β-Peptide coatings by surface-initiated polymerization

Article abstract of DOI:10.1039/c2cc33854h

Homopolymers of β-lactams can be grown by surface-initiated polymerization. These surface-linked β-peptides are living polymers with the potential to be utilized as tunable, protease-resistant interfaces in multiphase structural composites where the chara

Full text of DOI:10.1039/c2cc33854h

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COMMUNICATION
b-Peptide coatings by surface-initiated polymerizationw
ab
Li Chen,ab Yong Lei,ab Abbas G. Shilabin,z James D. Delaney,c George R. Baran*b and
Scott McN. Sieburth*a
Received 29th May 2012, Accepted 1st August 2012
DOI: 10.1039/c2cc33854h
Homopolymers of b-lactams can be grown by surface-initiated
polymerization. These surface-linked b-peptides are living
polymers with the potential to be utilized as tunable, protease-
resistant interfaces in multiphase structural composites where
the characteristics of the interface influence bulk properties.
Scheme 1 b-Lactam formation and base-catalyzed polymerization.
Biopolymers play many roles, including formation of the
structural components of organisms. They can be the structure
(cell walls1), can guide structure formation (diatom frustule2)
or can be a minor but critical component of a composite. This
last case is exemplified by substances such as seashell nacre
where polypeptides present on surfaces of aragonite platelets
comprise less than 5% of the composite by weight, and yet
contribute to an increase in toughness by several orders of
magnitude over the toughness of aragonite alone.3 Poly-
peptides can also perform as biocompatible4,5 or antibiotic
surfaces.6,7 Polypeptides are, however, readily degraded in vivo
by unbiquitous proteolytic enzymes.
the ‘‘grafting to’’ method cannot be used because they undergo
self-association to give insoluble oligomers, making grafting
impossible. The self association of b-peptides has been a long
standing obstacle to the study of these polymers.13,14 Use of
surface-initiated polymerization has the potential to circumvent
this solubility issue.
Polymerization of b-lactams 3 directly yields b-peptides
without the need for additional activation of the monomer
and without byproducts. b-Lactams are readily prepared by
cycloaddition of chlorosulfonyl isocyanate 2 with alkenes,
Scheme 1.15 Under basic conditions, attack of deprotonated
lactam 4 on another monomer yields N-acylated b-lactam 5.
Subsequent attack of 4 on activated b-lactam 5 (arrow) leads
to b-peptide formation. Growth of b-peptides under basic
catalysis occurs almost exclusively from N-acylated b-lactams
such as 5. Gellman16 and others14,17 have demonstrated that
an added N-activated b-lactam can act as an efficient initiating
group, leading to low polydispersity. Based on this precedent,
we have prepared an activated b-lactam initiating group 10 to
study surface-initiated polymerization of b-lactam monomers,
Scheme 2.
Treating b-lactam 618 with n-butyllithium followed by
succinic anhydride gave acid 7. Condensation of 7 with
N-hydroxy succinimide then gave bifunctional 8. While
reaction at either terminus would provide a polymerization
intiation site,16 when 8 was treated with a primary amine
the hydroxysuccinimide was cleanly displaced. Treatment of
b-Peptides, homologs of the naturally occurring a-peptides,
are polymers of b-amino acids that incorporate an additional
carbon between the amine and the carbonyl group of the
monomers. b-Peptides form all of the secondary structures
found with the a-peptides, yet are highly resistant to proteo-
lytic enzymes.8,9 We report here the base-catalyzed growth of
b-peptide homopolymers on glass and silica particles, by sur-
face-initiated polymerization of readily available b-lactams.
Surface-attached polymers can be formed by linking a pre-
formed polymer (‘‘grafting to’’) or by growing a polypeptide
from a polymerization initiating group secured to the surface
(‘‘grafting from’’). This latter method, surface-initiated
polymerization, is a powerful process for creating a densely
coated surface.10–12 In the case of b-peptide homopolymers,
a Department of Chemistry, Temple University, 1901 N. 13th Street,
Philadelphia, PA 19122, USA. E-mail: scott.sieburth@temple.edu;
Fax: +1 215-204-1532; Tel: +1 215-204-7916
b Department of Mechanical Engineering, Temple University,
1947 N. 12th Street, Philadelphia, PA 19122, USA.
E-mail: george.baran@temple.edu; Fax: +1 215-204-6936;
Tel: +1 215-204-8824
c Department of Applied and Computational Mathematics and Statistics,
University of Notre Dame, Notre Dame, IN 46556, USA
w Electronic supplementary information (ESI) available: Preparation
