Hydrophobicity Dependence of Oxidation of Tetrapeptides of Elastin Sequences with Mn(III): Synthesis, Characterization, Kinetics, and Mechanistic Study

Article abstract of DOI:10.1002/kin.10015

The analogues of elastin sequences, glycyl-glycyl-alanyl-proline (GGAP), glycyl-glycyl-phenylalanyl-proline (GGFP), and glycyl-glycyl-isoleucyl-proline (GGIP) were synthesized by classical solution phase method and characterized. The kinetics of oxidation of these tetrapeptides (TETP) by Mn(III) has been studied in the presence of sulphate ions in acidic solution at 25 deg C. The reaction was followed spectrophotometrically at λ_max = 500 nm. A first-order dependence of rate on both [Mn(III)] and [TETP] was observed. The rate is independent of the concentration of the reduction product, Mn(II), and hydrogen ions. The effects of varying the dielectric constant of the medium and addition of anions such as sulphate, chloride, or perchlorate were studied. Activation parameters have been evaluated using Arrhenius and Eyring plots. The oxidation products were isolated and characterized. A mechanism involving the reaction of TETP with Mn(III) in the rate-limiting step is suggested. An apparent correlation was noted between the rate of oxidation and the hydrophobicity of these sequences, where increased hydrophobicity results in increased rate of oxidation.

Full text of DOI:10.1002/kin.10015

Hydrophobicity
Dependence of Oxidation
of Tetrapeptides of Elastin
Sequences with Mn(III):
Synthesis, Characterization,
Kinetics, and Mechanistic
Study
B. K. KEMPE GOWDA, H. S. PRASAD, K. S. RANGAPPA, D. CHANNE GOWDA
Department of Studies in Chemistry, University of Mysore, Manasagangotri, Mysore-570006, India
Received 2 February 2001; accepted 9 July 2001
ABSTRACT: The analogues of elastin sequences, glycyl-glycyl-alanyl-proline (GGAP), glycyl-
glycyl-phenylalanyl-proline (GGFP), and glycyl-glycyl-isoleucyl-proline (GGIP) were synthesized
by classical solution phase method and characterized. The kinetics of oxidation of these
tetrapeptides (TETP) by Mn(III) has been studied in the presence of sulphate ions in acidic
solution at 25 C. The reaction was followed spectrophotometrically at
= 500 nm. A first-
max
order dependence of rate on both [Mn(III)] and [TETP] was observed. The rate is independent
of the concentration of the reduction product, Mn(II), and hydrogen ions. The effects of vary-
ing the dielectric constant of the medium and addition of anions such as sulphate, chloride,
or perchlorate were studied. Activation parameters have been evaluated using Arrhenius and
Erying plots. The oxidation products were isolated and characterized. A mechanism involving
the reaction of TETP with Mn(III) in the rate-limiting step is suggested. An apparent correlation
was noted between the rate of oxidation and the hydrophobicity of these sequences, where
C
increased hydrophobicity results in increased rate of oxidation. 2001 John Wiley & Sons, Inc.
Int J Chem Kinet 34: 39–48, 2002
to aging and disease processes [1,2]. Peptides and
proteins represent major targets for modification in
these reactions, and identification of sites and struc-
tures of modifications may lead to mechanistic un-
derstanding and approaches for prevention. In this ac-
count, oxidation by Mn(III) is of special importance
due to its biological relevance [3]. Manganese(III)
porphyrins have been studied as possible models for
INTRODUCTION
Oxidative reactions play an important role in a variety
ofbiochemicaleventsrangingfromnormalmetabolism
Correspondence to: D. Channe Gowda; e-mail: dcgowda@
yahoo.com.
c
2001 John Wiley & Sons, Inc.
ODI 10.1002/kin.10015

40
KEMPE GOWDA ET AL.
closely related biologically significant systems [4].
