Polypeptide phase separation

Several parenteral microencapsulated products have been commercialized the cote materials ate polypeptides with hormonal activity. Poly(lactide— glycohde) copolymers ate the sheU materials used. The capsules ate produced by solvent evaporation, polymer-polymer phase separation, or spray-dry encapsulation processes. They release their cote material over a 30 day period in vivo, although not at a constant rate. 

This polypeptide is structurally identical to ABA-type triblock copolymer with a central hydrophdic elastomeric end-block capped with two hydrophobic plastic end-blocks and exhibits amphiphilic characteristics. The end-blocks of the polymer were chosen in such a way that their LCST would reside at or near room temperature. Thus the polymer exhibits phase separation, which is analogue to conventional TPEs, and offers TPE gels under physiological relevant conditions [104]. Glutamic acid residue is placed periodically in the elastomeric mid-block to increase its affinity towards the aqueous... 

Polypeptide antibiotics, such as gramicidin A and polymyxin B, are capable of increasing the permeability of bacterial membranes. As is to be expected, they change the phase transition, much like cholesterol [130]. These substances induce a tightening of fluid membranes and an increase in the fluidity of rigid membranes. It has been shown that polymyxin B produces phase separation and forms a Dimyris-toylphosphatidylcholine (DMPG)-rich phase in DMPG/DMPC membranes [131]. 

Studies with larger peptides, e.g., polypeptide hormones, have indicated that the resolving power of reversed-phase separations may decrease as the peptide chain length increases. These results stress the importance of using elution conditions which generate adequate relative... 

When the polypeptide concentration exceeds the critical concentration at the A point, the solution separates into two phases. The polypeptide-rich phase is birefringent and separates from the more dilute medium initially in the form of spherical liquid droplets or spherulites. If the two-phase solution is cooled or further concentrated in polymer, the spherulites grow in size and coalesce forming a continuously birefringent fluid at the B point. 

As is known, the polypeptide a-helix molecules are rod-shaped if the helical internal structures are smeared out. Therefore, we may expect a phase separation in their solutions also. Indeed, Robinson (27) found in 1956 a phase separation in several solutions of the a-helix, poly-y-benzyl-L-glutamate, in which the second phase, separated out as small droplets, showed an optical birefringence. ITie critical concentration is of course a function of the molecular length. 

These stationary phases separate enantiomers on the basis that one isomer fits in the pocket and the other does not. In this fashion, the relative speed of the isomers is different, and separation results. There are three main types of inclusion chiral stationary phases a-cyclodextrin, /Tcyclodextrin, and y-cyclodextrin. From these three native cyclodextrins, several derivations can be made to alter the selectivity of the inclusion complex, including formation of acetates, esters, and carbamates. Astec produces all three native cyclodextrin stationary phases as well as several derivatized phases (called the Cyclobond series), and as with their macrolide polypeptide phases, they are covalently bonded. 

Native dastin derivatives and thdr precursors undergo revasible, temperature-dependent, hydrophobic assembly from aqueous solution, which results in spontaneous phase separation of the polyp>eptide above a critical temperature, T that depends on polypeptide sequence and environmental conditions. This process, known as coacervation, is formally analogous to the responsive phase behavior observed for smart polymas such as poly(N-isopropylacrylamide) and poly( ethylene glycol) in aqueous solution above the lower critical solution temperature (LCST). Howeva, while similarities are observed between the two processes, the coacervation of native... 

The term inverse transition was first used in connection with the increase in order of the antibiotic stendomycin on raising the temperature (D.W. Urry and A. Ruiter, Conformation of Polypeptide Antibiotics. VI. Circular Dichroism of Stendomycin. Biochem. Biophys. Res. Commun., 38,800-806,1970). The term became specifically inverse temperature transition in relation to coacervation of elastin fragments that exhibited a phase separation with increased order on raising the temperature (B.C. Starcher, G. Saccomani, and D.W. Urry, Coacervation and Ion-Binding Studies on Aortic Elastin. Biochim. Biophys. Acta, 310, 481 86,1973, and D.W. Urry, B. Starcher, and S.M. Partridge, Coacervation of Solubilized Elastin Effects a Notable Conformational Change. Nature, 222,795-796,1%9). 

Another test of the theory is to compare the experimentally determined dependence of Up, the volume fraction at which the nematic phase separates, on the axial ratio." The agreement between theory and experiment is particularly good with the alpha-helical polypeptide poly(y-benzyl-L-glutamate).(94-96) Studies of solutions ofthe polymeric aramides, such as poly(p-benzamide) and poly(/ -phenylene tereph-thalamide) indicate a qualitative accord between theory and experiment. Studies... 

In reversed-phase chromatography, polypeptides are separated according to their polarity. Polar compounds do not interact with the stationary phase and migrate faster than nonpolar molecules, which are withheld. There is no correlation with the molecular weight, so it is necessary to know the characteristics of the ingredients in the mixture. The equipment is expensive, but the method is particularly suited for proteins whose molecular weight is less than 10,000. 

Furthermore, suppose the unfolding/folding mechanism of a polypeptide chain in water can be simulated by the phase separation of a hydrocarbon (mineral oil) from water. This is accomplished by assigning the U state (unfolded state) to the monodisperse solution of oil in water, and the N state (folded or native state) to separated phases (i.e., an oil drop on the water surface). Each of the above four terms (two enthalpy and two entropy terms) that constitute AG,jansfef will be analyzed in light of the net formation/breakdown of noncovalent bonds and the overall increase/decrease of conformational entropy of protein and that of the solvent. A5, v,a, , which is equal to is negative because a monodisperse solution (U) is less ordered... 

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