Peptide isomers

Common posttranslational modifications of proteins. Frequently modified amino adds for each moiety are described. Most modifications involve low-mass substituents (<500amu), but some modifications are small polypeptides of considerable mass such as ubiquitin or highly related peptide isomers like SUMO-1, SUMO-2, and SUMO 3. SUMO is the abbreviation for similar to ubiquitin methyl organizer , SUMO has different family members (SUMO 1, 2, and 3 that can combine with proteins). 

Direct analysis with the on-line ORD detector linked to the HPLC was hampered by peak broadening, which led to incomplete resolution of the two peptides. Diffusion in the large flow cell may be a major factor in this difficulty. Because detector response can be either positive or negative, it is important to have an excellent signal-to-noise ratio and baseline HPLC resolution of the peptide isomers to be able to quantitate the degree of racemization by this technique. While the on-line ORD detector is promising, further improvements in resolution as well as sensitivity are required for it to be useful for routine peptide analysis. 

Tadey, T. and W. C. Purdy, Capillary electrophoretic resolution of phosphorylated peptide isomers using micellar solutions and coated capillaries. Electrophoresis, 16, 574-579, 1995. 

A practical system for analytical scale high-resolution separations was first demonstrated by Culbertson and Jorgenson. Narrow diameter capillaries and LIF detection were used to resolve analytes with extremely small differences in mobility such as peptide isomers. Misleveling of the buffer reservoirs, using a siphoning action to provide a counter flow in large diameter capillaries, was also reported with modest resolution improvements. ... 

Regardless of the method used to construct the main pseudodipeptidic backbone, separation of different diastereoisomers has been achieved in several cases by selective crystallization at dipeptide [74, 91], tripeptide [74, 92, 93], or even tetrapeptide level [94]. The isomer corresponding to the natural peptide is usually crystallized out selectively nevertheless, in some tripeptides it has been reported that non-natural peptide isomers are prone to selective crystallization in less polar solvents (Et20 or CHCI3), probably caused by differences in the intramolecular hydrogen bonding in these media (Scheme 16) [92, 93]. 

The second application of the CFTI approach described here involves calculations of the free energy differences between conformers of the linear form of the opioid pentapeptide DPDPE in aqueous solution [9, 10]. DPDPE (Tyr-D-Pen-Gly-Phe-D-Pen, where D-Pen is the D isomer of /3,/3-dimethylcysteine) and other opioids are an interesting class of biologically active peptides which exhibit a strong correlation between conformation and affinity and selectivity for different receptors. The cyclic form of DPDPE contains a disulfide bond constraint, and is a highly specific S opioid [llj. Our simulations provide information on the cost of pre-organizing the linear peptide from its stable solution structure to a cyclic-like precursor for disulfide bond formation. Such... 

In the native protein these less stable ds-proline peptides are stabilized by the tertiary structure but in the unfolded state these constraints are relaxed and there is an equilibrium between ds- and trans-isomers at each peptide bond. When the protein is refolded a substantial fraction of the molecules have one or more proline-peptide bonds in the incorrect form and the greater the number of proline residues the greater the fraction of such molecules. Cis-trans isomerization of proline peptides is intrinsically a slow process and in vitro it is frequently the rate-limiting step in folding for those molecules that have been trapped in a folding intermediate with the wrong isomer. 

The structures of these ylide polymers were determined and confirmed by IR and NMR spectra. These were the first stable sulfonium ylide polymers reported in the literature. They are very important for such industrial uses as ion-exchange resins, polymer supports, peptide synthesis, polymeric reagent, and polyelectrolytes. Also in 1977, Hass and Moreau [60] found that when poly(4-vinylpyridine) was quaternized with bromomalonamide, two polymeric quaternary salts resulted. These polyelectrolyte products were subjected to thermal decyana-tion at 7200°C to give isocyanic acid or its isomer, cyanic acid. The addition of base to the solution of polyelectro-lyte in water gave a yellow polymeric ylide. 

Reversed micelles have also shown to be useful not only in bioconversions, but also in organic synthesis. Shield et al. (1986) have reviewed this subject and brought out its advantages in peptide synthesis, oxidation or reduction of steroids, selective oxidation of isomeric mixtures of aromatics, etc. In the oxidation of aromatic aldehydes to carboxylic acids with enzymes hosted in reverse micelles, the ortho substituted substrates react much more slowly than other isomers. 

Seebach, D., Abele, S., Gademann, K., Guichard, G., Hintermann, T., Jaun, B., Matthews, J. L., and Schreiber, J. V. (1998). /32- and /3 -peptides with proteinaceous side chains Synthesis and solution structures of constitutional isomers, a novel helical secondary structure and the influence of solvation and hydrophobic interactions on folding. Helv. Chim. Acta 81, 932-982. 

Stereospecific enzymes catalyse reactions with one type of optical isomer but may also react with a series of related compounds of the same configuration. Many proteolytic enzymes hydrolyse only peptide bonds linking laevorotatory (L-) amino acids. 

Chiral stationary phases for the separation of enantiomers (optically active isomers) are becoming increasingly important. Among the first types to be synthesized were chiral amino acids ionically or covalently bound to amino-propyl silica and named Pirkle phases after their originator. The ionic form is susceptable to hydrolysis and can be used only in normal phase HPLC whereas the more stable covalent type can be used in reverse phase separations but is less stereoselective. Polymeric phases based on chiral peptides such as bovine serum albumin or a -acid glycoproteins bonded to... 

Acid acetone extracts of human and rodent leukocytes (RBL-cells) have been found to contain immunoreactive SOM (iSOM) and immunoreactive SP (iSP) as determined by radioimmunoassay [144], Quantities of the peptides varied from 325 pg iSOM/107 cells for human monocytes and 272 pg iSOM/107 cells for RBL-cells to 4.4 pg iSOM/107 cells for human T cells. iSP was highest in murine bone marrow-derived mast cells (64 pg iSP/107 cells) and RBL-cells (23 pg iSP/107 cells) and lowest in human T and B lymphocytes (2.5 and 1.2 pg iSP/107 cells, respectively). Interestingly, the murine bone marrow-derived mast cells had the highest ratio of iSP to iSOM. Preliminary chromatographic results show a large and a small SOM (SOM-28 and SOM-14, respectively). SOM-14 (3 x 10 9 M) has been shown to inhibit histamine release and LTC4 generation from murine bone marrow-derived mast cells stimulated by anti-IgE serum [144]. 

Amino acids coordinated to (en)2Co(III) centers are the most useful in peptide synthesis, and general routes to the preparation of these important A/) O-chelates are described below. Many of the corresponding trien complexes are also available, usually as /32 isomers, but [Co(NH3)4(AAO)]2+ species are less accessible. The [Co(en)2(AAO)]2+ complexes have been reported for 19 of the 20 naturally occurring... 

Activated esters (see Section 2.9) Activated esters of peptides are rarely used because there is no general method available for converting an (V -protected peptide into the ester with a guarantee that it will be a single isomer. Attempts have been made to overcome this obstacle (see Section 7.8). However, solid phase synthesis allows the preparation of thioesters of segments (see Section 7.10). Once the ester is in hand, it can be aminolyzed without generation of a second isomer if suitable conditions are employed. 

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