Effect of residue insertion on the stability of polyproline‐I and II structures: Circular dichroism spectroscopic analyses of block‐type oligo‐prolines (Pro)m‐Gly/Ala‐(Pro)n.

The molecular conformation of oligo‐proline peptides composed of two oligo‐proline block sequences and a non‐proline linker residue, designated as (Pro)m‐Gly/Ala‐(Pro)n peptides, was analyzed by circular dichroism (CD) spectroscopy. The CD spectra in water and trifluoroethanol indicated that the two oligo‐proline blocks were separated by an inserted residue independent of polyproline‐II (PP‐II). In addition, the stability of the (Pro)m‐Gly/Ala‐(Pro)n peptides was analyzed using a conformational transition system, during transition from PP‐II to polyproline‐I (PP‐I) in aliphatic alcohols, methanol (MeOH), and 1‐propanol (1‐PrOH). Interestingly, the PP‐II/PP‐I transition was inhibited after a Gly/Ala was inserted at the center of the oligo‐proline; the inhibitory effect of Ala was stronger than that of Gly. When the position of the inserted Ala moved towards the C‐terminal, the (Pro)m‐Gly/Ala‐(Pro)n peptides displayed a PP‐II/PP‐I transition in 1‐PrOH. Our results confirmed that (Pro)m‐Gly/Ala‐(Pro)n peptides prefer to form PP‐II hairpin conformations even in MeOH and 1‐PrOH. Thus, our findings suggest that the insertion of Gly/Ala acts as a stabilizer in PP‐II in proline‐rich peptides. [ABSTRACT FROM AUTHOR]