Several studies have been reported on the kinetics of
oxidation of substrates with Mn(III) in different media
[5–7]. Extensive work has been reported on the kinetics
ofoxidationofaminoacidswithvariousmetalions, and
several other oxidants [8–12]. However, similar studies
on the kinetics of oxidation of peptides and proteins by
Mn(III) oxidant have not been reported in the literature
except the oxidative behavior of bromamine-B towards
glycyl-glycine [13].
using HCOONH4/Pd–C (10%) [21]. All coupling re-
actions were achieved with isobutylchloroformate. The
protectedpeptideswerepurifiedbyrecrystalizationand
characterized by physical and analytical techniques.
The purity of the free peptides was checked by paper
chromatography, HPLC and amino acid analysis.
EXPERIMENTAL
Elastic protein-based polymers have their origins
in repeating sequences of the mammalian elastic pro-
tein, elastin [14,15]. The most prominent repeating se-
quence occurs in bovine elastin; it can be written (Val1-
Pro2-Gly3-Val4-Gly5)n where n is eleven without a sin-
gle substitution. Another repeat first found in porcine
elastin is (Val1-Pro2-Gly3-Gly4)n but this repeat has
not been found to occur with n greater than 2 without
substitution [16]. The monomers, oligomers, and high
polymers of these repeats have been synthesized and
confirmationally characterized [17]. These polymers
have a number of medical and nonmedical applica-
tions [18,19]. The cross-linked polytetrapeptide matri-
ces based on the repeating amino acid sequences, Gly-
Gly-Ala-Pro, Gly-Gly-Ile-Pro, and Gly-Gly-Val-Pro
were tested for cell adhesion promoting activity in both
the absence and presence of fetal bovine serum [20].
The degree of cell attachment increased with increase
of hydrophobicity. In this context, it was thought inter-
esting to investigate the oxidative behavior of Mn(III)
towards these tetrapeptides and this is reported in
the present paper.
The peptides were synthesized by the classical so-
lution phase methods. The classical methods for syn-
thesis in solution are labor-intensive, time-consuming,
and skill-intensive in large part because the interme-
diates differ in solubility characteristics. With all of
these concerns, the solution method provides relatively
pure materials that do not require much purification at
the end of the synthesis. On the other hand, the solid-
phase synthesis always yields impure products that re-
quire extensive purification even at the component pep-
tide stage. During the course of our physical studies, a
largeamountofmaterialwasrequired. Forthispurpose,
an efficient, well-established and less expensive tert-
butyloxycarbonyl (Boc) chemistry was used instead of
more expensive 9-fluorenylmethyloxycarbonyl (Fmoc)
chemistry.
All the amino acids used except glycine are of
L-configuration unless otherwise specified. All
tert-butyloxycarbonyl (Boc) amino acids, amino
acid derivatives, 1-hydroxybenzotriazole (HOBt),
and trifluoroacetic acid (TFA) were purchased from
Advanced Chem. Tech., (Louisville, Kentucky, USA).
Isobutylchloroformate and N-methylmorpholine
(NMM) were purchased from Sigma Chemicals (St.
Louis, USA). All solvents and reagents were of ana-
lytical grade or were purified according to procedure
[22] recommended for peptide synthesis. Thinlayer
chromatography (TLC) was carried out on silica gel
plates obtained from Whatman Inc., with the fol-
lowing solvent systems: chloroform–methanol–acetic
acid (95:5:3), R1f ; chloroform–methanol–acetic acid
(90:10:3), R2f ; and chloroform–methanol–acetic acid
(85:15:3), R3f . The compounds on the TLC plates were
detected by UV light after spraying with ninhydrin or
by chlorine/tolidine. Paper chromatography was car-
riedoutonWhatmanNo. 1chromatographicpaperwith
the solvent system butanol–acetic acid–water (4:1:5,
upper phase). The compounds on paper were detected
by spraying with ninhydrin. The melting points were
determined using Selaco Can. No-103 melting point
apparatus and were uncorrected. Elemental analyses
was carried out by Mic Anal (Tucson, Arizona, USA).
The optical rotation was measured using a Perkin-
Elmer 243 digital polarimeter. Amino acid analysis
was performed on water HPLC Pico-Tag analyzer.
Hydrolysis of the sample was carried out using 6 N HCl
containing 1% phenol by volume at 110 C for 72 h in a
sealed tube under vacuum from which the air has been
removed using nitrogen. The product analysis was car-
ried out with gas chromatography (GC 15A, Shimadzu,
Japan).
