The effects of O-substituents of hexahomotrioxacalix[3]arene on potentiometric discrimination between dopamine and biological organic/inorganic cations.
As an interesting type of molecular recognition at a membrane surface, the tri-O-acetic acid ester (host 2) of hexahomotrioxacalix[3]arene, when incorporated into poly(vinyl chloride) (PVC) liquid membranes, displays a high potentiometric selectivity for dopamine over, not only other catecholamines (noradrenaline, adrenaline), but also quaternary ammonium guests (tetramethylammonium, choline, and acetylcholine) and inorganic cations (Na+, K+, NH4+). Interestingly, changes in membrane potential based on the host-guest complexation of host 2 that were observed dopamine/inorganic cation selectivity were not displayed by the related hosts 3 and 4, which contain amide substituents. This paper describes our efforts to separately estimate the two factors contributing to the dopamine selectivities, i.e., the guest lipophilicity factor and the host-guest complexation factor, in an attempt to understand the effects of the O-substituents of these hosts. The potentiometric experiments showed that, although the guests had roughly equal lipophilicity, the electromotive force (EMF) response for dopamine by host 2 was excellent. Furthermore, host 2 displayed ca. a 20-fold stronger complexation for dopamine, compared to noradrenaline, adrenaline, K+, and NH4+ cations. These results indicate that the high potentiometric selectivity of the ion-selective electrode for dopamine mainly reflect, not the guest lipophilicity factor but the host-guest complexation factor. On the other hand, host 3 displayed ca. a 3000-fold stronger binding to Na+ than dopamine, thus explaining the reasons for the lower dopamine-selectivities of host 3 compared to host 2. It is interesting to note that the high potentiometric selectivities for dopamine were displayed by not only host 2 but also host 5, regardless of the simple structure of the O-substituents.
Saijo
Ryosuke
R
Graduate School of Pharmaceutical Sciences, Nagoya City University, Nagoya, Japan.
Murakami
Hiroyuki
H
Tsunekawa
Saori
S
Imanishi
Souhei
S
Shirai
Naohiro
N
Ikeda
Shin-Ichi
S
Odashima
Kazunori
K
D047250:Calixarenes / Q000737:chemistry*; D002412:Cations / Q000737:chemistry*; D004298:Dopamine / Q000737:chemistry*; D015394:Molecular Structure; D011199:Potentiometry / Q000379:methods*
D002412:Cations; C520695:hexahomotrioxacalix(3)arene; D047250:Calixarenes; D004298:Dopamine