Solid Phase Synthesis and Secondary Structural Studies of (1→5) Amide-Linked Sialooligomers

Article abstract of DOI:10.1021/jo971415e

A series of dimeric through octameric (1→5) amide-linked sialooligomers were prepared using solid-phase peptide methods on Rink resin with Fmoc protecting group chemistry. The oligomers were conjugated to ε-amino caproic acid in order to model membrane-bound conformations. The secondary structure of the oliogomers was probed with NH/ND exchange rates determined by NMR, and with circular dichroism. The combined structural studies show that a tetramer is required for ordered secondary structure, and that secondary structure is stabilized upon elongation to pentameric and hexameric species. Interestingly, the heptamer shows rapid NH/ND exchange rates; however, ordered secondary structure is restored in the octamer. These studies provide the first evidence that oligomers composed of constrained carbohydrate-derived amino acids form stable secondary structures in water.

Full text of DOI:10.1021/jo971415e

1
074
J . Org. Chem. 1998, 63, 1074-1078
Solid P h a se Syn th esis a n d Secon d a r y Str u ctu r a l Stu d ies of (1f5)
Am id e-Lin k ed Sia looligom er s1
†
Lajos Szabo, Brenda L. Smith, Katherine D. McReynolds, Abby L. Parrill,
‡
Edwin R. Morris, and J acquelyn Gervay*
Department of Chemistry, The University of Arizona, Tucson, Arizona 85721
Received J uly 30, 1997
A series of dimeric through octameric (1f5) amide-linked sialooligomers were prepared using solid-
phase peptide methods on Rink resin with Fmoc protecting group chemistry. The oligomers were
conjugated to ꢀ-amino caproic acid in order to model membrane-bound conformations. The secondary
structure of the oliogomers was probed with NH/ND exchange rates determined by NMR, and with
circular dichroism. The combined structural studies show that a tetramer is required for ordered
secondary structure, and that secondary structure is stabilized upon elongation to pentameric and
hexameric species. Interestingly, the heptamer shows rapid NH/ND exchange rates; however,
ordered secondary structure is restored in the octamer. These studies provide the first evidence
that oligomers composed of constrained carbohydrate-derived amino acids form stable secondary
structures in water.
In tr od u ction
In his review on complex carbohydrates, Nathan
Sharon pointed out that neuraminic acid (NeuAc) is not
only a carbohydrate but it is also a naturally occurring
δ-amino acid (Figure 1).2 That observation inspired our
recent studies directed toward using NeuAc as an amino
acid equivalent.3 The idea of using sugar amino acids
as both glyco and peptido mimetics has gained consider-
F igu r e 1. N-Acetylneuraminic acid is a naturally occurring
amino acid.
4
able interest in the past two years. Amide-linked sugars
4
b,4c
have been shown to possess important biological
and
structural properties4 and more recently they have been
employed in combinatorial syntheses.5 Our interest in
amide-linked sugars is focused on preparing oligomeric
materials that have defined secondary structures in
a
F igu r e 2. O-Linked Oligomers of NeuAc Are Helical in
Solution.6
†
Department of Chemistry, Michigan State University, Lansing
Michigan.
water. The fact that O-glycoside oligomers of sialic acid
(Figure 2) are helical in solution prompted us to inves-
‡
6
Department of Food Research and Technology, Cranfield Univer-
sity, Silsoe Campus, Silsoe, Bedford, UK.
tigate whether amide-linked oligomers derived from
NeuAc would also be helical. While the secondary
structures derived from O-linked and amide-linked sug-
ars would not necessarily be the same, the possibility of
creating novel helical structures is an exciting proposi-
tion. One important application of these novel materials
would be as helical scaffolds in biomolecular recognition
events. Therefore we embarked upon an investigation
to determine if amide-linked oligomers of NeuAc could
be prepared, if higher order constructs would be water
soluble, and if so, whether the oligomers would have
defined secondary structures in water.
(
1) Presented at the 213th ACS National Meeting, San Francisco,
CA; April 1997, CARB 101.