Boc-Phe-Pro-OBzl (I). To Boc-Phe-OH (13.3 g,
0.05 mol) dissolved in acetonitrile (100 ml) and cooled
to 0 C NMM (5.5 ml, 0.05 mol) was added. The solu-
tion was then cooled to 15 C 1 C and isobutylchlo-
roformate (6.5 ml, 0.05 mol) was added under stirring
while maintaining the temperature at 15 C. After stir-
ring the reaction mixture for 10 min at this tempera-
ture, a precooled solution of HOBt (6.8 g, 0.05 mol)
The synthesis was carried out by the stepwise so-
lution phase method. The Boc group was used for
temporary N␣-protection and its removal was achieved
with 4 N HCl in dioxane or trifluoroacetic acid. The
C-terminus carboxyl group was protected by the benzyl
ester and its removal was effected by hydrogenolysis

OXIDATION OF TETRAPEPTIDES OF ELASTIN SEQUENCES WITH MN(III)
41
was added. The reaction mixture was stirred for
an additional 10 min and a precooled solution of
HCl H-Pro-OBzl (hydrochloride salt of Pro-OBzl)
(12.1 g, 0.05 mol) and NMM (5.5 ml, 0.05 mol) in
DMF (120 ml) was added slowly. After 20 min, the
pH of the solution was adjusted to 8 by the addition
of NMM and the reaction mixture stirred over night
at room temperature. Acetonitrile was removed un-
der reduced pressure and the residual DMF solution
was poured into about 600 ml ice-cold 90% saturated
KHCO3 solution and stirred for 30 min. The precipi-
tated peptide was filtered, washed with water, 1 N HCl,
and water and then dried. The crude peptide was recrys-
tallised from ether and petroleum ether to obtain 19.5 g
of I (yield, 86.5%).
with water, and dried over Na2SO4. The solvent was
removed under reduced pressure and triturated with
ether, filtered, washed with ether, and then dried to
obtain VI–VIII.
Gly-Gly-Xaa-Pro. (Xaa is Ala for IX, Ile for X,
and Phe for XI). Each peptide (VI–VIII, 0.009 mol)
was deblocked with TFA (10 ml/g of peptide) for
40 min. The solvent was removed under reduced pres-
sure, triturated with ether, filtered, washed with ether
to obtain TFA salt of IX–XI (yield, 100%).
Preparation of Mn(III) Sulphate
A 0.05 M solution of Mn (III) sulphate was prepared by
the electrolytic oxidation of Mn (II) sulphate in aque-
ous sulphuric acid by the procedure reported [24] pre-
viously. Even though the solution appeared to be stable
for more than 1 month at [H+] > 5.0 M, it was prepared
afresh daily. All other reagents were prepared from AR
grade chemicals.
Boc-Gly-Phe-Pro-OBzl (II). I (13.6 g, 0.03 mol)
was deblocked with 4.1 N HCl/dioxane (10 ml/g of
peptide) for 1.5 h. Excess HCl and dioxane were re-
moved under reduced pressure, triturated with ether, fil-
tered, washed with ether, and dried (yield, 100%). The
HCl H-Phe-Pro-OBzl (hydrochloride salt of Phe-Pro-
OBzl) was neutralized with NMM (3.3 ml, 0.03 mol)
and coupled to Boc-Gly (5.3 g, 0.03 mol) in acetoni-
trile (50 ml) and NMM (3.3 ml) using isobutylchloro-
formate (4.1 ml, 0.03 mol) and worked up the same as
I to obtain 13.4 g of II (yield, 87.7%). The sample was
recrystallized from ether/petroleum ether.
