Isoleucine functional group

The vast differences in the functional groups possessed by the side chains allows for their chromatographic separation. However, closely related compounds can still be tough to resolve. For example, leucine and isoleucine tend to afford very tight resolution. The chemical differences in the side chains can be very problematic when it comes to sample preparation. This is especially true for the acid hydrolysis of proteins to liberate their constituent amino acids in the free form. The different functional groups exhibit different chemical stabilities in an acid environment. 

Polymers imprinted for amino acids with larger side groups generally displayed a higher, yet relatively modest, enantioselectivity. Also, polymers are able to separate amino acids with functional groups that are similar to the imprint amino acid. For example, polymers imprinted for L-phenylalanine displayed a selectivity of 1.45 for L-phenylalanine 1.36 for L-tyrosine and 0.99-1.08 for alanine, valine, leucine, and isoleucine. This indicates that the larger cavity created by phenylalanine does not have the ability to differentiate the smaller side chains of the smaller amino acids. Likewise, polymers imprinted with smaller amino acid side chains showed moderate selectivity for similar amino acids, but could not differentiate the larger amino acids. 

The helix-turn-helix scaffold is designed to dimerize into a four-helix bundle. Modification of the peptide with the nicotinoyl functionality did not significantly perturb the peptide structure. CD spectroscopy showed no loss in helici-ty from the parent peptide and NMR spectroscopy confirmed successful incorporation of the nicotinoyl group as well as maintenance of crucial NOE connectivities. In particular, the presence of long-range NOE signals between the side chains of phenylalanine 38 and leucine 12 or isoleucine 9, which lie near the C- and AT-termini, respectively, demonstrate that the supersecondary structure of the motif has been conserved. 

Lattice substitution requires that the incorporated impurity be of similar size and function to the primary crystallizing species. In other words, the impurity must fit into the lattice without causing significant dislocations. An example of such a system is found in the crystallization of L-isoleucine in the presence of trace quantities of L-leucine. The two species have similar molecular structures, differing only by one carbon atom in the position of a methyl side group. In this system, the incorporation of L-leucine in L-isoleucine crystals is proportional to the concentration of L-leucine in the mother liquor. Moreover, the shape of the recovered crystals changes as the content of L-leucine in recovered crystal increases. 

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