Monday, November 30, 2009

Supramolecular Chemistry.

Supra-molecular chemistry is a branch of chemistry concerned with the aggregation of molecules into large assemblies. The molecules in the assembly are held by the weak intermolecular forces such as hydrogen bonding, dipolar, Vander Waal's and hydrophobic interaction and also metal ion coordination. Just as in a molecule atoms are held together by strong covalent bonds, in Supra-molecular structure, molecules are held together by weak intermolecular forces. It is often described as chemistry beyond the molecule.

Supra-molecular chemistry involves self-assembly or spontaneous association of several (2 or more) molecular components into a discrete non-covalently bound aggregate with a well defined structure. When highly complex structures are obtained from such units, the process is sometimes called self-organization.

Self assembly involves molecular recognition processes (binding events). Molecular recognition relies upon complementary of size, shape and chemical functionalities.

The partners of supra-molecular species have been named molecular receptor and substrate. The substrate usually being the smaller component whose binding is being sought. It is also sometimes called host-guest interaction. The binding of a substrate S to a receptor R yields the supra-molecule RS and involves a molecular recognition process. In order to achieve high recognition it is desirable that the receptor and substrate be in contact over a large area. This occur when the receptor is able to wrap around its guest so as to establish numerous non-covalent binding interactions and to sense its molecular size, shape and architecture.

In addition to binding sites, the receptor also carries reactive functions, it may effect chemical transformation on the bound substrate, thus behaving as a supra-molecular catalyst.

A lipophilic membrane soluble receptor may act as a carrier effecting the translocation of the bound substrates across the membrane. Thus, molecular recognition, transformation and translocation represent some of the basic functions of supra-molecular species. Supra-molecular chemistry has thus contributed in the construction of receptors, transfer agents enzyme models and extended arrays.

The concepts of the molecular recognition and complementarity are common in biological systems. complementarity between H-bonding groups on the twin strands of DNA is what makes the double helix stable. Molecular recognition is also part of the so called 'lock and Key' mechanism of enzyme action and transport of ions across membranes.

Supra-molecular chemistry first synthetic receptors were able to recognize the simplest of the substrates, the alkali metal ions. Receptors that recognize these ions already exist in nature. An example is the ionophore valinomycin which is a cyclic peptide molecule that binds specifically K+ ions and transports them across cell membranes. Valinomycin's ability to recognize and bind K+ ions selectivity is due to its geometrical complementarity, the cavity being of the rigid size to bind K+ ions. Na+ ions are smaller and are not bound by valinomycin. In addition to geometrical complementarity, valinomycin is a good receptor because of the presence of several interaction sites (Carbonyl groups) so that strong binding is possible.

It was the search of the synthetic analogues of natural ionophores that led to the discovery of cyclic ethers or crown ethers as receptors for metal ions (pederson). Crown ethers contain oxygen atoms in a cyclic structure with a central cavity with right size to fit the target ion. e.g., 18-crown-6 and 15-crown-5

In 18-crown-6 a cavity of the right size to fit K+ ions. It can, therefore, recognise and selectively bind to K+.

Subsequently, bicyclic molecules were synthesized in which nitrogen atoms unite three ether chains. These receptor molecules are three dimensional receptors with roughly spherical cavities, that bind alkali metal ions more tightly than do the single ring crown ethers. These molecules were called Cryptands and their metal complexes Cryptates. The metal ion binding ability of cryptands is selective, i.e, they exhibit molecular recognition. The selectivity is dependent on the size of the cavity.

Tricyclic receptors have also been developed which could bind large cations such as Cs+ and also NH4+ ions in their cavity.

In all these systems, the preference for a particular metal ion depends on a subtle balance between several thermodynamic factors.
a) binding energy (enthalpy)
b) gain or loss of configurational freedom (entropy) when the solvent cage of the metal ion is replaced by the ether cage.

This is characteristic of all supra-molecular complexes.

The Crown ethers and Cryptands highlighted the features that help good molecular recognition of a substrate by a receptor and helped in the construction of new receptors. They showed forinstance, how increasingly strong selective binding results from an increasing degree of pre-organization of binding groups in the receptor.

