-
Indian Journal of ChemistryVol. 27 A. November 1988, pp.
979-982
Dissociation Constants of Esters of Glycine &Leucine in
Different Mixed Solvents
B P DEY & S C LAHIRI*Department of Chemistry, University of
Kalyani, Kalyani
Received 25 September 1985; revised 29 July 1987;rerevised and
accepted 23 February 1988
In order to determine the ratio of zwitterion to neutral forms
of the amino acids in different mixed solvents, the dis-,sociations
constants of.the methyl esters and ethyl esters of glycine and
leucine have been determined pH-metrically inmethanol + water,
ethanol + water, isopropanol + water and r-butanol + water
mixtures. These data have been used todetermine the values of the
different microscopic constants of the amino acids. The results
indicate that the neutralforms are present in negligible amounts
even in the high percentages of alcohols.
One of the important features of the amino acids isthat they are
present predominantly as "zwitterions"in water, though the presence
of slight amounts ofneutral forms is not ruled out. The
macroscopicconstants (K1 and K 21determined experimentallyare
resultant of the processes actually occurring inthesolutin(Eqs
1-5)1: ' ,
K,
NH;CHRC02H +H20 "'NH;CHRCO; + H30' ... (1)
K.
NH;CHRC02H+H20 i=' NH2CHRC02H+H30' ... (2)
K,
NH;CHRCO; + OB" ••• NH2CHRCO; + H20 .. , (3)
"'·u
NH2CHRC021:H OH - •• NH2CHRCO; + H20 ... (4)
.•. (5)
where,
111'-=-+-'«, «: s;KA s;and ,Kz =-, - = -KB KD
Thus, the knowledge of K z would be of help notonly in
determining the ratio of zwitterion to neutralform but also in
determining the microscopic con-stants utilising the values of K I
and K 2'
Since the neutral amino acid is non-existent, thedissociation
constant of the ester of amino acid,
.•. (6)
is regarded to be the K z for the reaction (5) and isutilised
for the determination of the microscopicconstants in water.
In the course of our studies on the effect of sol-vents on the
dissociation constants of the amino ac-ids, we found that the
values of K z of amino acid es-ters in aquo-organic mixtures are
particularly im-portant in obtaining information regarding the
mic-roscopic constants and the ratio of the 'Zwitterion'to neutral
form of amino acids in mixed solvents.
The ratio of neutral to zwitterionic forms (K z) ina few
aquo-organic mixtures has been determinedby Edsall et all assuming
p KE values of ester aminehydrochloride in water to remain
unchanged inaquo-organic solvents.
However, the pK-values of the 'isoelectric reac-tions' have been
found to change with solvent com-position", Therefore, it is
desirable to determine thepKE values of esters of the amino acid
hydrochlo-rides in aquo-organic mixtures to get
microscopicconstants and the extent of conversion from
zwitte-rionic to neutral species. These considerations ledus to
determine the dissociation constants of methyland ethyl esters of
glycine and leucine in aquo-or-ganic mixtures (methanol + water,
ethanol + water,isopropanol +water and r-butanol +water
mix-tures).
Materials and MethodsThe methyl and ethyl esters of glycine and
leucine
hydrochlorides (GR, Sigma Chemicals) were usedwithout further
purification. The compounds wereanalysed by titration of the total
amount of the acidpresent in the ester hydrochlorides. The
alcoholswere purified and the weight percentages of organicsolvents
were determined in the usual way='. Theslight traces of water
present, if any, in the organicsolvents were neglected.
