Band-Shape Analysis of the Electronic Spectra of the Schiff Bases of 5'-Deoxypyridoxal and n-Hexylamine in Cationic Micelles*
M. A N G E L E S GARCIA D E L VADO, G E R A R D O R. E C H E V A R R [ A , t MIGUEL A. VAZQUEZ S E G U R A , J O S E F A D O N O S O P A R D O , FRANCISCO M U N O Z , and FRANCISCO GARCIA BLANCO Department of Physical Chemistry, Faculty of Sciences, University of Alcald de Henares, E-28871 Alcald de Henares, Spain (M.A.G., G.R.E.); Department of Chemistry, Faculty of Sciences, University of the Balearic Islands, E-07080 Palma de Mallorca, Spain (M.A. V., J.D.P., F.M.); and Department o[ Physical Chemistry, Faculty of Pharmacy, Complutense University, E-28040 Madrid, Spain (F.G.B.)
We analyzed the UV-visible absorption spectra of the Schiff bases formed between 5'-deoxypyridoxal (DPL) and n-hexylamine (HEX) in aqueous media of various ionic strengths containing different concentrations of the cationic snrfactant hexadecyltrimethylammonium bromide (CTAB). According to the results obtained, the hydrophobic chains of the sur- factant interact with the hydrocarbon chain of the imine, thereby dis- placing the tantomeric equilibrium to the enol form. An increase in the electrolyte concentration also lowers the polarity of the environment where the Schiff base lies, thus also increasing the proportion of the enol form. The results obtained in this micellar medium suggest that the system can be used as a model for hydrophobic media.
Index Headings: Schiff base; DPL; CTAB.
R R
CH CH ~.
CH3 OH p K ~ , a
+1 +r~ - CH a 3 H H
B . 2 B.1
PK .16
INTRODUCTION
Pyridoxal 5'-phosphate (PLP) acts as a co-enzyme for a variety of enzymes that catalyze chemical reactions involved in amino acid metabolism, 1-3 as well as in gly- cogen phosphorylase. 4
5'-Deoxypyridoxal (DPL) is a highly representative analogue of PLP, as it possesses three chemical groups (CH=O, OH, and =N) that are considered to be essential for catalytic activity in amino acid metabolism; DPL reconstituted glycogen phosphorylase is active as well) ,6
In every PLP-dependent enzyme studied to date, PLP is bound to it, forming a Schiff base (imine) through the terminal amino group of a lysine residue, which lies in a more or less hydrophobic environment. 7-9 The Schiff bas- es of DPL possess the same protonable groups as those of PLP except for the phosphate group, so their proton- ation and tautomeric equilibria (Schemes 1 and 2) are simpler.
Finding model systems of amines and amino acids with PLP and DPL in order to simulate the state of PLP in enzymes is essential in order to accurately elucidate the role of this co-enzyme in biological catalysis. A substan- tial proportion of the information available on the state of the co-enzyme in the holoenzyme has been obtained by using UV-visible absorption spectroscopy. Band anal- ysis spectroscopic studies in this context have so far been carried out in aqueous media 1°-2° by using lognormal curves to characterize the bands and curve fitting meth- ods to evaluate the different molecular species involved.
Received 27 May 1992; revision received 20 July 1992. * Supported by DGICyT (Project PB88-0284). t Author to whom correspondence should be sent.
R R
~N
CH CH _~
_ .- 3 3
P K o -
B1 B o
SCHEME 1
Recently, we carried out a band analysis of 5'-deoxy- pyridoxal and its Schiff base with n-hexylamine (HEX) in partly aqueous and nonaqueous media 21,22 in order to investigate the tautomeric equilibrium of the chemical species involved in terms of the nature of the solvent and its polarity.
In order to obtain partly hydrophobic media without using nonaqueous solvents, in this work we carried out a spectroscopic study of the Schiff base of DPL and HEX in the presence of the cationic surfactant hexadecyltri- methylammonium bromide (CTAB) at various ionic strengths. This surfactant possesses a hydrocarbon chain of 16 carbon atoms that can interact with the 6-C alkyl chain of the DPL-HEX Schiff base, thereby modifying its environment. The tautomeric equilibrium occurring in this micellar medium was compared with that ob-
served in nonaqueous solvents so as to determine the effective dielectric constant of the micro-environment of the Schiff base in a micellar medium. The ultimate aim of this work was to determine the most suitable condi- tions for this model system of DPL and cationic micelles of CTAB to be able to promote amino acid transami- nations.