2) Sharon, N. Complex Carbohydrates Their Chemistry, Biosyn-
(
thesis, and Function; Addison-Wesley Publishing Co.: London, 1975;
p 16.
(
3) (a) Gervay, J .; Flaherty, T. M.; Nguyen, C. Tetrahedron Lett.
997, 38, 1493. (b) Gervay, J .; Ramamoorthy, P. S.; Mamuya, N. N.
Tetrahedron 1997, 32, 11039.
4) (a) Graf von Roedern, E.; Lohof, E.; Hessler, G.; Hoffmann, M.;
1
(
Kessler, H. J . Am. Chem. Soc. 1996, 118, 10156. (b) Suhara, Y.;
Hildreth, J . E. K.; Ichikawa, Y. Tetrahedron Lett. 1996, 37, 1575. (c)
Suhara, Y.; Ichikawa, M.; Hildreth, J . E. K.; Ichikawa, Y. Tetrahedron
Lett. 1996, 37, 2549. (d) Muller, C.; Kitas, E.; Wessel, H. P. J . Chem.
Soc., Chem. Commun. 1995, 2425. (e) Wessel, H. P.; Mitchell, C.;
Lobota, C. M.; Schmid, G. Angew. Chem., Int. Ed. Engl. 1995, 34, 2712.
(
f) Nicolaou, K. C.; Florke, H.; Egan, M. G.; Barth, T.; Estevez, V. A.
Tetrahedron Lett. 1995, 36, 1775. (g) Graf von Roedern, E.; Kessler,
H. Angew. Chem., Int. Ed. Engl. 1994, 33, 687. (h) Poitout, L.; Merrer,
Y. L.; Depezay, J .-C. Tetrahedron Lett. 1995, 36, 6887. (i) Apostol-
opoulos, C. D.; Couladouras, E. A.; Georgiadis, M. P. Liebigs Ann.
Chem. 1994, 781. (j) McDevitt, J . P.; Lansbury, P. T. J . Am. Chem.
Soc. 1996, 118, 3818. (k) Yoshimura, J .; Ando, H.; Sato, T.; Tsuchida,
S.; Hashimoto, H. Bull. Chem. Soc. J pn. 1976, 49, 2511. (m) Nicolaou,
K. C. US Patent Appl. 401039 (Chem. Abstr. 1996, 125(23), 301496h).
We were alerted to the fact that solubility might be a
7
problem since, until very recently, hexameric species
were the largest amide-linked pyranoses reported. We
reasoned that the pendent side chain of NeuAc would
afford greater solubility and thus began synthetic studies
(
6
n) Glycomed Inc. US Patent Appl. 438669 (Chem. Abstr. 1997 126(5),
0291a).
5) (a) P. Fugedi, Cs. Peto and K. Wlasichuk: 8th European
(6) (a) Michon, F.; Brisson, J .; J ennings, H. J . Biochemistry 1987,
26, 8399. (b) Yamasaki, R.; Bacon, B. Biochemistry 1991, 30, 851. (c)
Brisson, J .-R.; Baumann, H.; Imberty, A.; P e´ rez, S.; J ennings, H. J .
Biochemistry 1992, 31, 4996.
(7) Goodnow, R. A. J r.; Tam, S.; Pruess, D. L.; McComas, W. W.
Tetrahedron Lett. 1997, 38, 3199.
(
Carbohydrate Symposium, J uly 2-7 1995 Abs. A74 and A75 (b)
Sabesan, S. Tetrahedron Lett. 1997, 38, 3127. (c) Ramamoorthy, P. S.;
Gervay, J . J . Org. Chem. 1997, 62, 7801.
