Kinetics and mechanism of oxidation of glycyglycine by diperiodatoargentate(III) in aqueous alkaline medium

Shan Jinhuan, Wang Heye, Song Changying, Wang Fang, Shen Shigang
(College of Chemistry and Environmental Science, Hebei University, Baoding, Hebei 071002, China)

Abstract The kinetics of oxidation of glycyglycine by diperiodatoargentate(III) complex (DPA) was studied spectrophotometrically in aqueous alkaline medium in a temperature range of 293.2K-308.2K. The reaction was found to be first order with respect to DPA and 1.30-1.35 to glycyglycine. The observed rate constant (k_obs) decreased with the increase of the [IO₄^-], decreased with the increase of the [OH^-], and then increased with the increase of the [OH^-] after a turning point. The k_obs was independent of the ionic strength and no free radicals were detected. A possible mechanism involving a two-electron transfer is proposed and the rate equations derived from the mechanism can explain all experimental results. The activation parameters, as well as the rate constants of the rate-determining step have been calculated.
Keywords Glycyglycine; Diperiodatoargentate(III); Redox reaction; Kinetics; Reaction mechanism

1. INTRODUCTION
In recent years, many researchers have focused on the study of peptide, especially for its chemical modification ^[1-3]. Glycyglycine is one of the important peptides, the kinetics and mechanism of oxidation of glycyglycine can provide some valuable information for the chemical modification of peptide.
    Diperiodatoargentate(III) (DPA) is a powerful oxidizing agent in alkaline medium with the reduction potential of 1.74V^[4]. Jayaprakash Rao and other researchers have studied DPA as an oxidizing agent for the kinetics of oxidation of some organic substrates, such as amino acids, reducing sugars and amines ^[5-10]. But the kinetics of oxidation of small peptides by DPA has almost not been reported previously. In this paper, the kinetics and mechanism of oxidation of glycyglycine by dihydroxydiperiodatoargentate (III) is presented.

2. EXPERIMENTAL
2. 1 Materials and Reagents
All chemicals used were of A.R. grade and doubly distilled water was used throughout the work. The stock solution of glycyglycine was prepared by dissolving an appropriate amount of sample in doubly distilled water. Then glycyglycine was converted into potassium salt by addition of equivalent KOH solution. The glycyglycine was used from its stock solution. KNO₃ and KOH were used to maintain the ionic strength and alkalinity of the reaction, respectively. The stock standard solution of IO₄^- was prepared by dissolving KIO₄ in doubly distilled water and kept for 24h to attain the equilibrium.
2. 2 Apparatus Kinetic Measurements     
The kinetic measurements were performed on a UV-vis spectrophotometer (TU-1901, Beijing Puxi Inc., China), which had a cell holder kept at constant temperature (± 0.1^oC) by circulating water from a thermostat (BG-chiller E₁₀, Beijing Biotech Inc., Beijing). The kinetics measurement was carried out under pseudo-first-order conditions [Glycyglycine]₀>>[Ag(III)]₀ at 293.2K-308.2K. The reaction was initiated by mixing DPA with the glycyglycine solution which also contained KNO₃, KOH and KIO₄. The progress of the reaction was monitored spectrophotometrically at 362nm, which is the maximum absorption wavelength of DPA. It was verified that there was almost no interference from other species in the reaction mixture at this wavelength.
2.3 Product analysis and free radical detection             
A solution with known concentrations of [DPA] and [OH^-] was mixed with an excess of glycyglycine. The completion of the reaction was marked by the complete fading of DPA color. After completion of the reaction, the product of the oxidation was identified as the corresponding aldehyde acid by their characteristic spot test^[12]. The addition of acrylonitrile or acrylamide to the reaction mixture under the protection of nitrogen, neither changed the rate nor initiated any polymerization, showing no free radicals in the reaction.

3. RESULTS AND DISCUSSION
3. 1 Evaluation of Pseudo-First Order Rate Constants                  
Under the conditions of [Glycyglycine]₀>>[Ag(III)]₀, the plots of ln(A_t-A_∞) versus time are linear, indicating the reaction is first order with respect to [Ag(III)], where A_t and A_∞ are the absorbance at time t and at infinite time respectively. The pseudo-first-order rate constants k_obs were calculated by the method of least squares (r≥0.999). To calculate k_obs generally 8-10 values of A_t within three times the half-life were used. The values of k_obs were average values of at least three independent experiments and reproducibility of k_obs is within the experimental error ±5%.
3.2 Rate dependence on [glycyglycine]                        
The plots of lnk_obs versus ln[glycyglycine] are straight linear (r>0.99) at fixed [DPA], [IO₄^-], [OH^-], ionic strength (m) and temperature . From the slope, the observed reaction order (n_ap) was found to be 1.30-1.35, and [glycyglycine]²/k_obs versus [glycyglycine] are straight linear with a positive intercept (r > 0.999) (Fig. 1).

Fig.1 Plots of [glycyglycine]²/k_obs versus [glycyglycine]
[Ag(III)]=4.14×10^-5 mol·L^-1, [OH^-]=0.03mol·L^-1, [IO₄^-]=0.004mol·L^-1, m=0.064mol·L^-1.

