Unusually stable monoacylated anthocyanin from purple yam dioscorea alata

Article abstract of DOI:10.1016/0040-4039(91)80088-N

Alatanin C isolated from the tuber of purple yam Dioscorea alata. is the first discovered unusually stable monoacyalated anthocyanin in nuetral aqueous solutions. The stability is ascribed to the intramolecular stacking of sinapic acid and to the chiral self-association of anthocyanidin nuclei.

Full text of DOI:10.1016/0040-4039(91)80088-N

Tetrahed~on Letters, Vo132, No 40, pp 5579-5580, 1991
Printed m GreatBritain
0040-4039/91 $3 00 -~
Pergamon Press pie
U N U S U A L L Y S T A B L E M O N O A C Y L A T E D A N T H O C Y A N I N F R O M
P U R P L E Y A M D I O S C O R E A A L A T A l)
Kuml YOSHIDA,* Tadao KONDO,*+ and Toshto GOTO++
School of Home Economics, Sug~yama Jogakuen Umversuy, Hoslugaoka, Chikusa, Nagoya 464, Japan
Chemical Instrument Center, Nagoya Umversity, ++Laboratory of Orgamc Chemistry, Faculty of Agriculture, Nagoya
Umversity, Chlkusa, Nagoya 464-01, Japan
+
Abstract' Alatamn C ~solated from the tuber of purple yam Dtoscorea alata ts the first discovered unusually stable
monoacyalated anthocyanm m nuegal aqueous solutions The slabdity ~s ascribed to the intramolecular stacking of smapic
acid and to the chiral self-assocmtion of anthocyamdm nuclei
Polyacylated anthocyanin pigments,2a) which contain two or more cinnamic acid derivatives, are very
stable to decolorization xn neutral or weakly acidic aqueous solutions. Intramolecular sandwich-type stacking
of actds has been considered to prevent anthocyantdin nuclei from hydratton at its 2-positron and subsequent
loss of color.2) Recently we have isolated alatanin C from pigments of purple yam and found it to be
unusually stable anthocyanin although it xs only monoacylated 1) Here we propose a possible mechanism for
xts stabilizatton.
The alatanins are very resistant to hydration at pH 6 0 as shown in Fxg. I. Alatanln A (1)3) and B (2)4)
having two sinapic acid are stabdized by sandwtch-type stackmg 2) Surprtsmgly, monoacyalted alatanm C (3)
is also quite stable, despite the fact that monoacylated anthocyanms hke sh~sonin are generally decolonzed
quickly,
on
L--. ~ / . ~
o .
/ A 0 72
.
oc. , ^ 0 28
\
,..: 'oc..
alatan,n
C
(3)
In the circular dtchrotsm (CD) spectrum of 3 an exctton-type posmve Cotton effect in the visible region is
observed (Fig 2) indicating that the anthocyanxdm nuclet of 3 are aggregated clockwxse. This chtral self-
association was verified as follows; the exciton-type Cotton effect was shown to be concentration
dependent 5) and addition of the co-p~gment, flavocommehn, caused disappearance of the Cotton effect in the
CD.2a, 6) Simple anthocyanms i e. pelargonin and delphm are stabihzed by self-association at concentration
of >--5x10-3M and chtral stacking of these anthocyamdm nuclet is suggested from their concentration
dependent Cotton effects.5) At 5x10"SM most anthocyanms exist m a monomeric state, and no exciton-type
Cotton effect is observable m the visible region.
5
579

In alatanm C smaplc acid also plays an important stabthzmg role Deacylated 3 (4) 7) does not show
exc~ton-type Cotton effect m the visible region (Fig 2) and ~s rapidly hydrated at position 2 (Fig. i). Sinap~c
amd may allow 3 to undergo the self-assocmuon more eastly and the intramolecular stacking of acid residues
may also contribute to stabdtzatmn by stencally preventing hyd~auon of the anthocyanidin nuclm In actdtc
D20, 3 showed long dtstance NOEs between the s~gnals of the anthocyamdm nucleus and the slnap~c actd
residue, and the chemtcal shifts of the protons of the smap~c acid were observed at higher field than those of
methyl slnapate probably due to an antsotoroptc effect of the cyanidm ring current (see F~g. 3) The same
stacking of slnap;c acid may occur in neutral solution Thus, the unusual stabd~ty of 3 tmght be caused by the
cooperauon of untque self-assocmt~on and mtramolecular stacking (F~g. 3)
1
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400
500
600
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700
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Fig. i. Stabtiity ot-alatanms 5xlll-SM-m 0-.0-YM-
phosphate buffer (pH 6.0) at 20°C 1, • • 2,
:3, [ ] ' 4 , O:sh~sonm8).
Fig. 2. CT) of-J, 4andsihsonm m (f0-5M-pliosphate
buffer (pH 6 0) at 20*C.
•
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~ anthocyani di nnucl eus
o r
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aromatic acid
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(~ sugar
Fig 3. Proposed stackmg structure of alatanin C.
Acknowledgements" The authors thank Dr Harada of Tohoku Umversity for d~scussion on CD spectra. They
also acknowledge the financial support of the Salt Scmnce Research Foundation and the Ministry of
Education, Scmnce and Culture, Japan (Grand-m-Aid for Sctenttfic Research on Priority Areas No.
0
2250103)
References and Notes
1
"File lsoihuon and-structural-d~termmatJon of-alhtanm A, B andC~Js d~s[:rxbed m tile preceding paper ol-Ybsil~da, Ket al
(,'0.Qy',/oo_T,.gnndn,.T Ange.3~ Chem. l_nt_ EM_ Engl_~_kgO_L_~O~.l_7.~iQolft_T:_P. ro~ CJJ:2n_ Org Nat. P-rod_,.I~8-7.,.5Z,.
9
3
4
._
1
58/(b) Bromllard,
3-0-(6-O-(3-O-(~-D-glucopyranosyl)-6-O-(E)-slnapyl-~-D-glucopyranosyl)-~-D-glucopyranosyl)-7-O-(6-O-(E)-smapyl-~-D-
glucopyr,'mosyl)-2'-O-(~-D-glucopymnosyl)cyamd
R Phytochemzstry, 1983, 22, 13l 1
.
m
3-0-(6-0-(6-~-(E)-sinapy~-~-D-g~uc~pymn~sy~)-~3-D-g~u~pymn~W~)-7~(6-~-(E)~smapyl-~-D-g~c~pyran~sy~).2`-~(~-D-
gl ucopyranosyl)eyanldm
5
l-JrLshmo, T , Matsumoto~ T , Harada N, Goto, T Tetrahedran Lelt, 1981 22, 3621/' Goto, T , Tamura H, Kondo,
Letrahedrczn.LeJI_~.l~8.7 Z.% 5 .Q Q .7 ..~ Gntf~.T ¢.Y.oshlda,.g ,Yoshlk,'me~.M_;.g*)ndn~.T Zetrahe.drczn. L~eJt..l_g.qO. 3/., 7.13.
A d d m o n of flavocommehn reduced the magmtude of the Cotton effect and wah ~ts 8 cq Ihe Cotton effect disappeared
Deacylauon of 3 was achmved wtth l N NaOH-MeOH 1 1 at 0*C under mtrogen
T
6
7
8
.
.
.
K~mdo,.T;_T_am~tr~.H_~.Yosh~da. K_~.Qole,.'I: A~r,: 11!o1_ Chem.,.kqg.9_.~3~.7.Q'?.
(
Recetved in Japan 9 May 1991)