structure in a solvent of relatively low polarity, such as
CDCl3, as expected from a variety of investigations pub-
lished on closely related, Aib-rich, short peptides.[10b,c]
Laser flash photolysis experiments (Figures S3 and S4)
were carried out to characterize the triplet-state behavior of
SH and establish the reaction conditions necessary to maxi-
mize the yield of the desired intramolecular coupling prod-
uct(s) in a preparative experiment, and minimize intermo-
lecular coupling reactions. Pulsed laser excitation (248 nm;
ꢁ100 mJ; ꢁ20 ns) of a deoxygenated 58 mm acetonitrile so-
lution of SH led to strong transient absorptions centered at
lmax ꢁ525 nm, assignable to the triplet state of the l-Bpa
moiety based on the similarity of the spectrum to that ob-
tained from the model compound Boc-l-Bpa-OMe (lmax
ꢁ525 nm; tꢁ6.3 ms) under similar conditions; both spectra
are quite similar to that of the 4-methylbenzophenone trip-
let in acetonitrile.[12] The SH-triplet absorption decayed with
biexponential kinetics, characterized by an initial fast decay
component that comprised about 75% of the signal
Figure 1. HPLC profiles and retention times of SH (I), the final photore-
action mixture (after 40 min) showing SH and products A and B (IV),
and the purified products A (II) and B (III). Conditions: Vydac C18
A
a longer-lived component
ACHTUNGTRENNUNG
column; 2.5 mLminꢀ1; 20–80% of solvent mixture B in 25 min [A
=
of the Boc-l-Bpa-OMe triplet was quenched efficiently by
adding (0–1.5 mm) Boc-l-Met-OMe [kQ =(1.02ꢂ0.05)ꢃ
109 mꢀ1 sꢀ1]. We thus assign the short-lived component of the
SH-triplet decay to Bpa triplet moieties that are quenched
intramolecularly by the Met residue in the molecule, and
the long-lived component to Bpa moieties residing in a
minor proportion of non-quenching conformations. From
the various lifetimes and rate constants obtained in these ex-
periments, we estimate that at a concentration of 1 mm,
ꢃ80% of Bpa triplets produced on photoexcitation of SH
will be quenched intramolecularly by interaction with the
remote l-Met moiety in the molecule.
The Yang-photocyclization reaction was thus conducted in
a deoxygenated 0.8 mm acetonitrile solution of SH in a
12 mm diameter quartz tube, by irradiating it with 12
(350 nm each) lamps. The course of the reaction was fol-
lowed by HPLC-MS analysis. As the reaction proceeds, two
equally intense new peaks progressively emerge in the
HPLC profile at the expense of SH. After 40 min, the resid-
ual amount of SH is almost negligible (Figure 1). The photo-
products (A and B) corresponding to the two intense peaks
were chromatographically purified (Figure 1) and their
chemical and three-dimensional structures were separately
analyzed by mass spectrometry, UV/Vis, CD, FT-IR, and
NMR techniques, and in addition, for one of them (A) by
X-ray diffraction.
H2O/CH3CN 9:1; B = H2O/CH3CN 9:1 (TFA 0.05%)]; UV detector at
226 nm.
analogy of the chromophores in products A and B and their
diverging characteristics relative to those in SH (we inter-
pret this finding as mainly due to the onset of a new chiral
carbon atom in the vicinity of the aromatic groups of A and
B; see below). Moreover, the Cotton effects of A and B
are: i) quasi-mirror images, suggesting a diastereomeric rela-
tionship between the two compounds; and ii) very intense
and their shapes are quite unusual for short peptide mole-
cules, thus preventing any reliable conformational assign-
ment based only on this spectroscopic technique.
In the N–H stretching region of the FT-IR spectra of A
and B in CDCl3 solution (Figure S7), the relative intensities
of the two bands (weak at about 3425 cmꢀ1, associated with
free NH groups,[13] and strong at about 3315 cmꢀ1, associated
with intramolecularly H-bonded NH groups) are quite close,
concentration independent (in the range 1–0.1 mm), and
strictly comparable to that of SH (Figure S1). This finding
strongly supports the view that the three hexapeptides ex-
hibit similar helical propensities. In the C=O stretching
region the marked shoulder at 1677 cmꢀ1 shown by SH (Fig-
ure S8) is assigned to the contribution of the Bpa benzophe-
none carbonyl chromophore,[14] which appears to be absent
in both products A and B.
ESI-TOF mass spectrometry data unequivocally show
that SH and products A and B are isomeric compounds:
m/z calcd for [M+H]+: 855.4248; found: 855.4139 (SH),
855.4424 (product A), and 855.4427 (product B).
The near-UV spectra of A and B lack the band at 338 nm
in acetonitrile solution that is shown by the Bpa-containing
SH and due to the n!p* transition of its benzophenone
chromophore[3] (Figure S5), clearly indicating the absence of
this moiety in both products. The CD spectra in 2,2,2-tri-
fluoroethanol solution (Figure S6) further highlight the close
The 600 MHz NMR spectra of SH and the two products
A and B were recorded in CDCl3. In the 1D spectra the
proton signals are reasonably dispersed and produce mini-
mal overlaps. For the assignment of all proton resonances a
combination of NOESY and TOCSY experiments[15] was
used. It is evident that the Met e-CH3 proton signal at d=
2.1 ppm in the spectrum of SH is missing in those of the two
products (Figure 2). Concomitantly, new signals, assigned to
68
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 67 – 70