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)

AbstractThe 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 (kobs) decreased with the increase of the [IO decreased with the increase of the [OH], and then increased with the increase of the [OH] after a turning point. The kobs 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.

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. 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. KNO and KOH were used to maintain the ionic strength and alkalinity of the reaction, respectively. The stock standard solution of IO was prepared by dissolving KIO 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 (± 0.1C) 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]>>[Ag(III)] at 293.2K-308.2K. The reaction was initiated by mixing DPA with the glycyglycine solution which also contained KNO, KOH and KIO. 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. 1 Evaluation of Pseudo-First Order Rate Constants

Under the conditions of [Glycyglycine]>>[Ag(III)], the plots of ln(A-A) versus time are linear, indicating the reaction is first order with respect to [Ag(III)], where A and Aare the absorbance at time t and at infinite time respectively. The pseudo-first-order rate constants kobs were calculated by the method of least squares (r≥0.999). To calculate kobs generally 8-10 values of A within three times the half-life were used. The values of kobs were average values of at least three independent experiments and reproducibility of kobs is within the experimental error ±5%.
3.2 Rate dependence on [glycyglycine]
The plots of lnkobs versus ln[glycyglycine] are straight linear (r>0.99) at fixed [DPA], [IO], [OH], ionic strength () and temperature . From the slope, the observed reaction order (nap) was found to be 1.30-1.35, and [glycyglycine]/kobs versus [glycyglycine] are straight linear with a positive intercept (r > 0.999) (Fig. 1).
Fig.1 Plots of [glycyglycine]/kobs versus [glycyglycine]
[Ag(III)]=4.14×10-5 mol·L-1, [OH]=0.03mol·L-1, [IO]=0.004mol·L-1=0.064mol·-1

3.3 Effect of [IO
The concentrations of IO were varied from 2.0×10-3 to 4.5×10-3mol·L-1 at constant [DPA], [OH], [glycyglycine] and. It was observed that the rate constants decreased by increasing [IO]. The plots of 1/kobs vs. [IO] are straight lines with a positive intercept (r >0.999) (Fig. 2).
Fig.2 Plots of 1/kobs vs.1/[IO] at 303.2K
[Ag(III)]=4.14×10-5mol·L-1, [glycyglycine]=0.004mol·L-1, [OH]=0.03mol·L-1=0.064mol·L-1

3. 4 Effect of [OH] and Ionic Strength (μ
At constant [DPA], [glycyglycine], [IO],and temperature, the values of kobs decreased with the increase of [OH], and then increased with the increase of [OH]. The concentration of OH was approximately 0.02 mol·L-1at the turning point, at which the rate was the slowest (Table 1). The addition of KNO solution, to adjust the ionic strength of the reaction at constant [DPA], [glycyglycine], [IO], [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. kobs varying with [OH] at 303.2K

[OH] (mol·L-1 0.010 0.015 0.020 0.030 0.040 0.050
10obs (s-1 2.862 2.771 2.712 2.864 3.140 3.357

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

Table 2.obs varying with ionic strength at 303.2K

(mol·L-1 0.050 0.064 0.100 0.150 0.200
10obs (s-1 2.861 2.898 2.925 2.889 2.920

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

In periodate aqueous solution equilibria (1)-(3) were observed and the corresponding equilibrium constants at 298.2K were determined by Aveston [14]
2IO + 2 OHimage H104-                  log=15.05 (1)
IO+OH+Himage HIO2-               log= 6.21  (2)
IO + 2 OHimage HIO3-                  log= 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 IO species of periodate is neglected. The main species of periodate are HIO3- and HIO2-, consistent with the result calculated from Crouthamel’s data[15] by Murthy. Eqs. (4) and (5) can be obtained from(3) and(2):
image (5)
Here [IOex represents the concentration of original overall periodate ion and is approximately equal to the sum of [ HIO3- ] and [HIO2-]. Based on such distribution, the formula of Ag(III) periodate complex may be represented by either [Ag( OH)( HIO3- or the less protonated [Ag( OH) (HIO5-. 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)(HIO5- +OHimage [Ag(OH)(HIO)]3- +IO3-+O          (6)
A                                                                  B
R                                                                                           RH
[Ag(OH)(HIO)]3-+RH image[Ag(OH)(HIO)(RH)]3-       (8)
B                                                            adduct
[Ag(OH)(HIO)(RH)]3-image[Ag(OH)(HIO)(RH)]3-      (9)
[Ag(OH)(HIO)(RH)]3-imageproduct         (10)
The total concentration of Ag(III) at time t can be written as [Ag(III)]=[A]+[B]+[adduct]. Since reaction (9) is the rate-determining step, the rate of disappear of [Ag(III)] is represented as:

image               (11)

obsimage    (12)
Equation(13) can be obtained from equation(4) and (12)
kobs= image (13)
image         (14)

Mechanism II―in stronger alkaline medium:
[Ag(OH)(HIO5- +OHimage[Ag(OH)(HIO)]3- +IO3-+O (15)
A                                                                   B
[Ag(OH)(HIO)]3-image[Ag(OH)(HIO)(R)]4- (16)
B                                                          adduct
[Ag(OH)(HIO)(R)]4-image[Ag(OH)(HIO)(R)]4- (17)
[Ag(OH)(HIO)(R)]4-image product (18)
The total concentration of Ag(III) at time t can be written as [Ag(III)]=[A]+[B]+[adduct]. The reaction (17) is the rate-determining step, the rate of disappear of [Ag(III)] is represented as:
image                         (19)
obs =image                 (20)
Equation(21) and (22) can be obtained from equation (20) and (4)
image   (21)
image   (22)
image   (23)
Equation (13) and (22) can explain the values of kobs 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/kobs versus [IOex should be linear(evidenced by Fig.2). Equation (20) shows that the order in glycyglycine should be 1< nap <2 and Equation (23) shows that [glycyglycine]/kobs versus [glycyglycine] should be linear (confirmed by Fig.1). Meanwhile, the plots of [glycyglycine]/kobs versus [glycyglycine] were linear at different temperatures. From Equation (23), it can be seen that the slope of the linear plot of [glycyglycine]/kobs 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 negativesupports 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

T /K k/mol-1·L·s-1 activation parameters
293.2 1.49 =52.547 kJ·mol-1
50.048 kJ·mol-1
=-61.502 J·mol-1·K-1
298.2 1.85
303.2 2.26
308.2 2.52

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