Final Report Summary - HALOGEN (Understanding Halogen Bonding in Solution: Investigation of Yet Unexplored Interactions with Applications in Medicinal Chemistry)
Halogen bonding is the attractive interaction of an electron poor region of a halogen - iodine, bromine, chlorine, and in a few cases fluorine - and an electron rich functionality. The geometry, energy and the dominantly electrostatic nature of this chemical force resemble that of the hydrogen bond, which is to date the most frequently used molecular tool in pharmaceutical research, for instance. Although halogens are common in nature, are part of molecules that play key roles in the physiological regulation of our body, and are frequently present in our medicines, agrochemicals and in a variety of commonly used materials, the fundamental understanding behind their frequent utilization is still scanty. Accordingly, although the phenomenon of halogen bonding has been first recognized over one and a half centuries ago, its applicability remains largely unrecognized. Moreover, despite the fact that the majority of our physiological and industrial processes take place in solutions, halogen bonding has so far mostly been studied in solid state and by computational predictions whereas only a few of its studies in solutions were so far performed. The project HALOGEN took a major leap to provide improved understanding on the so far barely explored chemical force halogen bonding and on its behavior in solutions.
As part of the project we have developed model systems for studying halogen bonds within a molecule, in solutions. We have shown that a halogen may simultaneously form halogen bonds to two electron donors in a linear fashion. Such double halogen bonds are astonishingly strong, especially when they are formed by iodine and bromine. These two halogens form equally long and strong bonds to both electron donor sites. The symmetric geometry of their complexes remains even if the solvent they are dissolved is altered. Neither is the geometry affected by the change of the electrolyte, i.e. upon alteration of the ions that complex to the system. The chlorine –centered symmetric halogen bond was stabilized in this project for the very first time. This system is less stable and could only be maintained at low temperatures yet it was shown to be symmetric as well. In contrast, the analogous fluorine-centered system was revealed to be only weakly complexing and asymmetric. Using electronic effects, we have shown that the chlorous halogen bond can be stabilized in solution even at higher temperatures. Importantly, the halogen bonds of all four types of halogens were shown to behave differently as compared to the corresponding hydrogen bond. The latter forms a dynamic mixture of asymmetric systems in which the hydrogen jumps between the electron donor atoms. Halogen bonds in contrast are static geometries. This observation is of high importance because halogen bonds are being developed into a complementary molecular tool to hydrogen bonding for drug development strategies, for the development of new materials and as catalytic systems. Therefore, gaining a deeper understanding of the differences and similarities of halogen bonds to other existing molecular tools is of vast importance
Millions of patients are anesthetized each year using volatile anesthetic agents, such as sevoflurane. Surprisingly, the mechanism of action of such polyhalogenated anesthetic agents remains unrevealed. This is unfortunate, not at least as the lack of understanding of the mechanism of anesthesia hinders the development of strategies to avoid anesthesia related illnesses, such as the inherited mortal disease malign hyperthermia. The latter illness is known to be connected to the dysregulation of the calcium concentration in cells upon anesthetic treatment; however, its’ mechanism is so far very little understood. Calmodulin is a regulatory protein that is present in all human cells and that was previously proposed to play a key role in anesthesia. However, the interaction of this regulatory protein with anesthetic agents has not been studied. In the project HALOGEN we identified the previously unknown anesthetic agent binding sites of calmodulin. We have shown that each calmodulin binds two anesthetic agents and this only happens when calmodulins calcium binding sites are saturated with calcium ions. Surprisingly, volatile anesthetics do not influence the calcium binding affinity of the calcium regulating protein calmodulin directly. Instead, they compete with other proteins of the cell for calmodulin binding. The anesthetic agent binding site of calmodulin overlaps with its ryanodine receptor-1 binding site. Ryanodin receptor-1 is one of the most important regulators of the intracellular calcium level in cells, and is known to be involved in the lethal disease malignant hyperthermia, for example. The understanding of the mechanism how anesthetic agents modulate the interaction of the ryanodin receptor with its regulator calmodulin is therefore of vast importance.