EXPERIMENTAL
Materials. 5'-Deoxypyridoxal was synthesized from pyridoxine hydrochloride according to the method of Iwata. 23 All other reagents were purchased from Merck and were used as supplied.
Acetate, phosphate, and carbonate buffers were used in appropriate pH ranges. The buffer concentrations used were typically 0.02 mol L -1, and the ionic strength was kept constant and equal to 0.1, 1.0, or 2.0 tool L -1 by adding appropriate amounts of KC1 to the medium.
The pH measurements were made with a Crison pHmeter furnished with a Metrohm EA120 combined electrode that was previously calibrated with aqueous buffers at 25°C. As checked in every experiment, the difference between the initial and final pH in the reaction cell never exceeded 0.04 pH units.
DPL solutions were prepared in appropriate buffers and were stored in the dark. Their exact concentrations were determined by dilution 24 with 0.1 mol L -1 NaOH and were found to be in the region of 2 × 10 -5 mol L -1.
n-Hexylamine solutions were made by dissolving the required amount of n-hexylamine hydrochloride in an appropriate buffer. The working concentrations used ranged between 1 × 10 -2 and 5 × 10 -2 tool L -1.
Aqueous solutions of hexadecyltrimethylammonium bromide were prepared by dissolving an appropriate amount of the reagent in water with the aid of an ultra- sonic bath. The transparent solutions thus obtained were
[ CTAB ]. 10 4
FIG. 1. Variation of the macroscopic pK_1. of the DPL-HEX Schiff base as a function of the CTAB concentration in an aqueous medium at 25°C.
diluted in the reaction cell, and the resulting CTAB con- centrations ranged from 2.1 × 10 -4 and 2 × 10 -2 mol L -i.
On mixing, the solutions were allowed to stand for 25 min in order to ensure that equilibrium was reached. Data were acquired over the wavelength range 500 to 250 nm (v = 20.000-40.000 cm-1), absorbances being mea- sured to the nearest 0.001 unit.
Full spectra of the solutions were recorded on a Perkin- Elmer 330 spectrophotometer and on a Uvikon 940 spec- t rophotometer equipped with cells of 1.0 cm light path at a constant temperature of 25.00 + 0.05°C.
Methods. The overall reaction between the aldehyde and the amine can be schematized as follows:
kl R1-CHO + R2-NH2 ~ R1-CH=N-R2 + H20
k2
the equilibrium constant of which is
[Ble kpH [Pie [L]e (1)
where [B]e , [P]e, and [L]e denote the equilibrium con- centrations of the Schiff base, aldehyde, and imine, re- spectively. In the wavelength range of interest, the ab- sorption of the mixture formed by the aldehyde and its corresponding aldimines results exclusively from the ab- sorption of these two compounds rather than the free amine present. Therefore, at equilibrium and a given wavelength, the overall absorption of the sample will be given by
A(X) = [P]eEp(h) + [B]~E~(h) (2)
where Ep(h) and Es(h) are the molar absorptions of the
1 8 4 2 V o l u m e 4 6 . N u m b e r 1 2 , 1 9 9 2
E
0 4000
,Q
i 0
0 , 20 ,0
B_I:
B 0 :
/ /%,
, \ !
t ',
I
3 0 , 0
6000
5000
4OOO
3000
e. 0 n
2OOO
i o X
1000
o-p -~ 20 30 40
Wavenumber (kK)
FIG. 2. UV-visible absorpt ion spec t rum of the chemical species B and Bo of the D P L - H E X Schiff base in water a t 25°C; I k K = 103 cm -1.
aldehyde and Schiff base, respectively. The spectra of the Schiff base were acquired by using a computer-as- sisted method that fitted the experimental results to Eq. 2. The concentrations [P]e and [B]e were determined from Eq. 1; the Ep(h) and KpH values used for this purpose were either obtained in previous work 25-27 or taken from Harris e t a l . u
Spectra were deconvoluted into lognormal curves by using the method of Metzler e t a l . lo The wavenumber of maximum absorp/tion, the maximum molar absorption, and the bandwidth and its skewness were the four input data required by the computer in all instances. The pro- gram minimized the sum of the squares of the deviations and obtained the output data from the best fit, which allowed the area (integrated intensity) of the absorption band of each tautomer to be determined.
