P-Sheet formation

Figure 4.9 Model for the hierarchical selforganization of A-l. (a) E forms tapes comprising two covalently linked single parallel P-sheet domains with opposite orientation, (b) F-l aggregate into more flexible and less stable single antiparallel P-sheet tapes, (c) Tapes obtained by parallel P-sheet formation from A-D are polar...

Another interesting feature of this extended p-sheet model can be seen in Figure 3b, where we show the effect of temperature on the CD spectrum of Peptide 5. Curiously, when the sample temperature is decreased (to 5 C), the spectrum takes on more of a "random coil character (only 20% p-sheet) but, when the temperature is increased to 85 C, the spectrum exhibits significantly more P-sheet character (38% p-sheet). In other words, high temperatures induce structure and cold temperatures reduce structure. We believe that this represents an excellent example of cold denaturation (19), and it suggests that the thermodynamics of p-sheet formation may be more complex than currently appreciated. 

However, it can be difficult to couple to the hindered Hmb-protected amino acids so in cases where the peptide chain contains Ser or Thr, oxazolidine dipeptide derivatives 3 have been employed (Scheme 7). These pseudoproline derivatives induce a turn in the peptide chain disrupting P-sheet formation. 

Kemp, D.S. and Bowen, B.R. vl988) Synthesis of peptide-functionalized diacylaminoepindolidiones as templates for p-sheet formation. Tet. Lett. 29 5077-5080. 

K t breaks down gel state t induces p-sheet formation... 

Fig. 2 Supramolecular natural polymeric hydrogels discussed in this chapter, (a) Chemical structure of the most-repeated sequence in collagen, forming the a-chain that folds in a three-stranded superhelix [135]. These superhelices bundle to fram the collagen fiber, (b) Representative chemical structure of fibroin and the antiparallel p-sheet formation connected by hydrophilic linkers, (c) Chemical structure of alginic acid, cross-linked by calcium ions (highlighted), (d) Left Top view of two a-helixes of keratin forming a coiled coll by hydrophobic interactions. Right Overview of subsequent formation of the fibril. The left part is adapted from [57] with permission of The Royal Society of Chemistry...

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