Small peptides, applications

For many of the neuropeptides the receptors are known, and more recently non-peptidic, small-molecule antagonists have become available. Application of these antagonists will help to elucidate further the involvement of peptidic neurotransmitters within the nervous system, as well as their contribution to mental disturbances. It is also expected that some of the recently developed antagonists may become useful therapeutic agents. 

The first installment in this series (Volume 267, 1996) mostly covered peptide and peptidomimetic based research with just a few examples of small molecule libraries. In this volume we have compiled cutting-edge research in combinatorial chemistry, including divergent areas such as novel analytical techniques, microwave-assisted synthesis, novel linkers, and synthetic approaches in both solid-phase and polymer-assisted synthesis of peptides, small molecules, and heterocyclic systems, as well as the application of these technologies to optimize molecular properties of scientific and commercial interest. 

The identification of a pathology-associated antigen and the isolation of a specific binder (antibody, peptide, small organic molecule) does not automatically yield a novel imaging agent. Indeed, a number of conditions must be fulfilled in order to obtain a successful imaging result. In part, these conditions depend on the experimental modality chosen for the in vivo imaging application. 

In synthetic target molecules esters, lactones, amides, and lactams are the most common carboxylic acid derivatives. In order to synthesize them from carboxylic acids one has generally to produce an activated acid derivative, and an enormous variety of activating reagents is known, mostly developed for peptide syntheses (M. Bodanszky, 1976). In actual syntheses of complex esters and amides, however, only a small selection of these remedies is used, and we shall mention only generally applicable methods. The classic means of activating carboxyl groups arc the acyl azide method of Curtius and the acyl chloride method of Emil Fischer. 

For most free amino acids and small peptides, a mixture of alcohol with water is a typical mobile phase composition in the reversed-phase mode for glycopeptide CSPs. For some bifunctional amino acids and most other compounds, however, aqueous buffer is usually necessary to enhance resolution. The types of buffers dictate the retention, efficiency and - to a lesser effect - selectivity of analytes. Tri-ethylammonium acetate and ammonium nitrate are the most effective buffer systems, while sodium citrate is also effective for the separation of profens on vancomycin CSP, and ammonium acetate is the most appropriate for LC/MS applications. 

It is notable that small peptides were retained on a cross-linked PEI column [37] but polypeptides over 20 residues were weakly retained or not at all. Possibly, small peptides were able to penetrate into the depth of the bonded phase while larger peptides could not. These packings were more useful for the separation of oligonucleotides, i.e. more acidic compounds. Lawson et al. [39] have shown the applicability of PEI-silicas for assessing the purity of precursor blocks, monitoring the chemical synthesis and isolating reaction products after synthesis. 

Recent developments in drug discovery and drug development spurred the need for novel analytical techniques and methods. In the last decade, the biopharmaceutical industry set the pace for this demand. The nature of the industry required that novel techniques should be simple, easily applicable, and of high resolution and sensitivity. It was also required that the techniques give information about the composition, structure, purity, and stability of drug candidates. Biopharmaceuticals represent a wide variety of chemically different compounds, including small organic molecules, nucleic acids and their derivatives, and peptides and proteins. 

The third current approach is synthesis of peptide chains as models for the helical peptaibol (Section 8.2) and gramicidin (Section 8.3) ion channels. Considerable work has been carried out in the former area, involving synthesis of Aib-containing small peptides, in order to obtain conformational data applicable to the more complex oligopeptide antibiotics. By working with such fragments it has been possible to obtain valuable X-ray crystal structure information on the helical conformation of ala-methin 246), emerimicin 247), suzukacillin 248), and other members of the peptaibol series. 

This benefit comes at a cost, which arises from significantly reduced S/N and some interpretive difficulty as compared to IR. Developments on the latter front are bringing the theoretical prediction capability of VCD for small molecules to a level demonstrably superior to that for ECD (Freedman and Nafie, 1994 Stephens et al., 1994 Stephens and Devlin, 2000), especially for peptide spectra (Kubelka et al., 2002). Most previous protein applications of VCD used empirically based analyses (Keiderling, 1996, 2000). Theoretical methods are limited when applied to large molecules such as proteins however, a hybrid approach using ab initio determination of spectral parameters with modest-sized molecules for transfer to large peptides has made simulation of spectra for large peptides possible (Bour et al., 1997 Kubelka et al., 2002). Theoretical techniques for simulation of small-molecule VCD are the focus of several previous reviews (Stephens and Lowe, 1985 Freedman and Nafie,... 

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