The design of methylcarbamates was based on a natural product called physostigmine Physostigmine inhibits an enzyme called acetylcholinesterase, which catalyses the hydrolysis of a neurotransmitter called acetylcholine The organophosphates also target the acetylcholinesterase enzyme, and act as irreversible inhibitors. Imidacloprid, nicotine, and acetylcholine share structural features that are important in binding these agents to the receptor binding site However, this does not explain why imidacloprid binds 1, times more strongly to insect receptors than human receptors—a key reason for its selectivity. A major reason for this selectivity is the presence of the nitro group, which can interact with an arginine residue that is present in the binding site of insect receptors but not mammalian receptors.
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Crystal engineering has grown over time, with its practitioners now seeking specific answers to specialized questions. How does a molecular crystal nucleate and then grow?
Can its structure be predicted computationally? Can one design a crystal structure with knowledge-based inputs? Can a crystal structure be considered as a collection of modular entities which represent its microcosms?
What properties are characteristic of the crystal as a whole rather than of its constituent molecules? Can these properties be designed and is property design different from structure design?
Can one predict if a given compound will have polymorphs and pseudopolymorphs? Can one design the structures of multi-component crystals in which each component is a solid when taken separately under ambient conditions?
All these issues connect through the structural landscape of crystals and the exploration of this landscape, that is crystallization. The subject of crystal engineering covers not only purely organic solids but also organometallics and more significantly the metal organic framework solids MOFs or coordination polymers as they are also called.
Crystal engineering evolved from an intersection of crystallography and chemistry, especially after the s when it became much easier to determine crystal structures of small molecule Synthons retrosynthesis.
Is it important to have a definition of the term crystal engineering itself? This question resurfaces with regularity. This wording seems to have stood the test of time as it identifies the three concepts that are of note in the subject: Crystal packing is closely connected with the nature of intermolecular interactions, their strength, their directionalities and their distance dependence properties.
These three attributes determine how and to what degree of importance an interaction manifests itself in the packing of molecules in a crystal.
Papers have appeared in the journal on hydrogen bonding, halogen bonding and van der Waals interactions. The related area of charge density studies is closely connected to the study of intermolecular interactions and the importance of charge density studies to crystal engineering is now acknowledged.
One may also ask if it is an over simplification to consider a crystal as an ensemble of two-body interactions. In the limit, the molecular crystal is a holistic entity.
Each crystal structure is, rigorously speaking, a different story. Why then does the modular approach with supramolecular synthons work so well in crystal design? The design of crystal structures calls for some element of predictability in the manner in which one molecule recognizes another during assembly.
At the core of the supramolecular synthon approach to crystal design is the argument that one functional group in a molecule recognizes another functional group in the same or different molecule in a particular way. This recognition pattern defines the synthon. The role of the supramolecular synthon in crystal engineering may be likened to that of the molecular synthon in organic synthesis because both types of synthon, molecular and supramolecular, arise from kinetically preferred events.
Logic driven retrosynthesis may be used to design organic crystal structures.
A similar retrosynthetic analysis may be applied to the design of coordination polymers and MOF compounds. When the functional groups that recognize each other arise from the same type of molecule, the result is a single component crystal.
When they are from different chemical entities, the result is a multi-component crystal or a cocrystal. Property design is the third and final stage in crystal engineering.
It is interesting to note, around three decades after the term crystal engineering itself entered the chemical and crystallographic literature in a general way, that each of the three stages in the development of the subject, crystal packing analysis, crystal design strategies and targeting of properties needed a certain degree of maturity of the earlier stages before they could develop systematically.
It was certainly unfair to expect any serious property engineering say 15 or 20 years ago, although there was progress in the design of non-centrosymmetric crystals for optical devices.
Today there is an explosion of activity in the design of crystal properties, ranging from gas absorption and catalysis applications in MOFs to mechanical, photochemical and photophysical properties for pure organics.
All of us know that structure and properties are connected. One can think of form and function in the macromolecular crystallography context. But what exactly is the connection?Disconnection, Synthons, Introductory Example Abstract Retrosynthetic analysis as an imaginative process is introduced.
Retrosynthesis and Asymmetric Synthesis, Introductory Example 5. Formalism in the presentation of C–C bond disconnection comprises, besides. The information contained in this module specification was correct at the time of publication but may be subject to change, either during the session because of unforeseen circumstances, or following review of the module at the end of the session.
NOTE: Above study material is of very high standards unlike cut, copy & paste type of notes provided by other institutes. Hence the prices are according to the standards maintained. From a supramolecular retrosynthesis perspective, only those synthons that occur repeatedly in crystal structures, namely robust synthons of a particular set of functional groups, are useful in crystal design (Nangia & Desiraju, ).
CSIR-UGC National Eligibility Test (NET) for Junior Research Fellowship and Lecturer-ship CHEMICAL SCIENCES Inorganic Chemistry 1.
Chemical periodicity. Organic Synthesis: The Disconnection Approach (second edition) is written by Stuart Warren (Chemistry Department, Cambridge University, UK) and Paul Wyatt (School of Chemistry, University of Bristol, UK) and published by Wiley and Sons Inc. in