S0022-3263(97)01415-1 CCC: $15.00 © 1998 American Chemical Society
Published on Web 01/23/1998

Solid Phase Synthesis of (1f5) Amide-Linked Sialooligomers
J . Org. Chem., Vol. 63, No. 4, 1998 1075
composed of δ-amino acids are closely related to Gell-
man’s â-peptide foldamers except at the ends. Both the
carboxy and amino termini of the â-peptides are exposed
to solvent, and therefore both ends can fray.9 In contrast,
the carboxy terminus of the sialooligomers is protected
by the lipid linker designed to model a membrane-bound
oligomer which would not be capable of fraying. The
NMR experiments were performed in DMSO-d
6
at room
temperature with 10% D O added. For the 2-8mers
2
F igu r e 3. Target amide-linked oligomers.
there are four sets of NH protons corresponding to the
primary amide (two singlets at 6.7 and 7.3 ppm), the
secondary amide (triplet at 8.2) on the caproamide linker,
and the internal amides (7.6-7.8 ppm). The half-lives
corresponding to NH/ND exchange rates for the oligomers
are listed in Table 1. Both of the amides in the lipid
linker show pseudo-first-order kinetics as do the internal
amides in the dimer and trimer. The internal amide of
the dimer has a t1/2 ) 0.5 h. The two internal NH’s of
the trimer were resolved, and their half-lives were shown
to be 1 h and 7 h for the end amide and the end-1 amide,
respectively. These data suggest that the amino termi-
nus amide exchanges at comparable rates (0.5-1 h)
regardless of oligomer length. The fact that the end-1
amide exchanges slower suggests that it is protected,
perhaps through an internal hydrogen bond. The tet-
ramer showed three different internal NH protons, and
the half-lives of the slower two (analyzed after the end
amide had exchanged) were shown to be 8 h and 6 h for
the end-1 and end-2 amides, respectively. The internal
amide resonances were not resolved for the 5-8mers, but
a pseudo-first-order plot (analyzed after the end amide
at exchanged) gave t1/2 ) ∼11 h (Table 1) for these protons
except for the heptamer which undergoes relatively fast
exchange (t1/2 ) 1 h).
Our interpretation of these data is that the end amide
is exposed to solvent and therefore undergoes rapid
exchange. The end-1 amide of the trimer is protected
from solvent and undergoes slower exchange. The t1/2 of
the tetramer end-1 amide is 2 h slower than the end-2
amide, suggesting that it is involved in stronger H-
bonding interactions which may give rise to secondary
structure. Interestingly, the heptamer deviates from the
above trends with all internal hydrogens exchanging with
a t1/2 ) 1 h.10 This behavior is difficult to explain without
further experimentation, however one possibility is that
the heptamer can engage in more than one type of
intramolecular hydrogen bonding interaction, creating
the opportunity for conformational fluctuation. This
behavior is not observed in the octamer which reverts
back to an ordered secondary structure.
on the preparation of functionalized sialic acids for
incorporation into amide-linked oligomers. Several ana-
logues of NeuAc, capable of undergoing solution phase
amidation, were prepared, but the coupling yields were
disappointingly low.3 Therefore we decided to explore
solid-phase methods in order to improve efficiency.
Reported herein is the solid-phase synthesis and struc-
tural studies of amide-linked sialooligomers ranging in
size from dimers to octamers.
Resu lts a n d Discu ssion
The target molecule for our investigations is shown in
Figure 3. Incorporation of an ꢀ-amino caproic acid linker
at the reducing end was deemed important for several
reasons. First, it was reasoned that coupling may be
facilitated by placing a linker between the resin and the
first residue. Second, the caproamide functionality could
serve as a potential site for conjugation of the sialooli-
gomers at a later stage in the synthesis. Finally, and
perhaps most importantly, the lipid chain could serve to
protect the reducing end amide from fraying when placed
in an aqueous environment. In this manner the soluble
oligomers would model membrane bound conformations
where the reducing end amide would not be exposed to
water.