979
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INDIAN J. CHEM., VOL. 27 A, NOVEMBER 1988
Table 1-The p K 1, P K 2, P K E and Other Microscopic Constants
of Esters of Glycine in Methanol-Water,Ethanol-Water,
2-Propanol-Water and r-Butanol-WaterMixtures
Wt%of 1/£ x 102 Glycine Glycine Ethyl Ester Glycine Methyl
Esteralcohol
pK, pK2 pKE logKz pKo pKE 10gKz pKD0 1.27 2.33 9.59 7.70 5.37
4.22 7.65 5.32 4.27
(2.34) (9.60) (7.73)
Methanol- Water8.0 1.33 2.57 9.60 8.09 5.52 4.08 7.69 5.12
4.48
16.0 1.40 2.63 9.63 8.12 5.49 4.14 7.72 5.09 4.5425.2 1.48 2.66
9.67 8.37 5.71 3.96 7.64 4.98 4.6934.4 1.57 2.90 9.74 8.49 5.59
4.15 7.62 4.72 5.0244.7 1.69 3.18 9.75 8.43 5.25 4.50 7.60 4.42
5.3354.2 1.84 3.36 9.78 8.37 5.01 4.77 7.59 4.23 5.5564.1 2.01 3.50
9.81 8.30 4.80 5.01 7.55 4.05 5.7675.9 2.25 3.64 9.74 8.28 4.64
5.10 7.38 3.74 6.00
Ethanol- Water8.0 1.35 2.55 9.59 7.74 5.19 4.40 7.73 5.18
4.41
16.4 1.42 . 2.66 9.68 7.76 5.10 4.58 7.65 4.99 4.6925.3 1.53
2.70 9.74 7.88 5.18 4.56 7.63 4.93 4.8134.4 1.71 2.88 9.74 7.86
4.98 4.76 7.61 4.73 5.0144.0 1.90 3.05 9.75 7.68 4.63 5.12 7.53
4.48 5.2754.1 2.14 3.38 9.83 7.63 4.25 5.58 7.33 3.95 5.8864.7 2.44
3.53 9.84 7.59 4.04 5.80 7.25 3.72 6.1276.0 2.87 3.73 9.73 7.45
3.72 6.01 7.10 3.37 6.36
2-Propanol- Water8.0 1.37 2.40 9.59 7.89 5.49 4.10 7.71 5.31
4.28
16.3 1.50 2.51 9.58 7.98 5.47 4.11 7.68 5.17 4.4125.1 1.65 2.64
9.59 8.22 5.58 4.01 7.65 5.01 4.5834.3 1.86 2.81 9.63 8.38 5.57
4.06 7.53 4.72 4.9143.9 2.13 3.03 9.68 8.10 5.07 4.61 7.46 4.43
5.2554.0 2.52 3.17 9.74 8.00 4.83 4.91 7.35 4.18 5.5664.6 3.10 3.49
9.80 7.92 4.43 5.37 7.27 3.78 6.0275.8 3.87 3.66 9.81 8.14 4.48
5.23 7.19 3.53 6.28
t-Buianol- Water8.0 1.43 2.40 9.55 7.83 5.43 4.12 7.67 5.27
4.28
16.4 1.58 2.50 9.58 8.05 5.55 4.03 7.70 5.20 4.3825.0 1.80 2.62
9.60 8.15 5.53 4.17 7.58 4.96 4.6434.2 2.08 2.71 9.61 8.12 5.41
4.20 7.50 4.79 4.8243.8 2.50 2.73 9.63 8.05 5.32 4.31 7.35 4.62
5.0154.0 3.18 2.84 9.68 8.00 5.16 4.52 7.25 4.41 5.2764.5 4.44 2.97
9.78 7.97 5.00 4.78 7.20 4.23 5.5575.8 5.63 3.27 9.81 7.95 4.68
5.13 7.00 3.73 6.08
Caustic soda was of GR Grade (E Merck). It was the ester
hydrochlorides. The pH-meter readingsstandardised in the usual way.
Other chemicals were taken using a Systronics digital pH-meter
withwere also of guaranteed reagent quality. All solu- a combined
glass and calomel electrodes assemblytions were made in water,
doubly distilled from all- maintained at 298K in the way described
in our pre-glass distilling apparatus. vious cornmunicationsv'.
For the determination of the dissociation con-Results and
Discussionstants of the methyl and ethyl esters of glycine and
leucine hydrochlorides, a known concentration of The p K E for
the reaction,the ester hydrochloride was neutralised to different
+NHJCHRCOzEt + HP ". NH2CHRC02Et + HJO+ isextents with caustic soda
in different volumetricflasks of known volumes and the hydrogen ion
con- K =B+lo U +10 [+NH,CHRC02Et)cent rations were measured
pH-metrically. The pro- p F. g II g [NH2CHRC02Et)cess was repeated
with diferent concentrations of in mixed solvents.