Receptor molecule with rigid cavities have the advantage that the binding sites are already in place before a substrate is bound. The binding sites donot have to organize themselves during the process of binding. In other words, they have a high degree of pre-organization and hence exhibit good molecular recognition properties such receptor molecules with relatively rigid cavities are known in nature of particular interest is the class of naturally occurring carbohydrates called Cyclodextrins, alpha-, beta-, and gamma- which contains 6, 7 and 8 glucose units respectively, linked head to tail in a ring. These molecules have a cylindrical cavity running through their centre in which small molecules can fit. The outer face of a cyclodextrin is hydrophilic with hydroxyl groups that have H-bonding ability so that the molecule is soluble in water. The inner cavity is lined mainly with C-C and C-H groups and is hydrophobic. Hence the cavity is able to receive and bind hydrophobic molecules such as benzene and phenylmethyl ether. Such receptor-substrate complexes are also called host-guest complexes or inclusion complexes. The molecular recognition in them is governed by complementarity of size and affinity (hydrophobicity) between cyclodextrin cavity and the molecule. Cyclodextrins have been investigated as enzymes mimics that enhance the selectivity of a chemical transformation carried out on the bound substrate. It was found that the H-atoms of the benzene ring of phenyl methyl ether could be substitued by chlorine specifically in the para position when the molecule was bound with a cyclodextrin. Only the para-hydrogen protruding from the bottom of the cavity was exposed to chlorine attack.

Synthetic rigid cavity receptors are also known. For example, formaldehyde and phenol derivatives can form cyclic oligomers which adopt a cup-like shape and are called calixarenes. Synthetic methods of making a wide range of calixarenes with different numbers of phenolic groups in the calix(n)arenes, where n denotes the number of phenol groups in the ring. For e.x., calix(4)arene has four phenol groups in the ring. The lining of benzene rings in the bowl shaped cavities of calixarenes allows them to acts as 'molecular baskets' for hydrophobic substrate like toluene, benzene and xylene.

The strength with which the substrate is bound depends on the complementarity of size. For e.x., toluene is firmly embedded in the bowl of a calix(4)arene but it is only loosely bound in the large cavity of calix(8)arene. It has been found that calix(7)arene will exhibit molecular recognition towards carbon cage fullerene molecules C60 and C70. The former with a soccer ball shape is bound tightly in the cavity, whereas the latter, with a rugby ball shape, is not. This provides a means of separating the two molecules which are generally formed in an intimate mixture.

Metal ions can serve as templates, that guide ligands into a particular arrangement. They, there by, act as organizing centers for synthesis of large supra-molecular assemblies from many component parts. In other words metal templation offers an effective approach to the self assembly of recognition sites.

Metal ions that form 4 coordinate square complexes are well suited to act as 90degree corners of square supra-molecular structures.

Metal ions with tetrahedral coordination geometries have been used as organizing centers for the multistep assembly of double helices (helicates). The two ligands twist around several metal ions in a double helical fashion. The two strands in these contain metal bound bipyridine units linked together. The efficiency of the self assembly process is the result of positive co-operativity, whereby the binding of one metal ion pre-organizes the ligands to facilitate the binding of the second. In other words, the assembly process gets easier as it progresses.

Similar strategies have been used to obtain molecular assemblies described as catenanes and rotaxanes. An assembly in which one ring is linked through another is called a catenane. A rotaxane is a linear molecule threaded through a cyclic molecule. Unthreading is prevented by subsequently adding bulky end caps.

One of the goals of supra-molecular chemistry is to build synthetic systems that carry out some of the functions of biochemical systems - trans membrane ion transport, enzyme -like catalysis, conducts for channeling electrons and chemical sensors with high sensitivity and selectivity.

18 comments:

  1. good work ....keep it up!

    ReplyDelete
  2. Actually overwhelming labor with the blog. I do similar to your firm labor on and will stay for more post from you as post gave me enjoyment and gives some helps to do same work right here. Thanks a lot…………………………strong journalism statement of purpose

    ReplyDelete
  3. To start with the first line of my statement – I do worth any terribly to provides a large-scale as a results of the diary esteem. terribly it's associate outsize work by him that i detected associate honest facilitate by his/her huge facts and figures check cashing costa mesa. I just crave to inform, Brobdingnagian Brobdingnagian pleasure comprise it up your work. rarely ar throughout an edge to|i'll} be ready to establish with you're posting and alter. wanting ahead to your any mails.Thanks heaps.

    ReplyDelete
  4. Free hand writing is one in every of our Brobdingnagian efforts for college papers. Here you want to realize the service of writing services. So, be not to mention USA.

