J . A m . Chem. SOC.1980, 102, 5375-5378
5375
Chemistry of Anthocyanin Pigments. 6.’ Kinetic and Thermodynamic Study of Hydrogen Sulfite Addition to Cyanin. Formation of a Highly Stable Meisenheimer-Type Adduct Derived from a 2-Phenylbenzopyrylium Salt Raymond Brouillard* and Jean Michel El Hage Chahine Contribution f r o m the Institut de Topologie et de Dynamique des Systames de 1’UniversitZ Paris VII, associz au CNRS, 1 , rue Guy de la Brosse, 75005 Paris, France. Received December IO, 1979
Abstract: In acidic aqueous solution, the addition of the HS0,- anion to the AH+ cation of flavylium or 2-phenylbenzopyrylium salts results in a Meisenheimer-type adduct S, where Ks = [S]/[AH+][HSO> [HSO;], [B]:
(14) Eigen, M.; De Maeyer, L. In “Techniques of Chemistry”, Weissberger, A,, Hammes, G., Eds.; Wiley: New York, 1973; Vol. 6, Part 11. (15) Bernasconi, C. F. “Relaxation Kinetics”; Academic Press: New York, 1976.
J . A m . Chem. Soc., Vol. 102, No. 16, 1980 5377
Hydrogen Sulfite Addition to Cyanin Concentrations refer to equilibrium concentrations. Since [HSO!] cannot be measured directly, it is convenient to rewrite eq 7 in the form TS-'
= k-s
+ ks{a(Co'
-
C,)
+ b[AH'])
(8)
where
Co = [AH']
+ [B] + [SI
C,/ = [SO,]
+ [HS03-] + [SI
[H+lKl
a =
(K,
+ [H'I)(Kh + [H'I) 0
I
2
1
2Kl
b= Kl
+ [H'I
Kh = (1.1 f 0.1)10-2 M for cyanin a t 25 'C." The plot of 7s-I vs. (a(Cd - C,) b[AH+]] is linear (Figure 3). The slope gives ks = (4.20 f 0.05)102M-I s-l, and the intercept k_s = (3.85 f 0.05)10-3s-I a t 25 OC. Thus the stability constant Ks = k s / k _ s for the Meisenheimer-type adduct is (1.10 f 0.05)105M-'. This latter value is high but normalI6 and shows that the competitive binding of hydrogen sulfite and water for the flavylium structure strongly favors the sulfite adduct S. W e have no evidence for the addition of H S 0 3 - to the 2 or 4 position and we only observe one u adduct. However, on the basis of our relaxation experiments we cannot completely exclude the possibility that small amounts of a second neutral u adduct S' exist. In the Appendix we show that the relaxation amplitude associated with the formation of S' may be, in unfavorable cases, too small to give rise to a measurable signal. Scheme 111 is essentially valid in acidic media (pH