and spectra of 8 and representative procedures. See DOI: 10.1039/
c2cc33854h
Scheme 2 Preparation of surface-linked polymerization initiating
z Current address: Department of Chemistry, Wesleyan University,
Hall-Atwater Laboratories, Middletown, CT 06459-0180, USA.
group.
c
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Reactions run in triplicate for 2, 4, 8 and 12 h were analyzed
by TGA and the results are shown in Fig. 1. Extensive washing
of the product was undertaken to ensure that only covalently
attached polymer was retained. Polymers of 12 are known
to have good solubility properties,16,22 while 13 would be
expected to have limited solubility. Nevertheless, the average
weight gain with reaction time increased similarly for both
monomers.
Treatment of the glass particles with the initiator linking
unit 8 diluted by succinimidyl acetate 11 (8/11 1 : 1 and 1 : 9)
gave 50% and 10% initiator coverage, respectively.10 Poly-
merization of monomers 12 and 13 was then conducted with
the resulting glass using 8 h reaction times, under conditions
identical to those described above. The results are shown in
Fig. 2. For these reactions, run in triplicate, polymerization of
monomer 12 gave an average of 5.2% weight gain for the silica
particles without dilution of the initiator. At 50% and 10%
coverage the weight gains were 4.8% and 4.5%, respectively.
The change in weight gains for monomer 13 at 100%, 50%
and 10% coverage were 5.1%, 3.9% and 2.8%, respectively. In
each case, the same quantities of monomer and base were
used, e.g., 200 equiv. and 1000 equiv. of monomer relative to
initiator for 50% and 10% initiator coverage, respectively.
The nonlinear polymer weight gains with decreasing initiator
coverage may be due to an increase in the length of the
polymer chains with an increase in the monomer/initiator
ratio. This would be consistent with the work of Gellman,
et al. in their solution studies of monomer 12 polymerization,
where diminishing quantities of initiator gave a corresponding
increase in polymer length.16,17
Fig. 1 Monomers and polymerization weight gain (TGA) with
reaction time.
aminopropylsilane coated glass particles 9 (surface area 10 m2 gꢀ1
,
amine coverage19 7.8 mmol gꢀ1) led to polymerization sub-
strate 10. Reanalysis of surface amine density indicated that
>90% had reacted.
Two b-lactams were selected for initial studies, 12 and 13,
Fig. 1. Cyclooctane-fused 12 was found by Gellman to give
cyclooctane b-peptides that retained solubility, presumably
because of limited intermolecular associations. The closely
related cis-3,4-di-n-propyl azetidinone 13 lacks the conforma-
tional constraints of the fused cyclooctane ring and its cis
stereochemistry was expected to yield n-octane b-peptide oligo-
mers with conformations suitable for b-sheet interactions.20,21
Suspensions of initiator-linked glass particles 10, combined
with b-lactam 12 or 13 (100 equiv., 0.2 M in DMF) at 0 1C,
were treated with lithium hexamethyldisilazide (15 equiv., 1 M
in THF) and then quenched after a predetermined time with
sat. NH4Cl. The treated glass was washed three times each
with THF, water, acetone, chloroform and acetone, and
then dried overnight at ambient temperature under vacuum.
The carbonyl region of four infrared spectra are shown in
Fig. 3 using identically treated silica (vide infra). Little absorp-
tion is seen for the aminosilylated silica 9. After attachment of
the initiator and polymerization of monomers 12 and 13,
significant absorptions are seen for characteristic amide I
(ca. 1640 cmꢀ1) and amide II (1520–30 cmꢀ1). Little change
was noted after derivatization with methacrylate. This is
unsurprising as the amides far outnumber the esters and have
a larger extinction co-efficient.23
Fig. 2 Weight gain as a function of surface coverage.
It is notable that the carbonyl absorbances for poly-13 are
much less broad than poly-12. This is consistent with poly-13
having extensive hydrogen bonding interactions, whereas
poly-12 would not.16
The reaction parameters were surveyed using commercial
aminopropyl silane coated silica (surface area 4.5 m2 gꢀ1
,
amine coverage 6.5 mmol gꢀ1) and a fractional factorial
experimental design, Fig. 4. In these four experiments, TGA
of the commercial aminopropyl silane coated silica 9 found
ca. 0.3% organic material. A small increase in the volatiles was
observed after derivatization of the surface amino groups with
Fig. 3 IR spectra of aminosilylated silica (9) and after polymerization
of 12 and 13.
Fig. 4 TGA analysis of four experiments surveying monomers 12 and 13, reaction time and surface density of the b-peptide.
c
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Chem. Commun., 2012, 48, 9604–9606 9605

