivalents of dendron 11 to the dye
solution, did produce a red shift of approximately 10 nm for
lmax in the absorption spectrum, indicating that 11 can also
interact with the dye.
Additional insight into the interaction of small molecules
with 10 was obtained through fluorescence studies with 1-
anilino-8-sulfonic acid (ANS), a dye with low emission in
aqueous environments.[14] Upon addition of dendron 11
(3 equiv) or core 6 to a solution of ANS, a small increase in
the intensity of the ANS fluorescence spectrum was detected.
However introduction of dendrimer 10 to a similar ANS
solution produced a much larger increase in fluorescence
intensity, indicating that the solvatochromic dye was within a
more hydrophobic environment (Figure 3).
Scheme 3. Synthesis of dendron 11.
1
solution of 10, effecting (as determined by H NMR spec-
troscopy) its complete hydrolytic conversion into 6 and 9. In
drug delivery and related applications, it is important that the
hydrolysis byproducts are nontoxic. Toward this end, com-
pounds 9, 10, and 4a–c were tested in a standard XTT cell
viability assay.[7] All five compounds were found to be
nontoxic under standard physiological conditions. It was
To test whether 10 degrades in the same manner as 4,[15] an
excess of 35% (w/w) DCl in D2O was added to a [D8]THF
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Figure 3. Fluorescence spectra for encapsulation studies of 6, 10, and
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In summary, described herein is the synthesis and study of
a newclass of compounds that can undergo tunable hydrolytic
degradation to well-defined byproducts. The rate at which
these molecules decompose can be controlled through sub-
stituent effects. In contrast with degradable materials con-
taining esters, that provide acidic products, TAAs degrade to
give products containing basic amine groups. The TAA unit
possessed a branched architecture that was utilized for the
synthesis of a water-soluble dendrimer that binds Rose
Bengal and ANS and degrades in acidic conditions. TAAs
have considerable potential in biological applications, espe-
cially in cases where relevant pH gradients can exploited, such
as in the extracellular space of tumor tissue and cellular
endosomes.[2] The ability to buffer endosomes has also been
shown to be important for certain applications.[16] Current
efforts are directed toward developing TAA-based materials
for gene and drug delivery, as well as tissue engineering.
[7] See Supplementary Information for details.
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Received: May 12, 2008
Revised: July 29, 2008
Published online: September 18, 2008
Keywords: degradation · dendrimers · host–guest systems ·
.
hydrolysis · macromolecules
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