Preliminary Studies
The maximum absorption ( max) of Mn (III) sulphate
solution occurs at 500 nm. The standard reduction po-
tential E00 of Mn(III)/Mn(II), the oxidizing power of
the oxidant, generally decreases on complexation. The
standard redox potential was measured at the specified
experimental conditions. These details were reported
previously[24]. TheformalredoxpotentialE00 obtained
at different [H2SO4] and in presence of the complex-
Boc-Gly-Gly-Xaa-Pro-OBzl. (Xaa is Ala for III,
Ile for IV, and Phe for V). The peptides Boc-Gly-Ile-
Pro-OBzl and Boc-Gly-Ala-Pro-OBzl were prepared
as previously described [23]. Each peptide (Boc-Gly-
Phe-Pro-OBzl, Boc-Gly-Ile-Pro-OBzl, and Boc-Gly-
Ala-Pro-OBzl, 0.02 mol) was deblocked separately
with 4.1 N HCl/dioxane (10 ml/g of peptide) for
1.5 h. Excess HCl and dioxane were removed under
reduced pressure, triturated with ether, filtered, washed
with ether, and dried (yield, 100%). The HCl H-Gly-
Xaa-Pro-OBzl was neutralized with NMM (2.2 ml,
0.02 mol) and coupled to Boc-Gly (3.5 g, 0.02 mol) in
acetonitrile (35 ml) and NMM (2.2 ml, 0.02 mol) using
isobutylchloroformate (2.6 ml, 0.02 mol) and worked
up the same as I to obtain III–V. The samples were
recrystallized from ether/petroleum ether.
4
ing agents HSO4 , P2O7 and Cl is 1.51, 1.48 and
1.42 V respectively. Triply distilled water was used for
preparing aqueous solutions.
Kinetic Measurements
Mixtures of solution containing requisite amounts of
TETP, sulphuric acid (to maintain known acid concen-
tration), Mn(II), and water (to keep the total volume
constant) were prepared in stoppered boiling tubes. The
mixture was thermally equilibrated in a water bath at
25 C. To the solution in this tube an aliquot of pre-
equilibrated Mn(III) sulphate stock solution was added
to give a known overall concentration. The progress of
the reaction was monitored for two half-lives by mea-
suring the absorbance of unreacted Mn(III) at 500 nm
using a Spectrochem MK II spectrophotometer. The
reaction mixture was quenched appropriately. Plots
of log (absorbance) versus time were linear. The rate
constants kobs calculated from these plots were repro-
ducible within 3% error.
Boc-Gly-Gly-Xaa-Pro-OH. (Xaa is Ala for VI,
Ile for VII, and Phe for VIII). Each peptide
(III–V, 0.01 mol) was hydrogenolyzed in methanol
(10 ml/g of peptide) using ammonium formate (2.0
equiv.) and 10% Pd–C (0.1 g/g of peptide) for 30 min at
roomtemperature. Thecatalystwasfilteredandwashed
with methanol. The combined filtrate was evaporated
in vacuo and the residue taken in to CHCl3, washed

42
KEMPE GOWDA ET AL.
RESULTS
Dependence of Rate on [Mn(II)]
and Added Salts
Peptide Synthesis
The effect on the rate of varying concentrations of
Mn(II) (which is the reduction product of the oxidant)
wasinvestigated. Increasein[Mn(II)](0.01–0.1M)had
no effect on the rate. Similarly the effect of anions Cl
The purity of the protected peptides (I–VIII) was
checked by TLC and of the free peptides (IX–XI) by
paper chromatography and HPLC. The yields, physi-
cal and analytical data of protected peptides are listed
in Table I. The amino acid analysis of free peptides is
shown in Table II.
2
(0.01–0.5 M), SO4 (0.01–0.5 M), and ClO4 (0.01–
1.0 M) on the rate was insignificant. The reaction prod-
uct Mn(II) had no effect on the reaction, indicating that
the product is not involved in a preequilibrium with the
oxidant.
Dependence of Rate on [Mn(III)]
and [TETP]
Effect of Solvent Composition
All kinetic runs were performed under pseudo first
order conditions of [TETP] [Mn(III)]. Plots of log
[Mn(III)] vs. time were linear even beyond 75% of the
reaction, showing first order dependence of the rate on
[Mn(III)]. At constant [Mn(III)]0, [Mn(II)]0, [H2SO4],
[Na2SO4], and temperature, the rate increased with
increase in [TETP]0. Plots of log kobs vs. log [TETP]0
(Fig. 1) were linear with slopes of 1.01, 1.00, and 1.00
for Gly-Gly-Ala-Pro, Gly-Gly-Phe-Pro, and Gly-Gly-
Ile-Pro, respectively.
