Modern Polymer Spect..

5.4 Polvpeptida 259
Figure 5-2. Antiparallelchain
pleated sheet of
polyi L-alanine). The CH;
group is represented by
the largest circle. (From
[51)
Figure 5-3. Antiparallelchain
rippled sheet of
polyglycine. (From [5])
produce such a regular alternation with other side-chain-residue polypeptides.)
Thus, in principle we can have antiparallel-chain pleated sheet (APPS) (Figure 5-2),
antiparallel-chain rippled sheet (APRS) (Figure 5-3), parallel-chain pleated sheet
(PPS), and parallel-chain rippled sheet (PRS) extended-chain structures. Their
structural parameters are given in Table 5-5 [82].
In describing these structures, and in the vibrational analyses, we assume the
sheets to be infinite in two dimensions and therefore calculate only the in-phase unit
cell modes. Although modes of only a single sheet are treated, the TDC interactions
have been evaluated over a number of sheets (7-11). The effect of finiteness of a
single sheet on just the TDC interactions for the amide I mode has been examined
[83, 841.
Extended-chain regions are a major component of protein structures. They can
occui- in mixed antiparallel and parallel arrangements even within a sheet, the sheets
can be twisted, and complex ‘barrel’ arrangements are also found [2]. It is therefore