The synthetic route to the target molecule is shown in
Scheme 1. â-O-Methyl neuraminic acid (1)3 was treated
with 9-fluorenylmethyl chloroformate in dioxane to pro-
vide the Fmoc-protected amino acid 2 in 72% yield. This
material was coupled to Rink resin, possessing an
a
ꢀ
-amino caproic acid linker, by activating the acid (1.4
equiv) with BOP (1.4 equiv) and H u¨ nig’s base (4 equiv)
in NMP. While the linker was designed to serve several
different roles, it should be noted that coupling of 3
directly to the Rink resin was also possible. The resin
containing the monomeric species was readied for elon-
gation by reaction with hydrazine hydrate in methanol
which resulted in removal of the Fmoc protecting group
as well as the O-acetates. At this point, the beads were
split and one portion was cleaved from the resin to give
the monomer linked ꢀ-amino caproic amide 4 in 62%
yield. The other portion was coupled to 3 and the above
process reiterated. After removal from the resin, the
dimeric species 5 was obtained in 47% yield. The process
outlined above was repeated for trimer through octamer
syntheses, giving pure products after size exclusion
chromatography; the yields ranged from 44 to 55%. Each
of the oligomers was soluble in water, methanol, and
dimethyl sulfoxide, confirming our expectation that
NeuAc analogues would afford greater solubility than
glucose-derived amide-linked oligomers.
Circular dichroism studies in water also support
secondary structure for higher order constructs (Figure
4
). The monomer is distinctly different from the oligo-
mers, suggesting that the strong positive bands and the
crossover to negative values is due to the (1f5) amide
chromophore. These spectra also show that the ellipticity
per residue increases with oligomer length. The most
compelling evidence comes from analysis of the difference
between the peak and trough ellipticities for each oligo-
(8) Appella, D. H.; Christianson, L. A.; Karle, I. L.; Powell, D. R.;
Gellman, S. H. J . Am. Chem. Soc. 1996, 118, 13071 and references
Recently, Gellman and co-workers demonstrated that
differences in amide NH/ND exchange rates are indica-
tive of secondary structure in hexamers composed of
trans-2-aminocyclohexanoic acid.8 Our sialooligomers
therein.
(
9) Rohl, C. A.; Scholtz, J . M.; York, E. J .; Stewart, J . M.; Baldwin,
R. L. Biochemistry 1992, 31, 1263.
10) The exchange rates experiments were repeated numerous times
with careful control of the pH.
(

1
076 J . Org. Chem., Vol. 63, No. 4, 1998
Szabo et al.
Sch em e 1
Ta ble 1. t1/2 (h ) As Deter m in ed by NH/ND NMR
solid-phase synthetic methods. The oligomers were
prepared with lipid linkers to keep the reducing end from
fraying (as evidenced by slow exchange of the reducing
end amide) in aqueous media as a model for membrane
bound oligomers. A series of oligomers were prepared
ranging from dimeric to octameric species and all were
soluble in water, DMSO, and methanol. Combined NMR
and CD data suggest that higher order constructs have
defined secondary structural features that may vary with
length. Further investigations are underway in our
laboratory to unambiguously determine the secondary
structures of these novel compounds, and to ascertain
whether they are helical.
Exch a n geExp er im en ts
internal amides
linker
end
linker
linker
oligomer 2° amide amide end-1 end-2 1° amide 1° amide
n/aa
monomer
dimer
trimer
tetramer
pentamer
hexamer
heptamer
octamer
43
20
67
45
52
63
11
58
n/a
n/a
7.0
8.3
n/a
n/a
n/a
6.2
7.3
1.8
5.8
4.6
4.8
5.0
2.2
4.8
7.7
1.9
5.3
5.0
4.7
4.9
2.6
4.9
0.5
1.0
1.0
n/a
n/a
n/a
n/a
10.8
11.6
1.3
10.4
a
n/a ) not applicable. Assumed to be consistent.
Exp er im en ta l Section
mer. There is a large increase between dimer and trimer
and between trimer and tetramer. Tetramer through
hexamer are roughly the same (10.9) but upon going to
heptamer another increase is observed (11.5) and still
further for the octamer (12.0). These data are consistent
with our NMR data and support the possibility that there
is a change in secondary structure occurring at the hepta-
mer length. Our interpretation of the combined NMR
and CD data is that four residues are required in order
to initiate a stable secondary structure. Incorporation
of the fifth and sixth residues leads to additional stabiliz-
ing interactions. The heptamer may be able to engage
in more than one type of H-bonding pattern as evidenced
by the change in CD and NMR data. Structural regular-
ity is then restored in the octamer whose secondary
structure may differ from the smaller oligomers, perhaps
forming a more extended secondary structure.
