Chain reversal

These structures show that RpII essentially consists of the small core of four-stranded )9-sheet and three relatively large loops. Residues 6-16, 23-30, and 35-40 form loops 1, 2, and 3, respectively, and the chain reversals are accomplished by tight turns involving residues D8-D11, E28-E31, and V36-P39. Segments involved in )5-sheet strands and loops alternate in the primary sequence of Rp toxins. As mentioned above, these structures indicate that the )5-sheet is highly twisted in order to form the correct disulfide bonds. Comparison of different refined structures of RpII indicate that the structure of loop 1 in RpII is less well defined than... 

Lewis, P. N., F. A. Momany, and H. A. Scheraga. 1973b. Chain Reversals In Proteins. Biochim. Biophys. Acta 303, 211-229. 

Tight turns can combine with other types of structure in a number of ways. In addition to their classic role of joining )8 strands, they often occur at the ends of a-helices (see Section II,A). A type II turn forms a rather common combination next to a Cl /3 bulge (see Section II,D). Isogai et al. (1980) have surveyed the occurrence of successive tight turns, which either form approximately helical features or else form more complex chain reversals than single turns. 

All but one of the above structures have four helices in the bundle, with + 1,+ 1,+ 1 connections. For the up and down topology on a cylinder, handedness can be defined by whether the chain turns to the right or to the left at the end of the first structure element (whether it is a helix or a /3 strand). With an even number of helices, reversing N to C direction of the chain also reverses handedness of the topology for an odd number of helices or strands handedness is invariant to chain reversal. For + 1, + 1, +1 topologies in general, handedness is not... 

Kim J, Liu Y, Ahn SJ, Zauscher S, Karty JM, Yamanaka Y, Craig SL. Self-assembly and properties of main-chain reversible polymer brushes. Adv Mater 2005 17 1749-1753. 

Yount WC, Juwarker H, Craig SL. Orthogonal control of dissociation dynamics relative to thermodynamics in a main-chain reversible polymer. J Am Chem Soc 2003 125 15302-15303. 

Of the two, classic ones are far less common however, those that do exist are frequently found at the loop end of 3-hairpinsJ81 Inverse y-tums generally do not lead to peptide chain reversal and are frequently situated at either the end of a-helices or within strands of p-sheets or adjacent to certain loop motifs. 91 They are generally well conserved during evolution and some are found at key positions within proteins. 

Turns are segments between secondary structural elements and are defined as sites in a polypeptide structure where the peptidic chain reverses its overall... 

One difficulty in characterizing the CD for a typical p turn has been the lack of well characterized model compounds and the variability of the p turn structure in terms of allowable backbone dihedral angles. Early classification schemes described three general classes of p turns, termed types I, n, and III (see Table I). Type III turns resemble a single turn of a 3jo helix (see below), while types I and II are the most common structures for chain reversal found in globular proteins. Types I and IT have also been identified, with the prime symbols indicating an approximate 180° rotation of the peptide group of the i+lth residue. 

P. J. Milburn, Y. Konishi, Y. C. Meinwald, and H. A. Scheraga, J. Am. Chem. Soc., 109, 4486 (1987). Erratum ibid., 109, 8123 (1987). Chain Reversals in Model Peptides Studies of Cystine-Containing Cyclic Peptides. I. Conformational Free Energies of Cyclization of Hexapeptides of Sequence Ac-Cys-X-Pro-Gly-Y-Cys-NHMe. 

Our next examples concern the characterization of /J-turns, which are structural elements that permit polypeptide chain reversals in proteins [65]. Tight turns in proteins and peptides, involving two residues as folding nuclei, have been widely investigated [66-69]. We have applied our GA technique for structure solution of the peptides Piv-LPro-Gly-NHMe and Piv-LPro-y-Abu-NHMe from powder diffraction data, in order to explore the structural properties of these materials (particularly with regard to the formation of /J-turns). 

Reversal of chain direction occurs by well-defined turns of the 310 or fi-type. A single turn of the 310 helix in globular proteins is frequently used to give rise to a chain reversal. It is stabilized by a main-chain 1 -4 hydrogen bond. A similar interaction between four consecutive amino acids is also observed in a structurally related series of so-called yS-turns [5911- Depending on the torsion angles at residues 2 and 3, three different types of fi-t urns, I, II, III, have been identified.1 Figure 19.8 shows only types I and II, type III is very similar to one turn of a 310 helix. 

New approaches based on the introduction of reactive species into reaction mixtures that tend to cap the growing chains reversibly allow, in many cases, production of well-defined polymers and copolymers with narrow polydispersi-ties. Up to few years ago, such a possibility was unobtainable by a classical free radical process. The proposed principle of control of macroradical reactivity is very interesting conceptually, and represents a very powerful tool to prepare block copolymers with well-controlled structures. However, it is clear that the true living character as demonstrated by some anionic polymerizations is still not obtained and much more work needs to be done to understand and control this new process better. 

According to the recommendation of Ham [87], the calculation can be improved by the use of the principle of chain reversibility as treated in Chap, ter 5, Sect. 5.5. Let us consider the copolymerization of two monomers, 1 and 2, in the presence of the transfer agent 3. According to the reversibility principle, the probability of termination of some sequence order by the transfer agent and initiation of new chain growth is equal to the probability of occurrence of all partial reactions in reverse order... 

Ham s method of calculating kjkp is certainly interesting. So far, however, the principle of chain reversibility is not supported by any rational justification, and the calculation of the probability P is subject to some error the error in a product of three P values, is correspondingly larger. 

Chain reverse transcription-polymerase chain reaction (RT-PCR) as a sensitive... 

A number of important conclusions were drawn from this study, as follows. Electrochemical reversibility in electroactive self-assembled monolayers depends upon concentration and polarity of a covalently attached redox probe. Reversible surface electrochemistry is observed for the well-diluted ferrocenyl ester. However, reversibility decreases with steric congestion of redox probe because higher redox probe concentrations lead to disorder due to cross-sectional mismatch of the redox probe and the alkyl chain. Reversibility also decreases with a nonpolar redox probe the alkylferrocene (System 4) yields broad peaks with long tails positive of E°, consistent with kinetic dispersion of the redox probes and their differential solvation in the SAM. 

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