The solvent composition of the medium was varied
by adding methanol (0.0–40%) to the reaction mix-
ture. The rate increased with increase in methanol con-
tent. The plots of log kobs vs. 1/D (D = dielectric con-
stant of the medium) were linear with positive slopes
(Fig. 2). Measurement of rate constants were taken
both in the presence and absence of TETP oxidized
with Mn(III) and used for the calculation of effective
kobs, although the rate of oxidation of methanol in the
absence of TETP is negligible under the present con-
ditions employed.
Dependence of Rate on [Acid]
Kinetic measurements were performed in H2SO4–
NaHSO4 solutions at different [H+]. The effective [H+]
used was evaluated with the aid of a standard curve [25]
of [H2SO4] vs. [H+]. Increase in [H+] (0.1–2.0 M),
had no effect on the rate.
Activation Parameters
To determine the activation parameters, the reactions
were carried out at different temperatures (20–40 C).
The Arrhenius plots of log kobs vs. 1/T (Fig. 3)
Table I Physical and Analytical Data of Protected Peptides
Elemental Analysis (%)
M.P.
( C)
Yield
[␣]2D5
Molecular
Formula
Peptides
R1f R2f R3f (%) (C,1; MeOH)
C
H
N
Boc-Phe-Pro-OBzl (I)
100 0.73 0.82
86 0.66 0.74
56 0.63 0.70
77 0.61 0.69
96 0.59 0.65
–
–
–
–
–
86.5
87.7
89.6
89.6
89.6
0.52
72
C26H32N2O5
68.90 7.07
(69.00) (7.10)
66.10 6.94
(65.99) (6.92)
57.90 7.12
(58.08) (7.10) (11.61)
60.66 7.49 10.69
(60.88) (7.56) (10.70)
63.71 6.69
(63.58) (6.75)
51.02 7.02
(50.99) (7.04) (13.99)
54.40 7.73 12.63
(54.28) (7.74) (12.66)
57.77 6.78 11.74
(57.97) (6.76) (11.75)
6.12
(6.19)
8.22
(8.24)
11.61
Boc-Gly-Phe-Pro-OBzl (II)
C
C
C
C
C
C
C
28H35N3O6
24H34N4O7
27H40N4O7
30H38N4O7
17H28N4O7
20H34N4O7
23H32N4O7
Boc-Gly-Gly-Ala-Pro-OBzl (III)
Boc-Gly-Gly-Ile-Pro-OBzl (IV)
Boc-Gly-Gly-Phe-Pro-OBzl (V)
Boc-Gly-Gly-Ala-Pro-OH (VI)
Boc-Gly-Gly-Ile-Pro-OH (VII)
79
75
15.3
10.6
66
9.86
(9.88)
14.00
66
99
–
–
–
0.46 0.49 92.4
0.36 0.44 91.3
0.39 0.48 93.4
Boc-Gly-Gly-Phe-Pro-OH (VIII) 105
33
aCalculated elemental values are in parenthesis.

OXIDATION OF TETRAPEPTIDES OF ELASTIN SEQUENCES WITH MN(III)
43
3
Figure 1 Effect of varying reactant concentration on the rate. [Mn(II)] = 0.02 mol dm 3, and [H2SO4] = 0.2 mol dm at
temperature 25 C.
were found to be linear. The activation energies (Ea)
were calculated from the slope of the plots. From
this value, the thermodynamic parameters 1H#, 1S#,
1G# and the frequency factor (log A) were evaluated
(Table III).
the in situ formation of free radical species during the
reaction sequence.
Reaction Stoichiometry
ThemixturescontainingTETP(0.001M), acid(0.1M),
and excess Mn(III) (0.01 M) were kept for 24 h at
25 C. The unconsumed Mn(III) was then determined,
8 moles of oxidant were sufficient to oxidise 1 mole of
TETP leading to aldehydes, carbon dioxide, ammonia,
Mn(II), and hydrogen ion.
Test for Free Radicals
Addition of the reaction mixture to aqueous acrylamide
monomer solution initiates polymerization, indicating
Table II Amino Acid Composition of Free Peptides
Based on these results the following stoichiometric
equations are suggested.
