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T
he interaction of a synt
hetic enantiopure azamacrocyclic receptor (
L) wit
h biologically important c
hiraldicarboxylates (A,
1-
7)
has been studied by means of potentiometric titrations in 0.15 M NaCl aqueoussolution in a wide pH range. T
his macrocycle forms strong complexes of t
he type [H
nLA]
(n-2) (wit
h n =0-5). As a general trend, t
he binding is muc
h tig
hter at basic or neutral pH t
han in acidic medium.Interestingly, nonprotected excitatory amino acids (Asp and Glu) are strongly bound even at acidic pH.Regarding selectivity, t
he receptor s
howed stereoselective binding toward t
hose substrates bearing anH-bonding donor at C
![](/images/gifc<font color=)
hars/alp
ha.gif" BORDER=0>, being
S-selective in most of t
he cases, except for glutamic acid. T
hus,
L displayedan excellent enantioselectivity for (
S)-malate dianion (
KS/
KR = 11.50 at pH 10.0 and
KS/
KR = 6.86 at pH7.0) and ex
hibited moderate enantiopreference for (
S,
S)-tartrate (
KSS/
KRR = 3.01 at pH 10 and
KSS/
KRR= 1.70 at pH 7.0). For t
his last anion, a very good diastereopreference was also observed (
KSS/
KRS =8.46 at pH 10 and
KSS/
KRS = 4.99 at pH 7.0). On t
he contrary,
L is smoot
hly
R-selective toward (
R)-Glu(
KR/
KS = 3.22 at pH 10 and
KR/
KS = 2.05 at pH 7.0) due to its longer and more flexible molecularstructure. T
he stereoselectivity of t
he corresponding complexes decreased w
hen decreasing pH values.For t
he
hydroxy derivatives, mass spectrometry also reflected t
he trends observed by potentiometry andconfirmed t
he receptor:dicarboxylate 1:1 stoic
hiometry of t
he supramolecular complexes. Additionalexperimental tec
hniques were used to study t
he most stereoselective example. Solution studies by NMRsuggested a good geometrical complementarity between t
he malate dianion and t
he receptor, w
hic
h s
howeda predominant
helical conformation in solution. Besides, self-diffusion rates (PGSE) of t
he diastereomericcomplexes wit
h malate also agree wit
h binding data. Circular dic
hroism was also used in t
his case atdifferent pH values, s
howing a very good correlation between t
he
helical content of t
he receptor and t
hestereoselectivity of t
he molecular recognition process.