3.3 Effect of [IO₄^-]           
The concentrations of IO₄^- were varied from 2.0×10^-3 to 4.5×10^-3mol·L^-1 at constant [DPA], [OH^-], [glycyglycine] and m. It was observed that the rate constants decreased by increasing [IO₄^-]. The plots of 1/k_obs vs. [IO₄^-] are straight lines with a positive intercept (r >0.999) (Fig. 2).

Fig.2 Plots of 1/k_obs vs.1/[IO₄^-] at 303.2K
[Ag(III)]=4.14×10^-5mol·L^-1, [glycyglycine]=0.004mol·L^-1, [OH^-]=0.03mol·L^-1, m=0.064mol·L^-1.

3. 4 Effect of [OH^-] and Ionic Strength (μ)           
At constant [DPA], [glycyglycine], [IO₄^-], m and temperature, the values of k_obs decreased with the increase of [OH^-], and then increased with the increase of [OH^-]. The concentration of OH^- was approximately 0.02 mol·L^-1 at the turning point, at which the rate was the slowest (Table 1). The addition of KNO₃ solution, to adjust the ionic strength of the reaction at constant [DPA], [glycyglycine], [IO₄^-], [OH^-] and temperature, had no effect on the rate(Table 2). It showed that there was no salt effect, which was consistent with the common regulation of the kinetics^[13].

Table 1. k_obs varying with [OH^-] at 303.2K

[Ag(III)]=4.14×10^-5mol·L^-1, [glycyglycine] = 0.004mol·L^-1, [IO₄^-]=0.004mol·L^-1, μ=0.064mol·L^-1.

Table 2. k_obs varying with ionic strength at 303.2K

[Ag(III)]=4.14×10^-5mol·L^-1, [glycyglycine]=0.004mol·L^-1, [OH^-]=0.03mol·L^-1, [IO₄^-]=0.004mol·L^-1

4. REACTION MECHANISM           
In periodate aqueous solution equilibria (1)-(3) were observed and the corresponding equilibrium constants at 298.2K were determined by Aveston ^[14]
2IO₄^- + 2 OH^- H₂I₂O₁₀^4-                  log b₁=15.05 (1)
IO₄^-+OH^-+H₂O H₃IO₆^2-               log b₂= 6.21  (2)
IO₄^- + 2 OH^- H₂IO₆^3-                  log b₃= 8.67   (3)
    The distribution of all species of periodate in aqueous alkaline solution can be calculated from equilibria (1)-(3). In the [OH^-] range used in this work the amount of dimer and IO₄^- species of periodate is neglected. The main species of periodate are H₂IO₆^3- and H₃IO₆^2-, consistent with the result calculated from Crouthamel's data^[15] by Murthy. Eqs. (4) and (5) can be obtained from(3) and(2):
(4)
(5)
    Here [IO₄^-]_ex represents the concentration of original overall periodate ion and is approximately equal to the sum of [ H₂IO₆^3- ] and [H₃IO₆^2-]. Based on such distribution, the formula of Ag(III) periodate complex may be represented by either [Ag( OH)₂( H₃IO₆)₂]^3- or the less protonated [Ag( OH)₂ (H₂IO₆)₂]^5-. We preferred to use the latter to represent DPA because it is closer to that suggested by Mukherjee^[16].
    Based on the above discussion, two simultaneous reaction mechanisms were proposed:
Mechanism I—in weaker alkaline medium:
[Ag(OH)₂(H₂IO₆)₂]^5- +OH^- [Ag(OH)₂(HIO₆)]^3- ＋H₂IO₆^3-＋H₂O          (6)
          A                                                                  B
^－OOCCH₂NHCOCH₂NH₂＋H₂O －OOCCH₂NHCOCH₂NH₃^＋＋OH^-    (7)
         R^-                                                                                           RH
[Ag(OH)₂(HIO₆)]^3-＋RH [Ag(OH)₂(HIO₆)(RH)]^3-       (8)
             B                                                            adduct
[Ag(OH)₂(HIO₆)(RH)]^3-＋R^-[Ag(OH)₂(HIO₆)(RH)]^3-R^-      (9)
[Ag(OH)₂(HIO₆)(RH)]^3-R^- product         (10)
    The total concentration of Ag(III) at time t can be written as [Ag(III)]_t=[A]_e＋[B]_e ＋[adduct]_e. Since reaction (9) is the rate-determining step, the rate of disappear of [Ag(III)]_t is represented as:

-d[Ag(III)]_t/dt=k·[adduct]_e·[R^-]
              (11)

k_obs=    (12)
    Equation(13) can be obtained from equation(4) and (12)
   k_obs= (13)
         (14)

