Chemistry Reference
In-Depth Information
TABLE 4.1
Chemical Shifts and Isomeric Shift Differences of
(All-
E
) and (13-
Z
) Astaxanthin
d
(All-
E
)
Astaxanthin (ppm)
d
(13-
Z
)
Astaxanthin (ppm)
Proton
Dd
(ppm)
H (7)
6.36
6.36
—
6.36
—
H (7′)
H (8)
6.52
6.52
—
6.52
—
H (8′)
H (10)
6.38
6.38
—
H (10′)
6.38
—
H (11)
6.77
6.77
—
6.77
—
H (11′)
H (12)
6.50
7.12
0.62
6.50
—
H (12′)
H (14)
6.40
6.24
−0.16
6.40
—
H (14′)
H (15)
6.79
6.97
0.18
6.74
H (15′)
−0.05
(dD
d
all-
E
) comparing the 13-
Z
and all-
E
spectrum are expected in the area around the 13-
Z
arrangement.
In the
1
H-NMR spectrum of the (13-
Z
) astaxanthin isomer, Figure 4.17b, the “convex-side”
protons 14 (6.24 ppm) and 15
=
d
Z
−
′
(6.74 ppm) are shifted to higher i eld, with Dd values of
−
0.16 ppm
). However, the “concave-side” protons 12 (7.12 ppm) and 15 (6.97 ppm) are
shifted to lower i eld with Dd values of 0.62 ppm (12) and 0.18 ppm (15). The protons far from the
cis
bond are unaffected from the stereochemical behavior.
The isomerization shifts conform to those that are described in literature (Englert and Vecci
1980, Englert 1995). All chemical shifts and isomeric shift differences of the olei nic region are
listed in Table 4.1.
Overall, the combination of HPLC together with NMR is a very efi cient tool to elucidate struc-
ture of different stereoisomers found in complex natural mixtures.
(14) and
−
0.05 ppm (15
′
4.5 CONCLUDING REMARKS
In summary, NMR spectroscopy is an extremely versatile tool useful that enables researchers to
understand the structure of natural products such as carotenoids. For a full structural assignment,
the compound of interest has to be separated from coeluents. Thus, it is a prerequisite to employ
tailored stationary phases with high shape selectivity for the separation in the closed-loop on-line
LC-NMR system. For the NMR detection, microcoils prove to be advantageous for small quantities
of sample. Overall, the closed-loop system of HPLC and NMR detection is very advantageous for
the structural elucidation of air- and UV-sensitive carotenoids.
ACKNOWLEDGMENT
The authors gratefully acknowledge the help of Jan Peter Mayser in preparing the i gures.
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