RESULTS AND DISCUSSION
Band-Shape Analysis. The absorption spectra of the Schiff bases of DPL and HEX in an aqueous medium recorded at different pH values, CTAB concentrations, and ionic strengths were used to obtain the correspond- ing absorption spectra of the different chemical species and determine their macroscopic pK values. The Schiff base of DPL and HEX can occur as four different chem- ical species depending on the pH (Scheme 1). The fully protonated species (B_2) has a macroscopic pK~ = 3.39 in water, which corresponds to the deprotonation of the hydroxyl group at position 3. Species B_I has a macro-
Wavenumber (kK)
FIG. 3. UV-visible absorp t ion s p e c t r u m (deconvoluted into lognormal curves) of the chemical species B_I of the D P L - H E X Schiff base ob- t a ined in water conta in ing 9.8 x 10 -3 mol L -~ C T A B at 25°C; 1 k K = 103 cm-k
scopic pK_I, = 6.23 which corresponds to the deproton- ation of the pyridine nitrogen. In highly basic media (pKoB = 11.81) the last deprotonation of the Schiff base takes place with the release of a proton from the imine nitrogen. 18,22,25,26
Figure 1 shows the variation of pK_ls for the Schiff base of DPL and HEX at an ionic strength of 0.1 mol L -1 and various CTAB concentrations. As can be seen, the pK decreases with increase in the CTAB concentration as a result of the local decrease in the proton concentra- tion near the ionic groups of the surfactant. 2s,29 This be- havior is similar to that observed for pK_~ and PKoB on changing the surfactant concentration and was previ- ously reported for analog systems by Behme and Cordes. 3°
Figure 2 shows the absorption spectra of the chemical species B_I and Bo of the Schiff base of DPL and HEX in water. As can be seen, both spectra show two strong bands at ~415 and 280 nm which correspond to the zwitterionic tautomers of the chemical species B_I and Bo, respectively (Scheme 2).
The sets of absorption spectra obtained for the chem- ical species B_I and Bo at each CTAB concentration and ionic strength assayed were deconvoluted into lognormal curves by using the method of Metzler e t a l . ~°
Figure 3 shows the deconvoluted absorption spectrum of the chemical species B_I obtained in the presence of a surfactant concentration of 9.8 × 103 mol L -1. This chem- ical species, which occurs as two tautomers (see Scheme
APPLIED SPECTROSCOPY 1843
8000 1,0
• " 6000
° E
'=E v
m J~
_= O 2O0O =E
0 20 30 40
)
0
0.8
0,6
0,4
0,2
J
0,0 0
• •
i • ! !
5 100 150 200
Wavenumber (kK) FIG. 4. UV-visible absorption spectrum (deconvoluted into lognormal curves) of the chemical species B0 of the DPL-HEX Schiff base obtained in water containing 1.8 × 10 '~ mol L 1 CTAB at 25°C; 1 kK = 103 cm-'.
[CTAB].IO FIG. 5. Variation of the mole fractions of the tautomers of the chemical species B_I of the DPL-HEX Schiff base as a function of the CTAB concentration at 25°C. (m) Enol form; (A) zwitterionic form.
2) was analyzed on the basis of three lognormal curves: bands I and II of the zwitterionic form centered at ~417 and 279 nm, respectively; and band I of the enol formed centered at about 340 nm. A band corresponding to band II of the enol form was included at - 248 nm in order to correctly fit the absorption spectrum in the region 290 to 250 nm.
Figure 4 shows the deconvoluted absorption spectrum of the chemical species B0 obtained in the presence of a surfactant concentration of 1.18 × 10 ~ mol L-L This chem- ical species possesses three tautomers (zwitterionic, enol, and dipolar; Scheme 2) and was analyzed on the basis of four lognormal curves: bands I and II of the zwitter- ionic form, centered at about 418 and 278 nm, respec- tively; band I of the enol form, centered at ~339 nm; and band I of the dipolar form, centered at about 355 nm. Correct fitting of the absorption spectrum was ac- complished in this case by including a band at about 245 nm corresponding to band II of the enol form.
Table I lists the areas of band I obtained for each tautomer as a function of the surfactant concentration and ionic strength. As can be seen, increasing surfactant concentrations result in increasing areas of the bands corresponding to the enol forms of the chemical species B_I and Bo and decreasing areas of the bands of their zwitterionic and dipolar forms.