Peptides
Gly
Proa
Ala
Ile
–
Phe
–
GGAP (IX)
2.07
(2.00)
2.10
(2.00)
1.98
1.00
0.98
(1.00)
–
GGIP (X)
1.00
1.00
0.97
(1.00)
–
–
H2N CH2 CO NH CH2 CO NH CH(R)
GGFP (XI)
–
1.02
(1.00)
CO N CH CO2H + 4Mn(OH)2+ + 4Mn3+
(2.00)
+ 3H2O → 2HCHO + RCHO + NH3+(CH2)3CHO
Calculated values are in parenthesis.
aPro taken as 1.0.
+ 4CO2 + 3NH4+ + 8Mn2+
(1)

44
KEMPE GOWDA ET AL.
3
2
Figure 2 Effect of varying dielectric constant on the rate. [Mn(III)] = 2 10 mol dm3, [TETP] = 1.0 10 mol dm3,
[Mn(II)] = 0.02 mol dm3, and [H2SO4] = 0.2 mol dm3 at temperature 25 C.
R:CH3 for Gly-Gly-Ala-Pro; CH3 CH2
CH(CH3) for Gly-Gly-Ile-Pro
column chromatography on silica gel (60–200 mesh)
using gradient elution (dichloromethane to chloro-
form). Aldehydes were analyzed qualitatively by gas
chromatography. The retention values of formalde-
hyde, acetaldehyde, isobutyraldehyde, phenylacetalde-
hyde, and 4-aminobutyaraldehyde are 6.0, 5.14, 27.4,
31.09, and 32.1 respectively, which are identical with
authentic samples. Ammonia and CO2 were detected
by the conventional tests.
and C6H5 CH2 for Gly-Gly-Phe-Pro.
Product Analysis
After the reaction was completed, the reaction prod-
ucts were extracted with ether and subjected to
Table III Activation Parameters for the Oxidation of TETP by Mn(III)
1
1
1
1
1
Substrate
Ea (kJ mol
)
1H# (kJ mol
)
1S# (J K mol
)
1G# (kJ mol
)
Log A
GGAP
GGFP
GGIP
59.93
54.70
58.44
57.42
52.18
55.92
110.05
132.49
119.10
90.73
92.32
92.00
7.33
6.31
7.02
3
2
[Mn(III)] = 2 10 mol dm 3, [TETP] = 1.0 10 mol dm3, [Mn(II)] = 0.02 mol dm3, and [H+] = 0.2 mol dm3.

OXIDATION OF TETRAPEPTIDES OF ELASTIN SEQUENCES WITH MN(III)
45
3
Figure 3 Temperature dependence of the oxidation of TETP by Mn(III). [Mn(III)] = 2 10 mol dm3, [TETP] = 1.0
2
10 mol dm3, [Mn(II)] = 0.02 mol dm3, and [H2SO4] = 0.2 mol dm3 at different temperatures.
species.
DISCUSSION
Mn3+(aq) + H2O
Mn(OH)2+(aq) + H+ (2)
Data published by Diebler and Sutin [26], Packler
and Chawla [27], and Davies [28], have shown
that in the presence of F ion, aqueous solutions
of Mn(III) consist of hexaaquomanganese (III)
{[Mn(H2O)6]3+}, Mn(III)(aq), hydroxopentaaquoman-
ganese (II) {[Mn(OH)(H2O)5]2+}, Mn(OH)2+(aq), and
MnF2+(aq). Along the same line it may be assumed
with justification that the Mn(III) species present in
The hydrolysis constant calculated was Kh =
0.93 0.03. The absorption spectra of both Mn3+
(aq)
and Mn(OH)2+ have been reported to be similar in
(aq)
both the visible and UV regions. Our observation of
the electronic absorption spectra is consistent with the
values reported.