As part of the investigation of anesthetic agents a new technique was developed to study the interaction of proteins and pharmacons or new bioactive compounds. In this method pharmacons are labelled with a paramagnetic tag, a molecular moiety designed to have unusual magnetic properties. The complex of the labelled pharmacon and a protein can then be studied using the technique nuclear magnetic resonance spectroscopy. This method helps to identify the molecular targets of a bioactive compound by allowing rapid scanning of a large number of cellular proteins for interaction with a certain compound. We have demonstrated that this new technique has an increased sensitivity of detection and thereby permits faster and safer detection and detailed description of pharmacon binding sites, which facilitates the development of new medicines and also the understanding of the mechanism of existing pharmacons.
As part of the project we have developed model systems for studying halogen bonds within a molecule, in solutions. We have shown that a halogen may simultaneously form halogen bonds to two electron donors in a linear fashion. Such double halogen bonds are astonishingly strong, especially when they are formed by iodine and bromine. These two halogens form equally long and strong bonds to both electron donor sites. The symmetric geometry of their complexes remains even if the solvent they are dissolved is altered. Neither is the geometry affected by the change of the electrolyte, i.e. upon alteration of the ions that complex to the system. The chlorine –centered symmetric halogen bond was stabilized in this project for the very first time. This system is less stable and could only be maintained at low temperatures yet it was shown to be symmetric as well. In contrast, the analogous fluorine-centered system was revealed to be only weakly complexing and asymmetric. Using electronic effects, we have shown that the chlorous halogen bond can be stabilized in solution even at higher temperatures. Importantly, the halogen bonds of all four types of halogens were shown to behave differently as compared to the corresponding hydrogen bond. The latter forms a dynamic mixture of asymmetric systems in which the hydrogen jumps between the electron donor atoms. Halogen bonds in contrast are static geometries. This observation is of high importance because halogen bonds are being developed into a complementary molecular tool to hydrogen bonding for drug development strategies, for the development of new materials and as catalytic systems. Therefore, gaining a deeper understanding of the differences and similarities of halogen bonds to other existing molecular tools is of vast importance
Millions of patients are anesthetized each year using volatile anesthetic agents, such as sevoflurane. Surprisingly, the mechanism of action of such polyhalogenated anesthetic agents remains unrevealed. This is unfortunate, not at least as the lack of understanding of the mechanism of anesthesia hinders the development of strategies to avoid anesthesia related illnesses, such as the inherited mortal disease malign hyperthermia. The latter illness is known to be connected to the dysregulation of the calcium concentration in cells upon anesthetic treatment; however, its’ mechanism is so far very little understood. Calmodulin is a regulatory protein that is present in all human cells and that was previously proposed to play a key role in anesthesia. However, the interaction of this regulatory protein with anesthetic agents has not been studied. In the project HALOGEN we identified the previously unknown anesthetic agent binding sites of calmodulin. We have shown that each calmodulin binds two anesthetic agents and this only happens when calmodulins calcium binding sites are saturated with calcium ions. Surprisingly, volatile anesthetics do not influence the calcium binding affinity of the calcium regulating protein calmodulin directly. Instead, they compete with other proteins of the cell for calmodulin binding. The anesthetic agent binding site of calmodulin overlaps with its ryanodine receptor-1 binding site. Ryanodin receptor-1 is one of the most important regulators of the intracellular calcium level in cells, and is known to be involved in the lethal disease malignant hyperthermia, for example. The understanding of the mechanism how anesthetic agents modulate the interaction of the ryanodin receptor with its regulator calmodulin is therefore of vast importance.
As part of the investigation of anesthetic agents a new technique was developed to study the interaction of proteins and pharmacons or new bioactive compounds. In this method pharmacons are labelled with a paramagnetic tag, a molecular moiety designed to have unusual magnetic properties. The complex of the labelled pharmacon and a protein can then be studied using the technique nuclear magnetic resonance spectroscopy. This method helps to identify the molecular targets of a bioactive compound by allowing rapid scanning of a large number of cellular proteins for interaction with a certain compound. We have demonstrated that this new technique has an increased sensitivity of detection and thereby permits faster and safer detection and detailed description of pharmacon binding sites, which facilitates the development of new medicines and also the understanding of the mechanism of existing pharmacons.