Table II gives the average width (o~) and skewness (p) of the characteristic bands, which are consistent with
previously reported values for analog systems 12-16,1s-2°,22 and hence testify to the goodness of the fits.
Tautomeric Equilibrium. The areas of bands I (Table I) were used to calculate the mole fractions of the tau- tomers corresponding to each chemical species from the following equation:
ai x /= -- (3) a o
where ai is the band area of form i, a ° its molar area as reported elsewhere, 22 and xi the mole fraction of the tau- tomer.
Figures 5 and 6 show the variation of the mole fractions of the tautomers of the chemical species B_I and B0, respectively, as a function of the CTAB concentration at a constant ionic strength of 0.1 tool L -1. In the absence of surfactant, the zwitterionic form ( - l a ) accounts for 90 % of the species; as CTAB is gradually added to the medium, the tautomeric equilibrium is displaced to the enol form, which amounts to 36.2% at a surfactant con- centration of 2 × 102 mol L -1.
As with chemical species B_I, the tautomeric equilib- rium of species Bo in water is displaced to the zwitterionic form. The addition of CTAB eliminates the dipolar tau- tomer, which was already a minor form in pure water, and substantially increases the proportion of the enol form at the expense of, chiefly, the zwitterionic form (Fig. 6). Thus, the enol form accounts for only 15.7% of the
1844 Volume 46, Number 12, 1992
TABLE I. Variation with the CTAB concentration of the area (Mm tool-') of band I for the tantomers of the chemical species B_, and Bo of the DPL-HEX Schiff base.
c h e m i c a l s p e c i e s i n a n a q u e o u s m e d i u m ; as t h e s u r f a c t a n t is a d d e d , i t s p r o p o r t i o n i n c r e a s e s t o 8 7 . 2 % a t a C T A B c o n c e n t r a t i o n o f 2 x 102 m o l L -1.
T h e d i s p l a c e m e n t o f t h e t a u t o m e r i c e q u i l i b r i u m t o t h e e n o l f o r m o f c h e m i c a l s p e c i e s B 1 a n d Bo i n t h e p r e s e n c e o f t h e s u r f a c t a n t a r i s e s f r o m a c h a n g e i n t h e e n v i r o n m e n t
0 m
f J
0 =i
1,0
0,8
0,6
0,4 •
0,2
0,0 Wll 0
• • • • •
5 100 150 200
[ C T A B ] . I O
Fro. 6. Variation of the mole fractions of the tautomers of the chemical species B0 of the DPL-HEX Schiff base as a function of the CTAB concentration at 25°C. ( I ) Enol form; (0) dipolar form; (A) zwitterionic form.
o f t h e S c h i f f b a s e . T h e a p o l a r c h a i n o f t h e i m i n e m o l e c u l e ( - ( C H 2 ) 5 - C H 3 ) i n t e r a c t s w i t h t h e a p o l a r c h a i n m o i e t i e s o f s u r f a c t a n t m o l e c u l e s , t h e r e b y l o w e r i n g t h e p o l a r i t y a r o u n d t h e c a r b o n - n i t r o g e n b o n d o f t h e i m i n e . T h u s , t h e z w i t t e r i o n i c f o r m , w h i c h p o s s e s s e s t w o c h a r g e s ( o n e o n t h e i m i n e n i t r o g e n a n d t h e o t h e r o n t h e p h e n o l i c o x y g e n ) is r e n d e r e d u n s t a b l e i n t h e p r e v a i l i n g e n v i r o n m e n t . O n t h e o t h e r h a n d , t h e e n o l f o r m , w h i c h is u n c h a r g e d i n t h e v i c i n i t y o f t h e 6 - C a t o m c h a i n , is c o m p a r a t i v e l y m o r e s t a b l e i n t h i s h y d r o p h o b i c m e d i u m .
T h e t a u t o m e r i c e q u i l i b r i u m o f c h e m i c a l s p e c i e s Bo is m u c h m o r e m a r k e d l y d i s p l a c e d t o t h e e n o l f o r m t h a n is t h a t o f s p e c i e s B _ I . T h i s d i f f e r e n t i a l b e h a v i o r a r i s e s f r o m
TABLE II. Average bandwidth (w) and skewness (p) of the bands obtained for the chemical species of the DPL-HEX Schiff base.