The electrochemical studies of Biederman and
Polombari [30] also indicated a significant amount of
Mn(OH)2+, as shown in Eq. (3).
sulphuric acid solution are Mn(III)(aq), Mn(OH)2+
,
(aq)
and MnSO4+. Therefore, it was shown [29] that
manganese (III) sulphate in aqueous sulphuric acid
contains Mn3+
and Mn(OH)2+
as reaction
Mn(OH)2+ + H2O → Mn(OH)2+ + H+
(3)
(aq)
(aq)

46
KEMPE GOWDA ET AL.
dm 3). The high value of ε has been attributed to the
presence of the hydrolyzed species Mn(OH)2+. There-
fore, it is probable that Mn(OH)2+ is the likely reactive
species.
The following Scheme I accounts for the observed
experimental results:
But a fresh solution of Mn(III) sulphate was always
prepared and used immediately after cessation of the
electrolysis, thereby, eliminating any reaction due to
Mn(OH)2+. The absence of a sulphate effect on the re-
+
action rate indicates that MnSO4
is not the active
(aq)
species under the present condition. Since there is no
hydrogen ion dependence on the rate, this suggests that
k
1
Mn(OH)2+ + TETP → X
(i) slow and rds
(ii) fast
Mn3+ and Mn(OH)2+ are the reactive species [31].
(aq)
k
2
X
→ Products
The molar absorption coefficient ε ranges between 131
1
1
and 110 dm3 mol cm at [H+] = 1.20–2.50 (mol
SCHEME 1 Hence, Rate = k0 [Mn(OH)2+] [TETP]
MECHANISM:
H3N+ CH2 CO NH CH2 CO NH CH(R) CO
N
CH CO OH + MnOH2+
→
[H2N CH2 CO NH CH2 CO NH CH(R) CO
N
CH
C
||
OH]3+ + H2O
↓
A
Mn
O
A
→ H 2 N CH2 CO NH CH2 CO NH CH(R) CO
N
CH
C
OH + Mn2+
||
B
O.+
B + Mn3+ → H2N CH2 CO NH CH2 CO NH CH(R) CO
N
CH+ + CO2 + H+ + Mn2+
C
C + 2H2O → H2N CH2 CO NH CH2 CO NH CH(R) CO2H ++NH3(CH2)3CHO
D
D + MnOH+2 + H+ → [H2N CH2 CO NH CH2 CO NH CH(R)
C
||
OH]3+ + H2O
↓
E
Mn
O
E
→ H 2 N CH2 CO NH CH2 CO NH CH(R)
C
OH + Mn2+
||
F
O.+
F + Mn3+ → H 2 N CH2 CO NH CH2 CO NH +CH(R) + CO2 + H+ + Mn2+
G
G + 2H2O → H 2 N CH2 CO NH CH2 COOH + RCHO + NH4+
H
H + MnOH2+ + H+ → [NH2 CH2 CO NH CH2
C
||
OH]3+ + H2O
↓
I
Mn
O
I
→ H 2 N CH2 CO NH CH2
C
OH + Mn2+
||
J
O.+
J + Mn3+ → H 2 N CH2 CO NH +CH2 + CO2 + H+ + Mn2+
K
K + 2H2O → H 2 N CH2 COOH + NH+4 + HCHO
L
L + MnOH2+ + H+ → [H2N CH2
C
||
OH]3+ + H2O
↓
Mn
O
M
M
→ H 2 N CH2
C
OH + Mn2+
||
O.+
N
N + Mn3+ → H 2 N + CH2 + CO2 + H+ + Mn2+
O
O + H2O → CH2O + NH+4
SCHEME 2 R: CH3 for GGAP, CH3CH2CH(CH3) for GGIP, C6H5CH2 for GGFP

OXIDATION OF TETRAPEPTIDES OF ELASTIN SEQUENCES WITH MN(III)
47
Amis [32] has shown that plots of log kobs vs. 1/D
give a straight line with a positive slope for positive
ion-dipole interaction. The positive dielectric effect in
the present investigation shows charge dispersal in the
transition state, pointing towards a positive ion-dipole
reaction and hence supports Scheme II.
7. Rangappa, K. S.; Chandraju, S.; Made Gowda, N. M. Int
J Chem Kinet 1998, 30, 7.
8. Rangappa, K. S.; Manjunathaswamy, H.; Ragavendra,
M. P.; Channe Gowda, D. Carbohydr Res 1998, 307,
253.
