The protonation constants of new group of peptidomimetic cyclophanes with valine or phenylalanine moieties incorporated into the macrocyclic skeleton as well as their linear analogues were determined by potentiometric measurements in solutions of methanol-water mixtures at 25°C and constant ionic strength. The influence of cavity size, location of protonation sites, and attached substituents of the macrocyclic ligands on the protonation constants were discussed on the basis of potentiometric measurement as well as H1-NMR results.
The acidity constants of organic reagent play an important role in many analytical procedures such as solvent extraction, complex formation, ion transport, and acid-base titration. The influence of acid-base properties affects the toxicity of the compounds [
In recent years the interest of peptidomimetic compounds arouse in different areas of research. This class of compounds found applications as the receptors in molecular recognition of cations and anions [
In this context finding out acid-base equilibria of this class of compounds seemed to be very crucial for their further possible application and environmental conditions necessary for proper activity.
All investigated linear and cyclophane ligands were synthesized according to the procedures previously described [
The main problem during pH-metric titration in mixed solvent solutions with glass electrode, as the working electrode, is its calibration. There are several procedures reported in the literature based on different methods used to gain the full characteristic of the electrode. We have used two methods. The first one is based on prepared buffer solutions [
In a typical measurement carried out at 25°C 10 ml of
All calculations were done by our computation program based on the following assumption.
In each point of titration curve of weak base by strong acid the following equation must be satisfied:
If the base concentration, the titration fraction, and the values of protonation constants are known this equation could serve us for calculation of pH in arbitrary point of titration curve. The easiest way to gain this aim is the application of the quick convergent Newton-Raphson method. The iterative equation in its modified form [ The initial set of protonation constants is input into the software program. For each of the experimental points of titration curve the pH value is calculated on the basis of the current set of protonation constants. In this way the calculated titration curve is obtained. Then the sum of square deviation between the calculated and experimental curve is computed: To the set of protonation constants the corrections are introduced in such a way to lower the SD value. Then steps (
In the computation, the applied value of the ionic product solvent was determined experimentally from the titration curve of strong base with the strong acid in each of the solvents mixtures.
The computation of protonation constants is based on the fitting of the experimental curve with the theoretical one. In the case of our study we have tried different models of protonation equilibria but the fitting parameters were acceptable only for the presented models (Scheme
The schematic presentation of protonation of the investigated ligands.
Structures of investigated linear ligands are presented in Figure
Logarithm values of successive protonation constants of LVal(
Linear ligands |
|
|
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LVal4a | 8,44 ± 0,02 | 8,17 ± 0,02 |
LVal3a | 8,17 ± 0,03 | 7,80 ± 0,03 |
LVal2a | 8,14 ± 0,05 | 8,15 ± 0,03 |
LPhal4a | 8,35 ± 0,02 | 7,86 ± 0,02 |
LPhal3a | 8,11 ± 0,02 | 8,00 ± 0,01 |
LPhal2a | 8,04 ± 0,04 | 7,82 ± 0,03 |
Structure of investigated linear ligands.
The proposed model of protonation of these derivatives is presented on Scheme
As can be noticed, the values of successive protonation constants for each presented in Table
Introduction of two amide groups into ligand molecule significantly reduce value of successive protonation constants for all ligands. Amide group contains oxygen atom, which is a very strong acceptor of hydrogen bond and nitrogen atom which can be a donor during the forming of such bond. The presence of these groups in molecule separated from each other by the chain of suitable length can lead to intramolecular hydrogen bond. But existence of intermolecular hydrogen bond depends not only on a suitable length of separation chain, but also on properties of the solvent and, as it is in this case, on the presence of hydrophobic substituents within the ligand structure. The suitable length of separation chain provides a free rotation of amide groups and possibility of formation of hydrogen bond between them. This possibility was studied in literature [
The second group of investigated compounds was macrocyclic ligands. Each structure of these cyclic ligands consists of two amide groups, two secondary amine groups, and endocyclic pyridine or benzene ring. All these moieties are part of intramolecular cavity of the investigated cyclic ligands. Furthermore, each of the macrocyclic compounds, similarly as linear ligands, possesses hydrophobic substituents (isopropyl or benzyl). Structures of investigated cyclic ligands are presented in Figure
Structures of investigated cyclic ligands.