Tautomers
Species B_I Species Bo
Zwitterionic Enol Zwitterionic Dipolar Enol
Band I Band II Band I Band I Band II Band I Band I
(cm ' x 10 -3) 3.71 _ 0.03 5.03 _+ 0.05 3.65 +_ 0.05 4.05 _+ 0.04 4.85 + 0.06 4.35 + 0.05 4.53 + 0.04 3.70"; 3.74 b 5.04"; 5.03 b 3.68"; 3.65 b 4.28"; 4.09 b 4.80"; 4.61 b 3.65"; 4.35 b 3.68"; 4.68 b 3.74~; 3.90 c 5.02" 3.60a; 4.00 ¢ 4.00 ¢ 1.5 ¢
p 1.43 + 0.04 1.23 _+ 0.04 1.30 + 0.02 1.39 _+ 0.03 1.31 _ 0.02 1.20 _+ 0.04 1.23 _+ 0.03 1.47~; 1.43 b 1.21% 1.2P 1.22a; 1.33 b 1.43"; 1.40 b 1.30"; 1.23 b 1.45"; 1.18 b 1.22~; 1.27 b 1.54"; 1.6 c 1.33 ~ 1.16"; 1.33 ° 1.5 ¢
"From Ref. 19. b From Ref. 20. c From Ref. 14.
APPLIED SPECTROSCOPY 1845
¢: o
o
14.
0
0 ,45
0,35 '
o
0,25 •
o
0,15
÷
0,05 0,0
o
! !
1,0 2,0
t - O m
o
M.
0
1,0
O,S
016
0,4
÷
0,2
x
0,0 0,0
& • & •
• tl
o 13
13 []
0 0 0
! !
1 , 0 2 , 0
Ionic Strength ( I / M ) FIG. 7. Variation of the mole fraction of the enol tautomer of chemical species B_I of the D P L - H E X Schiff base as a function of the ionic s t rength at 25°C and various CTAB concentrations: (x) 0 tool L-l; (+) 1.18 × 10 -3 mol L-~; (0) 4.19 × 10 _3 mol L-I; (O) 7.22 × 10 3 mol L-l; (m) 9.80 × 10 -3 tool L-~; ([::]) 1.20 × 10 2 mol L-~; (A) 2 × 10 -2 mol L -1.
the different polarity of these two chemical species. In- asmuch as the zwitterionic and enol forms of the chemical species B_I have a protonated pyridine nitrogen, whereas the corresponding tautomers of species Bo do not, the environment of this latter species must be less polar or more hydrophobic, so the enol form must occur in a higher proportion.
Figures 7 and 8 show variation of the mole fractions of the enol forms - lb and 0c as a function of the ionic strength at various CTAB concentrations. Increasing ionic strengths result in increasing proportions of the enol forms at a given concentration of surfactant. This ob- servation can be ascribed to two facts. First, an increase in the electrolyte (KCI) concentration gives rise to a sig- nificant decrease in the critical micelle concentration (CMC) of the surfactant and an increase in the molecular weight of the cationic micelles, which results in the for- mation of larger micelles at CTAB concentrations below 9.2 x 10 t mol L -t (the CMC of CTAB at ionic strength 0.017 mol L-l). 29,3° Thus, at low CTAB concentrations, the hydrophobic chains of the micelles will interact with the 6-C atom alkyl chain of the Schiff base, thereby decreasing the polarity of the imine environment and the probability of hydrogen bonds being formed in order to stabilize the zwitterionic form. Second, inasmuch as KC1 is solvated by water molecules, an increase in its con- centration will decrease the concentration of free water
Ionic Strength ( I / M ) FIG. 8. Variation of the mole fraction of the enol tau tomer of chemical species Bo of the D P L - H E X Schiff base as a function of the ionic s t rength at 25°C and various CTAB concentrations: (x) 0 mol L -I, (+ ) 2.1 x 10 -4 mol L-~; (O) 4.0 × 10 4 mol L 1; (O) 7.9 × 10 -4 mol L-l; (B) 1.18 × 10 3 mol L-~; (I-1) 2.48 × 10 -3 mol L 1; (A) 7.22 × 10 -3 mol L-t; (A) 2.0 × 10 2 mol L -t.
molecules available to solvate and hence stabilize the zwitterionic forms (the net effect is a lower polarity in the chromophore environment).
As can be seen, the enol form 0c, which has an unpro- tonated pyridine nitrogen, is much more sensitive to changes in the ionic strength than is the enol form - l b . This factor can be ascribed to the higher polarity of chemical species B_I, which must be more extensively solvated by water or other polar molecules in the me- dium, so the polarity of its environment is reduced to a lesser extent.
