P-turns

In the absence of skidding, the coefficient of static friction applies at each instant, the portion of the tire that is in contact with the pavement has zero velocity. Rolling tire friction is more of the type discussed in Section XII-2E. If, however, skidding occurs, then since rubber is the softer material, the coefficient of friction as given by Eq. XII-5 is determined mainly by the properties of the rubber used and will be nearly the same for various types of pavement. Actual values of p, turn out to be about unity. 

Abdreh-maschine, /. lathe (Ceram.) finishing machine, -spane, m.p/. turnings, -spindel,/. (Ceram.) finishing machine. 

There are fundamental differences between the behaviors of the two types of transformer. The voltage transformer is shown diagrammatically in Figure 17.6. The system voltage Fp is applied across the primary winding, which has A p turns. The secondary voltage Vs is ... 

However, if A commutes with H according to Eq. III.81, it is possible to determine the coefficients Ct so that P turns out to be also an eigenfunction to A associated with the eigenvalue Xk. This implies OkP = P and, by letting Ok operate on the relation III.92, we hence obtain... 

Figure 5-7. A p-turn that links two segments of antiparallel p sheet. The dotted line indicates the hydrogen bond between the first and fourth amino acids of the four-residue segment Ala-Gly-Asp-Ser.

All X-Pro peptide bonds—where X represents any residue—are synthesized in the trans configuration. However, of the X-Pro bonds of mature proteins, approximately 6% are cis. The cis configuration is particularly common in P-turns. Isomerization from trans to cis is catalyzed by the enzyme proline-CM,tr(2 r-iso-merase (Figure 5-9). 

In an attempt to design the p-turn-peptide-mimics, aspartic acid (an amino acid also known as aspartate) and lysine (an amino acid especially found in gelatin and casein) were attached to each amine group of 1,3-diaminoada-mantane in the form of amide bonds. The term p-turn refers to a peptide chain that forms a tight loop such that the carbonyl oxygen of one residue is hydrogen... 

P Turn A structure in which the polypeptide backbone folds back on itself. Turns are useful for connecting helices and sheets. 

Figure 2.7 (a) The P-bend or p-turn is often found between two stretches of antiparallel p-strands. (b) It is stabilized in part by hydrogen bonding between the C=0 bond and the NH groups of the peptide bonds at the neck of the turn... 

Moreover incorporation of a-methylproline into a peptide to replace proline in a p-turn... 

Binding to lysozyme stabilizes a conformation in site D compatible with the alternative mechanism. (i) Hg of residue D does not interact with residue E but forms a strong hydrogen bond to the mainchain 0 of residue 57, a residue involved with the unusual buried p turn in lysozyme. (ii) The value of 0 between D and E, which was -54 in the initial structure, stabilized at -62 , near the optimum of -60 for stereoelectronic assistance. 

To illustrate protein conformations in a clear (but extremely simplified) way, Richardson diagrams are often used. In these diagrams, a-helices are symbolized by red cylinders or spirals and strands of pleated sheets by green arrows. Less structured areas of the chain, including the p-turns, are shown as sections of gray tubing. 

Secondary structures are regions of the peptide chain with a defined conformation (see p. 68) that are stabilized by H-bonds. In insulin (2), the a-helical areas are predominant, making up 57% of the molecule 6% consists of p-pleated-sheet structures, and 10% of p-turns, while the remainder (27%) cannot be assigned to any of the secondary structures. 

Jiang S, Agoston GE, Chen T, Cabal M-P, Turns E (1995) BF3 Et20-promoted allylation reactions of allyl(cyclopentadienyl)iron(II) dicarbonyl complexes with carbonyl compounds. Organometallics 14 4697 -709... 

If the dominant conformation of the cyclic peptide is already known, the tendency for p-turns can, at least to a certain extent, be exploited to bring the termini of the linear precursor into closer proximity. Correspondingly, the peptide bond in/+ // +2-position usually is a bad choice, whereas the peptide bond i+2li+3 often represents one of the best cyclization sites (Scheme 5). 

In principle every compound with an amino and a carboxy group can be used for such purpose ranging from simple co-amino acids [e.g., 5-aminopentanoic acid (6-aminovaleric acid) (1, n = 3)]1541 or 6-aminohexanoic acid (e-aminocaproic acid) (1, n = 4)]135,57,4 791 and related derivatives of 3-aminobenzoic acid 14801 or more sophisticated structures. A few examples (1-6) are shown in Scheme 28. Numerous cyclic turn mimetics have been developed in the past years and for details on this subject the reader is directed to Vol. E 22c, Section 12. To explore the rigidification introduced by nonnatural amino acids or equivalent structures into cyclic peptides, a careful NMR conformational analysis is required, since frequently the so-called p-turn mimetics do not enable such turns to be established, conversely other secondary structure elements may be induced.14811... 

The next three sections (Sections 7.7.1, 7.7.2, and 7.7.3) cover fluorescence spectroscopy, I15-18 infrared, and circular dichroism, three powerful approaches to characterize the structure and conformational considerations of synthetic peptides. Section 7.7.1 deals with the use of fluorophores and broad aspects of fluorescence spectroscopy to characterize conformational aspects of peptide structure. In a similar manner, Section 7.7.2 covers a broad aspect of the uses of infrared (IR) techniques to study peptide conformations 19-22 Many IR techniques are discussed, as are approaches for the study of specific peptidic structures including amyloid, p-turn, and membrane peptides. Finally, there is a section on circular dichroism (Section 7.7.3) that covers the major issues of concern for peptide synthetic chemists such as the assignments of a-helix, 310-helix, -sheets and P-turns, and polyproline helices 23-25 There is also a brief description of cyclic peptides. 

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