Gen er a l. Resins and Fmoc-amino acids were purchased
from Advanced ChemTech. All air and moisture sensitive
reactions were performed under an argon atmosphere in flame-
dried reaction flasks. CH
2 2 3
Cl and CH CN were dried over
2 5
P O
, and all solvents were freshly distilled under an argon
atmosphere prior to use. For flash chromatography, 400-230
mesh silica gel 60 (E. Merck No. 9385) was employed. All
1
13
compounds described were >95% pure by H- and C NMR.
â-O-Meth yln eu r a m in ic Acid . â-O-Methyl-N-acetylneur-
aminic acid methyl ester (400 mg) was dissolved in 2 M NaOH
water solution (20 mL). The mixture was kept at 90-95 °C
for 48 h. After cooling to room temperature, the pH was set
+
to 7 with Dowex H resin. The resin was filtered, and the
supernatant was freeze-dried to give brownish amorphous
solid. (284 mg, 85.0%). Without any purification, the glycoside
was used as the starting material for the next reaction.
â-O-Me t h yl-N -(9-flu or e n ylm e t h oxyca r b on yl)n e u r -
a m in ic Acid . â-O-Methylneuraminic acid (190 mg, 0.67
mmol) was dissolved in a mixture of 10% NaHCO
and dioxane (15 mL). The mixture was cooled to 0 °C, and
-fluorenylmethyl chloroformate (190 mg, 0.737 mmol, 1.1
equiv) in dioxane (5 mL) was added dropwise over 30 min. The
reaction mixture was kept at 0 °C for 4 h and at room
3
(20 mL)
Con clu sion
9
In summary we have demonstrated that (1f5) amide-
linked sialooligomers can be efficiently prepared using

Solid Phase Synthesis of (1f5) Amide-Linked Sialooligomers
J . Org. Chem., Vol. 63, No. 4, 1998 1077
F igu r e 4. CD Spectra of amide-linked sialooligomers in H
structure in higher order constructs (4-8mers).
2
O. Analyses of peak and trough ellipticities indicate stable secondary
temperature for 16 h. The mixture was diluted with water
200 mL) and was extracted with diethyl ether (3 × 20 mL).
J
FmocCH-FmocCH2 ) 9.0 Hz, 1H, Fmoc CH), 4.11 (m, 2H, Fmoc
CH ), 4.02 (dd, J 6-H-5-H ) 12.4 Hz, J 6-H-7-H ) 7.2 Hz, 1H, 6-H),
3.96 (dd, 1H, 9′-H), 3,74 (dd, 1H, 5-H), 3.22 (s, 3H, OCH ),
2.46 (dd, J 3eq-H-3ax-H ) 13.1 Hz, 2.16, 2.07, 2.05, 1.94 (4s, 12H,
(
2
The inorganic phase pH was set to 3 with 10% HCl solution.
The acidic solution was evaporated to 25 mL, and the residue
was extracted with ethyl acetate (3 × 10 mL). The organic
layer was washed with distilled water (3 × 5 mL) and
evaporated to give 244 mg (71.7%) yellow amorphous solid.
â-O -Me t h y l-4,7,8,9-t e t r a -O -a c e t y l-N -(9-flu o r e n y l-
m et h oxyca r b on yl)n eu r a m in ic Acid . â-O-Methyl-N-(9-
fluorenylmethoxycarbonyl)neuraminic acid (530 mg) was dis-
solved in pyridine (2 mL) and cooled to 0 °C, and acetic
anhydride was added dropwise. The yellowish solution was
kept at room temparature 1 day. After the TLC plate showed
only the peracetylated analogue, methanol (2 mL) was added.