980
J
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DEY & LAHIRI : DISSOCIATION CONSTANTS OF ESTERS OF GLYCINE
& LEUCINE
Table 2- The pK], pK2' pKE and Other Microscopic Constants of
Esters of Leucine in Methanol-Water,Ethanol-Water, 2-Propanol-Water
and r-Butanol-WaterMixtures
Wt%of lie x 102 Leucine Leucine Ethyl Ester Leucine Methyl
EsterAlcohol
pK) pK2 pKE logKz pKo pKE logKz pKo
0 1.27 2.44 9.59 7.62 5.18 4.41 7.55 5.11 4.48(2.36) (9.60)
(7.63)
Methanol- Water
8.0 1.33 2.60 9.48 7.71 5.11 4.37 7.60 5.00 4.4816.0 1.40 2.71
9.48 7.87 5.16 4.32 7.62 4.91 4.5725.2 1.48 2.83 9.47 8.02 5.19
4.28 7.62 4.79 4.6834.4 1.57 2.98 9.48 8.12 5.14 4.34 7.54 4.56
4.9244.7 1.69 3.15 9.49 8.03 4.88 4.61 7.48 4.33 5.1654.2 1.84 3.30
9.48 8.02 4.72 4.76 7.42 4.12 5.3664.1 2.01 3.50 9.50 7.95 4.45
5.05 7.30 3.80 5.7075.9 2.25 3.79 9.51 7.93 4.14 5.37 7.20 3.41
6.10
Ethanol- Water
8.0 1.35 2.53 9.55 7.66 " 5.13 4.42 7.58 5.15 4.4016.4 1.42 2.60
9.53 7.73 5.13 4.40 7.61 5.01 4.5225.3 1.53 2.80 9.51 7.89 5.02
4.49 7.50 4.70 4.8134.4 1.71 2.96 9.53 7.80 4.84 4.69 7.36 4.40
5.1344.0 1.90 3.13 9.52 7.73 4.62 4.90 7.18 4.05 5.4754.1 2.14 3.26
9.52 7.61 4.35 5.17 7.08 3.82 5.7064.7 2.44 3.42 9.54 7.55 4.13
5.41 6.92 3.50 6.0676.0 2.87 3.68 9.56 7.49 3.81 5.75 6.70 3.02
6.54
'I-Propanol- Water
8.0 1.37 2.53 9.50 7.88 5.35 4.15 7.69 5.16 4.3416.3 1.50 2.58
9.50 8.10 5.52 3.98 7.60 5.02 4.4825.1 1.65 2.81 9.50 8.13 5.32
4.18 7.42 4.61 4.8934.3 1.86 2.95 9.52 8.17 5.22 4.30 7.28 4.33
5.1943.9 2.13 3.13 9.52 7.83 4.70 4.82 7.04 3.91 5.6154.0 2.52 3.37
9.52 7.77 4.40 5.12 6.88 3.51 6.0164.6 3.10 3.57 9.55 7.84 4.31
5.24 6.77 3.20 6.3575.8 3.87 3.74 9.58 7.91 4.17 5.41 6.74 3.00
6.58
t-Butanol- Water
8.0 1.43 2.53 9.62 7.93 5.40 4.22 7.61 5.08 4.5416.4 1.58 2.69
9.64 8.15 5.46 4.18 7.45 4.81 4.8325.0 1.80 2.80 9.62 8.20 5.40
4.22 7.28 4.48 5.1434.2 2.08 2.91 9.65 8.17 5.26 4.39 7.15 4.24
5.4143.8 2.50 2.99 9.71 8.15 5.16 4.55 7.00 4.01 5.7054.0 3.18 3.13
9.75 8.10 4.97 4.78 6.83 3.70 6.0564.5 4.44 3.28 9.79 8.08 4.80
4.99 6.73 3.45 6.3475.8 5.63 3.74 9.82 8.07 4.33· 5.49 6.65 2.91
6.91
B is the pH meter reading in mixed solvents, log UH The pKE
values of the methyl and ethyl esters ofis the correction factor to
be added algebraically to amino acid hydrochlorides are recorded in
Tables 1the pH meter reading to have appropriate [H+] ion and
2.concentrations in mixed solvents. The pKE values of the esters of
glycine hydroch-log U H was determined with each set of measure-
loride and leucine hydrochloride have been foundments in the way
described in our previous commu- to change with the change int he
composition of thenications?" . solvent, i.e., p K E values are
solvent-dependent. But
The use of neutral electrolytes was carefully it is difficult to
derive useful conclusions regardingavoided as the solute-solvent
interactions of un- the solvent effect on the p K E values.known
magnitude are likely to mask the 'medium ef- In general, P K E
values of ethyl esters of glycinefects' and the ionie strengths
were kept low (3- and leucine change considerably (about 0.7-0.8
and7 x 10 - 3 M) so that the 'medium effects' could be 0.5-0.6 pKE
units respectively) in mixed solventsdetermined. and pKE values are
higher in mixed solvents com-
981
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INDIAN J. CHEM., VOL. 27 A, NOVEMBER 1988
pared to those in water except at higher percentagesof ethanol
and the change in pK E values is consider-ably less in ethanol
+water mixtures.