The dipolar form 0b disappears when it is in the pres- ence of the surfactant or when a given ionic strength is exceeded. This tautomer is unstable in hydrophilic and hydrophobic media and its occurrence is dictated by the formation of intermolecular hydrogen bonds with acid and/or basic groups in an appropriate environment, t4,17
By comparing the results obtained in this work and others previously reported for partly aqueous and non- aqueous media, we estimated the effective dielectric con- stant of the environment of the Schiff base in the pres- ence of the cationic surfactant.
The proportion of the enol form of species Bo in meth- anol (Table III) is identical with that obtained in water and a medium containing 1.2 x 10 -2 mol L -1 at an ionic strength of 0.1 mol L 1. Such a proportion is also obtained
1848 Volume 46, Number 12, 1992
TABLE l lI . Proportion of the enol form of species Bo in different media. 22
in m e d i a of h i g h e r ionic s t r e n g t h s b u t lower C T A B con- c e n t r a t i o n s . T h u s , for I = 2 mo l L -~ a n d [ C T A B ] = 2.48 × 10 -3 m o l L - i , t h e p r o p o r t i o n o f t h e eno l f o rm is 85 .5%, W i t h a less p o l a r m e d i u m such as t h a t p r o v i d e d b y an- h y d r o u s p r o p a n o l , t h e p r o p o r t i o n of eno l f o rm is v i r t u a l l y i d e n t i c a l w i t h t h a t o b t a i n e d a t a C T A B c o n c e n t r a t i o n of 1.2 × 102 m o l L -~ a n d an ionic s t r e n g t h of 2 mo l L -L
T h e p r o p o r t i o n of t h e a b o v e - m e n t i o n e d eno l f o rm in 1:1 w a t e r / d i o x a n e m i x t u r e s ( T a b l e I I I ) co inc ides w i th t h a t o b t a i n e d a t a s u r f a c t a n t c o n c e n t r a t i o n b e t w e e n 1.5 x 10 -3 a n d 2.48 x 10 .3 m o l L -1 a n d d i f f e r e n t ionic s t r e n g t h s . A m o r e n o n p o l a r m e d i u m such as t h a t r ep - r e s e n t e d b y 3:7 w a t e r / d i o x a n e gives r i se to a n eno l f rac- t i on w h i c h can a lso be o b t a i n e d a t a g iven C T A B con- c e n t r a t i o n a n d ionic s t r e n g t h .
S u c h a s c a r c e l y p o l a r m e d i u m as t h a t p r o v i d e d b y a n h y d r o u s d i o x a n e , w h i c h r e su l t s in an eno l f r a c t i o n of 99.5% ( T a b l e I I I ) , cou ld n o t be a c c o m p l i s h e d in t h e mi - ce l l a r s y s t e m used ; in fac t , t h e m a x i m u m p r o p o r t i o n of t h e eno l fo rm, 93 .8%, was r e a c h e d a t a C T A B concen - t r a t i o n of 2 x 10 -2 mo l L -1 a n d a n ionic s t r e n g t h of 2 m o l L -L A h i g h e r C T A B c o n c e n t r a t i o n a n d ionic s t r e n g t h w o u l d p r o v i d e a m o r e h y d r o p h o b i c e n v i r o n m e n t for t h e Sch i f f base , such as t h a t of a n h y d r o u s d i o x a n e (Fig. 8).
T h e r e su l t s j u s t g iven c l ea r ly d e m o n s t r a t e t h e i m p o r - t a n c e of t h e h y d r o p h o b i c i n t e r a c t i o n . A c c o r d i n g to t h e a b o v e r e su l t s , t h e n o n p o l a r a n d h y d r o p h o b i c m e d i a p ro - v i d e d b y p a r t l y a q u e o u s a n d n o n a q u e o u s m e d i a can b e r e p r o d u c e d b y us ing w a t e r a n d a p p r o p r i a t e C T A B con- c e n t r a t i o n s a n d ionic s t r e n g t h s . A ca t i on i c m i c e l l a r me- d i u m a l lows for t h e i n t e r a c t i o n of t h e h y d r o c a r b o n c h a i n of t h e imine , t h e r e b y a l t e r i n g t h e t a u t o m e r i c e q u i l i b r i u m of t h e c h e m i c a l spec i e s of t h e D P L - H E X Sch i f f base .
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