After 2 h the mixture was evaporated in vacuo. The residue
was dissolved in ethyl acetate (30 mL) and washed 3× with
3
1
3
3 3
CH COO) 1.92 (dd, 1H, 3axH); C NMR (in CDCl ) δ 171.05,
171.04, 171.01, 170.5 168.53 (COO), 155.98 (CONH), 144.14,
143.40, 141.23, 141.11 (quat Ar C), 128.24, 128.24, 127.82,
127.66, 126.99, 125.24, 124.89, 119.91 (Ar C), 98.75 (C-2),
71.69, 71.17, 68.63 and 68.15 (C-4, C-6, C-7 and C-8), 67.41
(Fmoc CH
2
), 62.51 (C-9), 51.61, 51.16 (C-5 and OCH
3
), 46.87
COO).
H37NO14 672.2292.
(Fmoc CH), 36.32 (C-3), 20.95, 20.77, 20.69, 20.68 (CH
FAB HRMS [M + H ] 672.2307, calcd C33
3
+
(1f5) Am id e-Lin k ed Neu r a m in ic Acid Oligom er s Con -
ju ga ted to E-Am in o Ca p r oa m id e. (1f5) Amide linked
sialooligomer assembly via Fmoc chemistry was performed
manually (0.07 mmol scale, 1.5 mL wash volumes), starting
with 4-(2′,4′-dimethoxyphenyl-Fmoc-aminomethyl)-phenoxy
resin (Rink ResinsAdvanced ChemTech) (0.52 mmol/g, 135
mg). Fmoc removal from the original resin and from the
coupled ꢀ-aminocaproic acid was accomplished with piperidine/
NMP (3:7, 2 × 10 min), followed by washing with NMP (6 ×
0
.1 M HCl solution and with water, and the pH was kept at 4.
The organic layer was dried, evaporated, and reevaporated
with benzene in vacuo to give 640 mg as yellow amorphous
2
0
1
solid: yield 90.5%; [R]
δ 7.76-7.30 (m, 8H, ArH), 5.43 (dd, J 7-H-8-H ) 1.2 Hz, J 6-H-7-H
4.6 Hz, 7-H), 5.23 (m, 1H, 8-H), 5.19 (ddd, J 3eqH-4-H ) 4.7
D
3
) +20.0° (c 1.4); H NMR (in CDCl )
)
1 min). The caproic acid (5 equiv) was coupled with BOP
i
Hz, J 3axH-4-H ) 11.3 Hz, J 4-H-5-H ) 11.3 Hz, 1H, 4-H), 4.92 (d,
NH-5-H ) 10.2 Hz, 1H, NH), 4.57 (dd, J 9-H-9′-H ) 11.0 Hz,
9-H-8-H ) 0.8 Hz, 1H, 9-H), 4.24 (dd, J FmocCH-FmocCH2 ) 4.1 Hz,
reagent (5 equiv) and Pr
Couplings of the Fmoc â-O-Me-neuraminic acid was achieved
by adding the reagents sequentially to the resin in order: 1.4
2
NEt (5 equiv) in 1.5 mL of NMP.
J
J

1
078 J . Org. Chem., Vol. 63, No. 4, 1998
Szabo et al.
equiv of Fmoc amino acid, 1.4 equiv of BOP reagent, and 4
using deionized water and stored at -20 °C between CD
i
-1
equiv of Pr
2
NEt in 1.5 mL of NMP. The mixture was agitated
analyses. Dilutions were as follows (mg mL ): monomer
by bubbling argon through the reaction mixture (frit) for 4 h.