The methyl esters, on the other hand, show slightincrease in pKE
values initially but these decreasesubsequently and the pKE values
are considerablylower in mixed solvents. The changes in pKE
valuesare about 0.5-0.6 and about 0.8-0.9 pKE units (forglycine and
leucine componds) in mixed solvents(except in methanol +water
mixtures where thechange in much lower).
The maxima in the pK E values of the ethyl estersin mixed
solvents are: 34 wt% methanol, 25-34 wt%ethanol, 34 wt% isopropanol
and 25% t-butanol.The maxima in the p K E values of the methyl
estersare usually in the region of 8-16 wt% of organic sol-vents,
though variations exist.
The measurements of pH is extremely sensitivetask in the region
studied and fluctuations in p K Eare expected. The errors in the p
K E values are± (0.02-0.3) at low percentages of alcohol (about44
wt%) and ± (0.03-0.04) at higher percentages.Moreover, slight
hydrolysis of esters in the pH-region cannot be ruled out which may
change thep K E values and the hydrolysis of esters is
usuallysolvent dependent. Moreover, we have not taken in-to
consideration the autoprotolysis constants of wa-ter in different
mixed solvents to calculate the pHvalues in mixed solvents. Under
these conditions, itis not possible to infer much regarding the
solventeffect on p K E of amino acid esters.
The determination of the values of the micros-copic constants
requires the value of anyone of theconstants Kz, KA, KB, etc. It is
customary to assumethat the equilibrium constant for the process,
Ca-tion ~ uncharged acid + H + , is almost equal to
thecorresponding ionisation constant K E 9-11 for the ba-sic group
of the methyl or ethyl ester of the acid(p K E = P K B)' However,
our results suggest thatpKE values of the different esters are not
the samebut vary considerably in mixed solvents. Bryson etal.'?
also suggested the relation pKB = pKE ± 0.2from a study of ten
N-arylglycines in aqueous solu-tions.
KB has been neglected so that KI can be taken equalto K A' This
is justifiable in view of the fact that p K(of glycine or leucine
ester hydrochloride) is of th~
982
order of 7.5-8.0 compared to values of 2.33-2.44for pK1 of
glycine and leucine. Thus, Kz (ratio ofzwitterionic to neutral
forms) can be easily calculat-ed from the reaction,
K =KIz KE. 1 1 1
Since -=-+-, K2 Kc Ko
and K = Koz Kc
1 «, 1 1we have -=-+-=-(K +1)
, K2 «; x; x; ZpK2 = pKo + log Kz,[neglecting 1 in
comparisontoKzlpKD = pK2 -lOb KzThe value of log Kz suggests that
the percentage ofneutral form is very low in water. Though the
per-centage of neutral form increases in mixed solvents,it is very
very small (the ratio i.e. Kz value changesfrom 105 to 103 from
water to mixed solvents).
The results also show that pKD value of the ami-no acids
increase almost continuously with the in-crease in the [organic
solvents l,obviously due to theincrease in the increase in the
concentration of neu-tral amino acids.
References1 Edsall J J & Blanchard M H, J Am chern Soc, 55
(1933)
2337.2 Hazra D K & Lahiri S C, Analyt chim Acca, 79 (1975)
335; J
Indian chemSoc, 53 (1976) 787.3 Bhattacharyya A, Mondal A K
& Lahiri S C, Electrochim
Acta, 25 (1980) 559; Indian J Chern, 19A (1980) 532.4 Dey B P,
Dutta S & Lahiri S C, Indian J Chern, 21A (1982)
886.5 Pal A, Dey B P & Lahiri S C, Indian J Chern, 25A
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322.6 Van Uitert L G & Haas co, JAm chern Sac, 75 (1953)
451.7 Bhattacharya UC & Lahiri S C, Z ph~ Chern (N.F.), 50
(1966) 131.8 Lahiri SC & Aditya S, !Indian chem Soc, 51
(1974) 319.9 Ebert L, Z phys Chern (Leipzig), 121 (1926) 385.
10 Greenstein J P & Winitz M, Chemistry of the amino acids.
VolI (John Wiley, New York, London) 1961, Chapter 4.'
11 GreenR W8.' TongH K,J Am chem 50(',78 (1956) 4896.12 Bryson,
Davies N R& Serjeant E P, Nature, 195 (1962) 996.