9. Rangappa, K. S.; Manjunathaswamy, H.; Ragavendra,
M. P.; Channe Gowda, D. J Org Chem 1998, 63, 531.
10. Rangappa, K. S.; Ragavendra, M. P.; Mahadevappa,
D. S. Carbohydr Chem 1997, 16, 359.
11. Mahadevappa, D. S.; Ananda, S.; Made Gowda, N. M.;
Rangappa, K. S. J Chem Soc Perkin Trans II, 1985, 11,
39.
12. Rangappa, K. S.; Chandaraju, S.; Mahadevappa, D. S.
Transition Met Chem 1996, 21, 519.
13. Asha Iyengar, T.; Mahadevappa, D. S. Indian J Chem
1992, 31A, 752.
14. Sandberg, L. B.; Leslie, J. G.; Leach, C. T.; Torres,
V. L.; Smith, A. R.; Smith, D. W. Pathol Biol 1985,
33, 266.
15. Yeh, H.; Ornstein-Goldstein, N.; Indik, Z.; Sheppard, P.;
Anderson, N.; Rosenbloom, J. C.; Cicila, G.; Yoon, K.;
Rosenbloom, J. Collagen Relat Res 1987, 7, 235.
16. Sandberg, L. B.; Soskel, N. T.; Leslie, J. B. Engl J Med
1981, 304, 566.
The rate of oxidation of TETP by Mn(III) was com-
pared with that of oxidation of tripeptides, Gly-Ala-
Pro, Gly-Ile-Pro and Gly-Phe-Pro, dipeptides, Ala-
Pro, Ile-Pro and Phe-Pro, and free amino acids by
Mn(III) under identical experimental conditions, and
it was found that the rate of oxidation of tetrapep-
tide was slower than either tripeptides, dipeptides, and
free amino acids. The change in each case is due to
the increased distance between the functional groups
and consequently weaker electrostatic effects. Hence,
the oxidation of the tetrapeptides is expected to be
slower than the monomers, dipeptides, and tripep-
tides. Further, an apparent correlation was noted be-
tween the rate of oxidation and the hydrophobicity
of these sequences, where increased hydrophobicity
results in increased rate of oxidation. The order of
rate of oxidation of tetrapeptides was found to be
GGFP > GGIP > GGAP which is well in agreement
with their hydrophobicity [33].
17. Urry, D. W.; Long, M. M. CRC Crit Rev Biochem 1976,
4, 1.
18. Urry, D. W.; McKee, L. D.; Williams, T.; Olsen, D. B.;
Cox, B. A. Medical Application of Bioelastic Materials,
in Biotechnological Polymers: Medical, Pharmaceuti-
cal and Industrial Applications, Technomic Publishing:
Atlanta, Georgia, 1993; pp. 82.
19. Urry, D. W.; McPherson, D. T.; Xu, J.; Daniell, H.;
Guda, C.; Channe Gowda, D.; Jing, N.; Parker, T.
M. Polymeric Materials Encyclopedia, Salamone, J. C.
(Ed.); CRC Press: Boca Raton, 1996; pp. 7263.
20. Nicol, A.; Channe Gowda, D.; Parker, T. M.; Urry, D. W.
J Biomed Mater Res 1993, Vol. 27, pp. 801.
21. Anwer, M. K.; Spatola, A. F. Synthesis 1980, 929.
Spectral Evidence for the Formation
of TETP–Mn (III) Complex
The study of UV–Visible spectra separately of
pure Mn(III), TETP (glycyl-glycyl-alanyl-proline,
glycyl-glycyl-phenylalanyl-proline,
and
glycyl-
glycyl-isoleucyl-proline), and a mixture of Mn(III)
and TETP show deviation in peak wave length(
and absorbance(Abs) as follows.
)
max
Substrate
in nm
Abs
Complex
in nm
max
Abs
max
Mn(III)
GGAP
GGIP
500
0.970
2.789
3.000
2.913
215.0
223.0
222.0
Mn(III) + GGAP
Mn(III) + GGIP
Mn(III) + GGFP
241.5
238.5
239.5
2.987
3.063
3.023
GGFP
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