Values of successive protonation constants for cyclic ligands determined in methanol-water mixtures (
Logarithm values of successive protonation constants CyPirVal(
Cyclic ligands |
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CyPirVal4a | 6,56 ± 0,02 | 5,11 ± 0,02 | 7,38 ± 0,01 | 6,29 ± 0,01 |
CyPirVal3a | 6,56 ± 0,02 | 5,40 ± 0,02 | 7,59 ± 0,01 | 6,50 ± 0,01 |
CyPirVal2a | 6,37 ± 0,02 | 5,30 ± 0,02 | 7,57 ± 0,03 | 6,37 ± 0,03 |
CyPirPhal4a | 6,18 ± 0,04 | 4,54 ± 0,05 | 7,31 ± 0,01 | 5,98 ± 0,01 |
CyPirPhal3a | 6,24 ± 0,02 | 4,88 ± 0,03 | 7,23 ± 0,01 | 6,02 ± 0,01 |
CyPirPhal2a | 6,15 ± 0,03 | 4,60 ± 0,03 | 7,13 ± 0,02 | 6,04 ± 0,02 |
m-CyVal4a | 6,95 ± 0,03 | 5,53 ± 0,03 | 7,63 ± 0,02 | 6,18 ± 0,02 |
m-CyVal3a | 6,91 ± 0,04 | 5,43 ± 0,04 | 7,99 ± 0,03 | 6,02 ± 0,04 |
m-CyVal2a | 6,81 ± 0,03 | 5,57 ± 0,03 | 7,93 ± 0,04 | 5,73 ± 0,06 |
m-CyPhal4a | — |
— |
7,57 ± 0,03 | 5,67 ± 0,04 |
m-CyPhal3a | — |
— |
7,23 ± 0,05 | 5,97 ± 0,06 |
m-CyPhal2a | — |
— |
6,90 ± 0,06 | 6,05 ± 0,07 |
—
Values of successive protonation constants show significant decrease of basic properties of studied cyclic ligands as compared to their linear analogues. This is an effect of the presence of diverse donor groups in macrocyclic cavity and their induction effect, as well as less flexible conformation of the ligands and properties of the solvent. The influence of amide group and pyridine ring introduced into cavity of macrocyclic molecules was already published [
1H NMR spectra of CyPirVal(
1H NMR spectrum of CyPirPhal4a in CD3OD.
1H NMR spectrum of CyPirVal3a in CD3OD.
It can lead to the assumption that structure of ligands is bent (boat conformation) on carbon atoms which bonds H-1 and H-1′′ protons. In the literature [
On that base, it can be found that benzyl substituents in CyPirPhal(
The protonation of the investigated compounds is realized through the amino groups present in the linear as well as the cyclic derivatives. The calculated values of successive protonation constants for all investigated ligands showed weak basic properties of these groups (from 6.18 to 8.44 for the first protonation constant). This is an effect of incorporation into ligands skeleton, the amide groups in the cases of linear derivatives, and also pyridine and benzene ring in cyclic ones. The highest values of the protonation constants are observed for linear ligands in methanol-water mixture
The change of the reaction medium from
In both types of ligands, the presence of hydrophobic substituents and polar properties of the mixed solvent solutions lead to intramolecular hydrogen bonds, which are more likely observed in derivatives with benzyl substituents, the lowest pKa values.
In the case of derivatives with pyridine moiety incorporated in the macrocyclic skeleton, there was no protonation of this fragment observed.
The calibration methods of the glass electrode used during the pH metric determination of protonation constants for the investigated compounds (IUPAC buffer solutions or titration of buffer solution described by Wróbel et al. [
The authors declare that they have no competing interests.