Mechanism II—in stronger alkaline medium:
[Ag(OH)₂(H₂IO₆)₂]^5- ＋OH^- [Ag(OH)₂(HIO₆)]^3- ＋H₂IO₆^3-＋H₂O (15)
         A                                                                   B
[Ag(OH)₂(HIO₆)]^3- ＋R^- [Ag(OH)₂(HIO₆)(R^-)]^4- (16)
        B                                                          adduct
[Ag(OH)₂(HIO₆)(R^-)]^4-＋R^- [Ag(OH)₂(HIO₆)(R^-)]^4-R^- (17)
[Ag(OH)₂(HIO₆)(R^-)]^4-R^- product (18)
    The total concentration of Ag(III) at time t can be written as [Ag(III)]_t=[A]_e＋[B]_e ＋[adduct]_e. The reaction (17) is the rate-determining step, the rate of disappear of [Ag(III)]_t is represented as:
-d[Ag(III)]_t/dt=k·[adduct]_e·[R^-]
                        (19)
k_obs =                 (20)
Equation(21) and (22) can be obtained from equation (20) and (4)
   (21)
   (22)
   (23)
    Equation (13) and (22) can explain the values of k_obs decreased rapidly with the increase of [OH^-] up to the lowest point. After the point, it increased gradually with the continuous increase of [OH^-]. Equation (21) shows that the plots of 1/k_obs versus [IO₄^-]_ex should be linear(evidenced by Fig.2). Equation (20) shows that the order in glycyglycine should be 1< n_ap <2 and Equation (23) shows that [glycyglycine]²/k_obs versus [glycyglycine] should be linear (confirmed by Fig.1). Meanwhile, the plots of [glycyglycine]²/k_obs versus [glycyglycine] were linear at different temperatures. From Equation (23), it can be seen that the slope of the linear plot of [glycyglycine]²/k_obs versus [glycyglycine] is 1/k. From the slopes in Fig.1, the values of k at different temperature were evaluated. The activation parameters were evaluated by the method given earlier^[17]. Rate constants and activation parameters are listed in Table 3. The negative DS^≠supports the view that the rate-limiting step consists in the formation of an intermediate complex and does not involve the breaking of a bond.

Table 3. Rate constants (k) and activation parameters of the rate-determining step at 298.2K

    The regression equation of glycyglycine is shown: lnk=23.06-6320.31/T, (r=0.997)

REFERENCES           
[1] Feeny RE. Int J Pept Proteins, 1987, 29: 145-161.
[2] Criado S et al. Journal of Photochemistry and Photobiology B: Biology, 2001, 65: 74–84.
[3] Lazzaro F et al. Tetrahedron Letters, 2004, 45 : 9237–9240.
[4] Sethuram B. Some Aspects of Electron Transfer Reactions Involving Organic Molecules. New Delhi: Allied Publishers (P) Ltd, 2003 :151.
[5] Rao P J, Sethuram B , Rao T N. React. Kinet. Catal. 1985, 29: 289-296.
[6] Krishna K V, Rao P J P. Transition Met Chem, 1995, 20 (1): 344-346.
[7] Kumar A, Vaishali, Ramamurthy P. Int J Chem Kinet, 2000, 32: 286-293.
[8] Sarala G, Rao P J P, Rao T N. Indian J.Chem, 1987, 26A: 475-480.
[9] Shan J H, Li S M, Huo S Y. Transition Met Chem, 2005, 30 (6): 651-654.
[10] Kumar A et al. Polyhedron, 1999, 18 : 773.
[11] Cohen G L, Atkinson G. Inorg.Chem, 1964, 3 : 1741.
[12] Feigl F. Spot Tests in Organic Analysis. New York : Elsevier Publishing Co, 1956: 208.
[13] Jin J J. Kinetics Principie of Chemical Reaction in Liquid Phase. Shanghai: Science Technique Press, 1984: 186-218.
[14] Aveston J. J. Chem. Soc (A), 1969, 273.
[15] (a) Crouthamel C E, Meek H V, Martin D S et al. J. Am. Chem.Soc, 1949, 71: 3031.
       (b) Crouthamel C E, Meek H V, Martin D S et al. J. Am. Chem.Soc, 1951, 73: 82.
[16] Mukherjee H G, Mandal B, De S. Indian J. Chem, 1984, 23A: 426.
[17] Shan J H, Liu T Y. Acta Chimica Sinica, 1994, 52: 1140.

碱性介质中研究二羟基二(过碘酸根)合银(III)氧化双甘肽的反应动力学及机理
单金缓 王合叶 宋常英 王芳 申世刚
(河北大学化学与环境科学学院，保定 071002)
摘要 采用分光光度法在293.2K-308.2K温度范围研究了碱性介质中二羟基二(过碘酸根)合银(III)配离子(DPA)对双甘肽的氧化还原反应动力学及机理。结果表明：反应对DPA是准一级反应，对双甘肽的反应级数为1.30-1.35；在保持准一级条件下([glycyglycine]₀>>[DPA]₀)，反应表观速率常数随着[OH^-]的增加先减小后增加，随着[IO₄^-]的增加而减小；无明显的盐效应且没有自由基检测出来。根据实验结果提出反应机理，由此得出反应速率方程式，并能解释所有的实验现象。同时根据温度对反应速率的影响，进一步求得速控步骤速率常数k，估算出反应的活化能并提出DPA氧化双甘肽的机理。
关键词 二羟基二(过碘酸根)合银(III) 双甘肽 氧化还原反应 动力学 反应机理　

[ Back ] [ Home ] [ Up ] [ Next ]