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Phototrophic purple sulfur bacteria as heat engines in the South Andros Black Hole - page 6 / 8

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Photosynth Res

There was an unknown carotenoid peak in BH-1 that eluted just after Bchl on the HPLC system. This unknown carotenoid was characterized as follows. The pigment extract was loaded on a column of DEAE-Toyopearl. Carotenoids were eluted with n-hexane/acetone (1:1, v/v) while Bchl and polar lipids were retained on the column. Then the unknown carotenoid was collected from the HPLC system. Its absorption spectrum indicates that it is an acyclic carotenoid with 13 conjugated double bonds, which is the same with that of spirilloxanthin (Takaichi 1999; Takaichi and Shimada 1992; Frank and Cogdell 1993). Its relative molecular mass is 612, which is 16 mass units heavier than that of spirilloxanthin, and it changes to a mono-acetyl derivative by acetylation. To obtain further structural information 1H-NMR spectroscopy was used to show that the structure is indeed spirilloxanthin with a hydroxyl group at C-2 (Table 2).

OCH3

2 1

Spirilloxanthin

  • O

    CH 3

2 OH

1

(2S)-Spirilloxanthin-2-ol

(2'S)-Plectaniaxanthin

H3CO

O

CH H3CO 3

OH

1'

OCH3

1'

  • O

    CH 3

OH

2'

1' OH

Carotenoid molecules in photosynthetic bacteria (purple, green sulfur, green filamentous, and heliobacteria; except for some aerobic photosynthetic bacteria) do not have chirality (Takaichi 1999). The stereochemistry of the novel carotenoid was probed by circular diochroism spectroscopy and the spectrum found to be the same as that of the chemically synthesized chiral molecule (20S)-plectania- xanthin (Dumont and Pfander 1984). Therefore, the hydroxyl group is (2S) and the novel carotenoid has been determined to be (2S)-spirilloxanthin-2-ol (also called (2S)- 2-hydroxy-spirilloxanthin) (Fig. 6). The IUPAC-IUB semi- systematic name is (2S)-1,10-dimethoxy-3,4,30,40-tetrade- hydro-1,2,10,20-tetrahydro-w,w-caroten-2-ol.

O

2'

1' OH

(2'S)-Deoxymyxol 2'-dimethyl-fucoside

Fig. 6 Structure of (2S)-spirilloxanthin-2-ol and other carotenoids

It is interesting to note that the structure of end group of 1-OH and (2S)-2-OH is the same as found with the (20S)- myxol groups of carotenoids from cyanobacteria, such as (20S)-deoxymyxol 20-dimethyl-fucoside (see Fig. 6 for the structure of this Car molecule) from Synechocystis sp. PCC 6803 (Takaichi et al. 2001), and not found in purple

Table 2 1H-NMR data of spirilloxanthin-2-ol in CDCl3 at 24C

Spirilloxanthin-2-ol

Spirilloxanthin (Lindal and Liaaen-Jensen 1997)

Proton

da

Proton

da

Proton

da

a d in ppm, multiplicity, and coupling constant in Hz

H3-16

1.149 s

H3-17

1.115 s

H3-18

1.929 s

H3-19

1.978 s

H3-20

1.987 s

H3C-O-1 3.268 s

H3-160 H3-170 H3-180 H3-190 H3-20 H3C-O-1 0

H-2 H-3 H-4 H-6 H-7 H-8 H-10 H-11 H-12 H-14 H-15

    • 4.08

      d (7.5)

    • 5.66

      dd (15.5, 7.5)

    • 6.39

      d (15.5)

    • 6.20

      d (11)

    • 6.60

      dd (15.5, 11)

    • 6.39

      d (15.5)

    • 6.23

      d (11)

    • 6.65

      dd (15.5, 11)

    • 6.38

      d (15.5)

H2-2

0

H-30 H-40 H-60 H-70 H-80 H-100 H-110 H-120

6.27 m

H-140

6.65 m

H-150

0

2.32 d (7.5)

H2-2

5.73 dd (15.5

, 7.5) H-3

6.16 d (15.5)

H-4

6.11 d (11)

H-6

6.60 dd (15.5

, 11) H-7

6.35 d (15.5)

H-8

6.23 d (11)

H-10

6.65 dd (15.5

, 11) H-11

6.38 d (15.5)

H-12

6.27 m

H-14

6.65 m

H-15

1.159 s

H3-16

1.159 s

H3-17

1.929 s

H3-18

1.978 s

H3-19

1.987 s

H3-20

3.235 s

H3C-O-1

1.16 1.16 1.93 1.97 1.98 3.23 2.32 5.72 6.16 6.11 6.60 6.35 6.23 6.65 6.38 6.27 6.65

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