Theoretical Study of The “Charge Center-Linker-Hydrogen Binding Site” Three-Component Signal Transducers: The Effect of Charge-Bearing Substituents and Counter Ions

• Main Authors: Pai-Chi Li, 李珮琦
• Other Authors: Chao-Tsen Chen
• Format: Others
• Language: en_US
• Published: 2001
• Online Access: http://ndltd.ncl.edu.tw/handle/11521827295153390809

碩士 === 國立臺灣大學 === 化學研究所 === 89 === We consider a charge-bearing molecule as a three-component system consisting of a charge center, a linker, and a hydrogen bond binding site. In the three-component system, the binding center is controlled by a reaction center (charge center). Basically, we want to maximize the long-range through-bond effect and turn the three-component system into an effective signal transducer. In real experiment, one can change the formal charge of the reaction center through protonation, oxidation, reduction, or chemical transformation of a functional group. In our theoretical study, protonation was calculated for the reaction site. For the linker part, fully conjugated alkenes, alkynes and -N=N- are good candidates for maximizing through-bond effects. For a binding site, pyrrole was chosen for its ability to donate electrons through conjugation with the linker. Our aim is to investigate how different reaction centers and counter ions affect the hydrogen bonding sites, so as to provide guidelines for the design of effective signal transducers. Calculation results at the HF/6-31+G* level showed that 1) for neutral species, the longer the linker unit, the larger its binding energy (5~6 kcal/mol); for charged species, the binding energy of the protonated cationic system was roughly double that of the neutral system (9~14 kcal/mol). 2) If the linker is -(CH=CH)n-, the longer the linker unit is, the less binding energy it has. With the azo linkers, we have demonstrated that it is even possible to amplify the signals when the charge center is an iminium group (-CH=NH2+). 3) Counter ions could decrease the binding energy of the binding site because of long-range electrostatic interactions and because some electron is transferred from the counter ion to the reaction center. The transducers containing iminium are greatly influenced by the counter ions, while the charge delocalized guanidinium group (-NHC(NH2)2+), whose electrons can not conjugate with the linker, seem to have better ability to preserve signal transduction in the presence of a counter ion (40~50%). The calculation results showed that the efficiency of changing counter ions to improve signal transduction is quite small. Experimentally, one could consider reducing the effect of counter ions by using anion-binding hosts.