Secondary chiral structural chemistry

The chemistry of secondary phosphine oxides, R2P(H)0 and their phosphi-nous acid tautomers, R2POH, has continued to attract attention. The study of the phosphinous acid tautomers has been aided by the development of stereoselective procedures for direct conversion of secondary phosphine oxides to the phosphinous acid-boranes (83). Treatment of the secondary phosphine oxide with either a base-borane complex or boron trifluoride and sodium borohyd-ride provides the phosphinous acid-borane with predominant inversion of configuration at phosphorus. The phosphinous acid tautomers are usually trapped as ligands in metal complexes and further examples of this behaviour have been noted. Discrimination of enantiomeric forms of chiral phosphinous acids, Ph(R)OH, coordinated to a chiral rhodium complex, has been studied by NMR. °° Palladium complexes of di(t-butyl)phosphinous acid have found application as homogeneous catalysts.A lithium salt of the tellurophos-phinite Ph2PTeH has been prepared and structurally characterised. ... 

One of the interesting questions of CyD chemistry is whether inclusion complex-ation represents a prerequisite for chiral recognition and, if not, which part of the CyD, external or internal, provides a more favorable environment for enantioselec-tive recognition The synthesis of highly crowded heptakis-(2-0-methyl-3,6-di-0-sulfo)-j8-CyD (HMdiSu-jg-CyD) with 14 bulky sulfate substituents on both primary and secondary CyD rims can provide insights to this problem [62] since the bulky substituents on both sides of the cavity entrance may hinder inclusion complex formation between chiral analytes and HMdiSu-yS-CyD. In one study, 27 cationic chiral analytes were resolved in CE using native f-CyD and HMdiSu-yS-CyD [63]. For 12 of 16 chiral analytes resolved with both chiral selectors the enantiomer migration order was opposite. Analysis of the structures of analyte-CyD complexes in solution indicated that in contrast to mainly inclusion-type complexation between chiral analytes and j8-CyD, external complexes are formed between the chiral analytes and HMdiSu-j8-CyD [63]. 

The most commonly utilized chiroptical method in supramolecular chemistry is CD spectroscopy. CD is typically utilized to study chiral molecules, with most of the research focused on large biological molecules to elucidate their secondary structure or the conformation of macromolecules, and their response to changes from external stimuli-like temperature or pH. The most widely studied CD signatures are the various secondary structural elements of proteins such as the a-helix and the jS-sheet. This is understood to the point that CD spectra in the UV can be used to predict the percentages of each secondary structural element in the structure of a protein. ... 

The two most prominent secondary structural features of protein chemistry are the a-helix and the p-sheet (the basic structures are described in Appendix 4). As mentioned earlier, all helices have an inherent chirality. In contrast, sheets are in a sense flat, and therefore, they are not inherently chiral even though the peptide building blocks themselves are chiral. In addition to the a-helix and the P-shcet, peptides and proteins can lack any defined shape, called a random coil. Once again, no inherent chirality would be associated with this structure, although the building blocks are chiral. This suggests that spectroscopic methods that probe chirality could be used to probe protein secondary structure. Circular dichroism is by far the one most commonly employed. 

Brenner V, Piuzzi F, Dimicoli I, Tardivel B, Mons M (2007) Chirality-controlled formation of p-tum secondary structures in short peptide chains gas-phase experiment versus quantum chemistry. Angew Chem Int Ed 46 2463... 

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