    ReplyDelete
  5. Most of the days, you'll get the loan quantity at intervals succeeding twenty four hours. you need higher to appear at out before you're taking the loan. though day loans unit little or no amounts, however if you are doing not pay them on time, you'll got to be compelled to be compelled to pay a massive quantity of additional cash which could be a terrible expertise. often scan the legal policies of your payday loans state and your company before you apply for day loans.

    ReplyDelete
  6. Another tip is to close the record you have the credit charges being deducted from if conceivable. You should begin another record before you close your present record and by no means whatsoever, by any stretch of the imagination, get payday credits under the new record. You need to treat the payday change slant like an abuse. Cash Advance

    ReplyDelete
  7. As you biking throughout town, you are activity to see a lot of advertisements for a lot of altered companies, products, and services. You accept alike apparently apparent those signs for companies adage they action payday loans.payday loans corona

    ReplyDelete
  8. Do you go from aperture to aperture allurement money from friends? That's added awkward than actuality bent with your neighbor's babe with your pants down. Not anybody has a advancement ability to tap into. So, don't feel addled if your accompany are not able to advice you.
    cash advance chicago

    ReplyDelete
  9. This comment has been removed by the author.

    ReplyDelete
  10. Investment in absolute acreage is a badly assisting business. This has induced the call of absolute acreage advance accommodation in the absolute advance business. The accommodation is loaded with abounding adorable appearance like lower absorption rate, greater borrowings and beyond acceptable claim duration. The commodity deals in means of availing the accommodation for the account of the investors. aaa1autotitleloans.com/chicago

    ReplyDelete
  11. Payday loans are best back you charge the money actual fast. Aback it is candy quickly, you will accept the money appropriate away, aloof afterwards several canicule afterwards actuality approved. In adverse with best lending companies, they will absorb several canicule aloof attractive at your cardboard and processing it. A acceptable archetype is that, back you administer for a payday accommodation on a Monday, you are abiding to get your money afore the end of that week.
    check cashing

    ReplyDelete
  12. Besides, the being has to be aloft 18 years of age and a aborigine of the United States and charge acquire a coffer annual to acquiesce absolute drop of money into his account. To prove that you accept a coffer account, you may accept to accommodate drop block at the time of applying for these affectionate of loans.
    Cash Advance

    ReplyDelete
  13. Both the web and the authentic angel are calamity with spots across you can get a change with admirable credit, no acclaim check, and no arrangement affirmation about what these present acclaim sharks don't let you apperceive is that their afflatus of avant-garde costs are aeriform to the point that one day you adeptness wind up with the payday avant-garde mafia advancing afterwards you!
    Cash Advance

    ReplyDelete
  14. The alive chic in any country is the courage of its economy. And best generally than not, it is this alive chic that needs banking advice from time to time. For emergency requirements, a payday accommodation is the greatest advice a government can accommodate its people. These loans accept become so important to abounding in the alive class.

    auto title loans

    ReplyDelete
  15. I've been caught in the payday loan otherwise known as payday advance web many times. Sometimes life's circumstances leave us very few choices and we decide to do what we feel like we have to do to buy gas and put food on the table. Like I said, i've been there before and as a survivor of payday loan debt I have knowledge that can help you.
    Payday Loans

    ReplyDelete
  16. I want you to ask yourself a question which is, "if you don't have enough money to get by what makes you think you can payoff a loan with 300%- 700% interest?" You might as well sell your first born child. Payday loans may seem good in the short run and you may get instant gratification and even a rush when you hold that cash in your hands or see it in your bank account but in the long run it is a race that many can't win.
    cash advance

    ReplyDelete
  17. Hey I’m Martin Reed,if you are ready to get a loan contact.Mr Benjamin via email: 247officedept@gmail.com ,WhatsApp:+1 989-394-3740 I’m giving credit to his Service .They grant me the sum 2,000,000.00 Euro. within 5 working days.Mr Benjamin work with  group investors into pure loan and debt financing at the  low ROI to pay off your bills or buy a home Or Increase your Business. please I advise everyone out there who are in need of loan and can be reliable, trusted and capable of repaying back at the due time of funds.

    ReplyDelete
  18. Grown-ups and professors always tell scholars to study, but they no way explain how they should learn. As a result, scholars end up codifying assignment help every time they come across any grueling assignment that they can not handle. In this composition, you ’ll find ten vital study tips that will ameliorate your grades without depending on online help.
    our services-
    paraphrasing tool
    assignment help brisbane
    paper help
    assignment writer

    ReplyDelete