Aqueous solution, polypeptides

Researchers studying polypeptide and polypeptide hybrid systems have also processed vesicles using two solvents. This method usually involves a common organic solvent that solubilizes both blocks and an aqueous solvent that solublizes only the hydrophilic block. The two solvents can be mixed with the polypeptide or polypeptide hybrid system at the same time or added sequentially. The choice of organic solvent depends heavily upon the properties of the polypeptide material, and commonly used solvents include dimethylformamide (DMF) [46, 59], methanol (MeOH) [49], dimethyl sulfoxide (DMSO) [50, 72], and tetrahydrofuran (THF) [44, 55]. Vesicles are usually formed when the organic solvent is slowly replaced with an aqueous solution via dialysis or removed through evaporation however, some vesicles have been reported to be present in the organic/aqueous mixture [49]. 

Raman optical activity is an excellent technique for studying polypeptide and protein structure in aqueous solution since, as mentioned above, their ROA spectra are often dominated by bands originating in the peptide backbone that directly reflect the solution conformation. Furthermore, the special sensitivity of ROA to dynamic aspects of structure makes it a new source of information on order-disorder transitions. 

Kallenbach and co-workers have recently demonstrated via CD spec-tropolarimetry and NMR spectrometry that a seven-residue alanine peptide adopts predominantly the PPII helical conformation in aqueous solution (Shi et al., 2002). Since alanine is nothing but backbone, such a finding indicates that the polypeptide backbone possesses an intrinsic... 

Chapman et al. [131] reported the synthesis of poly(ethylene oxide) (PEO) supported dendritic f-BOC-poly(a, c-L-lysines). These dendritic polymers termed as hydramphiphiles formed foams possessing good temporal stability in aqueous solution. Scrimin et al. [132] synthesized a three-directional polypeptide having uses in membrane permeability modulation. Decapeptide fragments were linked to TREN [tris(2-aminoethyl)amine] core. 

Another potential site of reactivity for anhydrides in protein molecules is modification of any attached carbohydrate chains. In addition to amino group modification in the polypeptide chain, glycoproteins may be modified at their polysaccharide hydroxyl groups to form ester derivatives. Esterification of carbohydrates by acetic anhydride, especially cellulose, is a major industrial application for this compound. In aqueous solutions, however, esterification may be a minor product, since the oxygen of water is about as strong a nucleophile as the hydroxyls of sugar residues. 

Transition metals such as iron can catalyze oxidation reactions in aqueous solution, which are known to cause modification of amino acid side chains and damage to polypeptide backbones (see Chapter 1, Section 1.1 Halliwell and Gutteridge, 1984 Kim et al., 1985 Tabor and Richardson, 1987). These reactions can oxidize thiols, create aldehydes and other carbonyls on certain amino acids, and even cleave peptide bonds. The purposeful use of metal-catalyzed oxidation in the study of protein interactions has been done to map interaction surfaces or identify which regions of biomolecules are in contact during specific affinity binding events. 

The Lys-25 polymer was crosslinked into a gel by treatment of a 5% (w/v) solution of the polypeptide in either aqueous phosphate buffer (pH 9) or anhydrous dimethylsulfoxide with a bis(A -hydroxysuccinimidyl) derivative of suberic acid. The reagents were mixed at 4°C, and the solutions were warmed to 25°C. After approximately 10 minutes, the solutions formed mechanically stable gels that could be physically manipulated. The gels formed in DMSO were optically transparent in contrast to the opaque gels formed in aqueous solution. 

Histidine residues are efficient nucleophiles in aqueous solution at pH 7, much more so than lysines, and this is the basis for the site-selective functionalization of lysine residues in folded polypeptides and proteins [24, 25]. p-Nitrophenyl esters react with His residues in a two-step reaction to form an acyl intermediate under the release of p-nitrophenol followed by the reaction of the intermediate with the most potent nucleophile in solution to form the reaction product. In aqueous solution the reaction product is the carboxylic acid since the hydroxide ion is the most efficient nucleophile at pH 7. If there is an alcohol present the reaction product will be an ester and the overall reaction is a transesterification reaction. 

Recently, it has been shown that fraction V of apo HDL, closely resembling the D polypeptides of apo VLDL, is moderately soluble in aqueous solutions of ethanol, a fact to be taken into consideration in the delipidation of serum lipoproteins with extracting mixtures containing this organic solvent. The relevance to recovery problems was pointed out previously (Sll). 

Vitalis, A., Pappu, R.V. ABSINTH a new continuum solvation model for simulations of polypeptides in aqueous solutions. J. Comput. Chem. 2009, 30, 673-99. 

Several peroxidative reactions initiated by OH radical that may be generated by either y- and X-radiolysis of aqueous solutions or by transition metal-catalyzed reduction of H2O2 have been identified in free amino acids and short peptides. In this respect we may distinguish oxidizing reactions that involve the polypeptide backbone on the one hand... 

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