If the Kaiser test was positive, the coupling was repeated with
0.4412; dimer 0.2986; trimer 0.5902; tetramer 0.3115; pen-
tamer 0.4437; hexamer 0.4675; heptamer 0.3374; octamer
0.3672. The CD spectra were acquired on an Aviv CD
Spectropolarimeter Model 62A DS. Each sample was run at
room temperature in a 2 mm path length quartz cuvette at a
total volume 600 µL. Three scans for each sample were
obtained from 320 to 190 nm with data points taken every 1.0
nm. The data were manually baseline-corrected by averaging
the mdeg readings from 260 to 320 nm, where the curve was
a flat line. This average value was then subtracted over the
entire data series. These corrected mdeg values were used to
calculate the molar ellipticity values [θ], using the equation
[θ] ) (θM)/(cl), where θ is the corrected mdeg value, M is the
molecular weight, c is the concentration in mg/mL, and l is
the path length in mm. These values were plotted against
wavelength (nm), and the peak maximum and trough minu-
mum values were found for each oligomer and subtracted to
determine trends in molar ellipticity values over the series.
0
4
.5 equiv of Fmoc amino acid, 0.5 equiv of BOP reagent, and
i
equiv of Pr NEt for 2-4 h (negative ninhydrin test obtained).
2
After coupling the monomer unit, the Fmoc and acetyl protect-
ing groups were removed with H NNH ‚H O/MeOH (4:1, 2 h)
while the 1f5 amide-linked sialooligomers remained anchored
to the resin. The excess H NNH was washed with MeOH (4
Cl
2
2
2
2
2
×
1 min), and NMP (4 × 1 min), and CH
2
2
(6 × 1 min). The
resin was split. On one part of the resin the coupling was
continued as above. The other part of the resin was cleaved
which was carried out with CF
The filtrate was vacuum distilled at 25 °C, dissolved in H
2 mL), and filtered, and the filtrate was freeze-dried. The
3 2 2
COOH/CH Cl (3:7, 10 mL).
2
O
(
white-yellowish residues were purified on Bio-Gel (the di, tri,
tetra, penta, and hexamer on P-2 gel, the heptamer and
octamer on P-10 gel) column to give the pure oligomers.
Characterization data is listed in Table 2 (Supporting Infor-
mation).
NMR Am id e NH/ND Exch a n ge Exp er im en ts. 1H NMR
spectra were collected on a Bruker AM-500 spectrometer at
Ack n ow led gm en t. We are indebted to Dr. Neil
J acobsen for his contribution to the NMR studies, and
to Dr. Arpad Somogyi, director of the UA Mass Spec-
trometry Facility. The assistance of Dr. Pak Poon at
the UCLA Center for Structure Determination is also
greatly appreciated. Financial support for this research
was gratefully received from The State of Arizona
Materials Characterization Program, The University of
Arizona Vice President for Research, and The National
Science Foundation Early Career Award (CHE9623583).
2
95 K using the standard Bruker kinetics program. Samples
were prepared at 0.002 M concentration in 0.5 mL of DMSO-
(99.96%) under an inert atmosphere. Initial spectra were
collected, followed by the addition of 10% D O by volume. A
d
6
2
total of 50 FIDs were obtained, 64 scans each, at increasing
time intervals for a period of 20 h. Data were transferred to
a Silicon Graphics Indy workstation and processed using Felix-
9
5 NMR processing software by BioSym/Molecular Simula-
tions. Data analysis and graphics generation were carried out
using CA-Cricket Graph III by Computer Associates. NMR
plots of NH/ND exchange for each oligomer are given in the
Supporting Information (Figures 5 and 6). Resolution of the
internal amides for the trimer and tetramer are also given in
the Supporting Information (Figure 7). Pseudo-first-order rate
plots are shown in Figures 8 and 9 of the Supporting Informa-
tion.
Su p p or tin g In for m a tion Ava ila ble: 1H NMR spectra of
oligomers, a table of characterization data (Table 2), amide
NH/ND exchange data and pseudo-first order rate plots (7
pages). This material is contained in libraries on microfiche,
immediately follows this article in the microfilm version of the
journal, and can be ordered from the ACS; see any current
masthead page for ordering information.
Cir cu la r Dich r oism . Samples of each oligomer were
weighed out to the tenth of a microgram utilizing a Cahn 29
Automatic Electrobalance. The oligomers were then diluted
J O971415E