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N-acylated b-lactam initiating groups. It appears that utilizing
this ‘‘grafted from’’ technology can overcome the propensity
of b-peptides to precipitate as insoluble matter.
This work was supported by the National Institutes of
Health grant DE 019885. We are grateful to Esstech and
HPF. The Mineral Engineers Division of Quarzwerke for
donation of materials.
Notes and references
y In this example, lactam 13 and its polymer are depicted as a single
enantiomer for simplicity. While it is very likely that the details of
polymerization and the properties of the polymer derived from the
racemic monomer will be different from that derived from a single
enantiomer, the ability of the polymer produced from 3,4-cis
disubstituted b-lactams such as 13 to adopt a b-sheet compatible
conformation should be similar in both cases. The use of racemic
a-amino acids to prepare b-sheets has been addressed theoretically24
and experimentally.25
Scheme 3 Elaboration of the living polymers.
8 and 8/11 mixtures. With cyclooctane monomer 12, 100%
initiating group coverage and 8 h polymerization time,
Graph A, an additional 5.4% surface organics resulted from
base-catalyzed polymerization. When the initiator coverage
was reduced to 10% and the reaction time was reduced to 2 h,
Graph C, the added cyclooctane polymer was lowered to
1.1%. Switching to monomer 13, with 100% initiator coverage
and 2 h reaction time, Graph B, the additional polymer was
1% of the silica. Returning to 8 h reaction time, with 10%
initiator coverage, Graph D, polymerization of monomer 13
gave 0.7% polymer weight.
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The polymerization reaction is depicted in Scheme 3.
A suspension of particles coated with initiator (10) is treated
with a large excess of monomer 12 or 13 and a catalytic
amount of strong base (lithium hexamethyldisilazide). In the
case of 13, attack of the b-lactam monomer anion on the
activated b-lactam in 10 leads to ring opening and generation
of a new activated lactam terminus. A second monomer anion
will attack the terminal carbonyl and generate 14 (n = 1).y
Termination of the polymerization by depletion of the monomer
or by quenching the process with mild acid will leave a reactive
terminal b-lactam 14.
A variety of nucleophiles can be used to open the b-lactam
in 14. Scheme 3 illustrates the use of 3-aminopropanol. Reac-
tion of the nucleophilic amine followed by capping of the
resulting terminal alcohol with methacryloyl chloride leads
to an acrylate-terminated b-peptide. Treatment of the silica–
b-peptide particles in this way leads to additional weight as
determined by TGA, of about 0.3%. The incorporation of
these silica–b-peptide–methacrylate particles into composites
and the resulting properties of these composites will be
reported elsewhere.
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b-Peptides are outstanding surrogates for naturally occur-
ring a-peptides, forming secondary and tertiary structures that
effectively mimic the natural products but are also stable
towards proteolytic enzymes. We have found that homopolymers
of b-lactams can be grown from surfaces functionalized with
25 I. Rubinstein, R. Eliash, G. Bolbach, I. Weissbuch and M. Lahav,
Angew. Chem., Int. Ed., 2007, 46, 3710.
c
9606 Chem. Commun., 2012, 48, 9604–9606
This journal is The Royal Society of Chemistry 2012