Polyproline helices stability

Although proline cannot participate in a-helical conformations, polypeptides composed only of proline can adopt a different type of helical conformation. This polyproline helix is not stabilized by hydrogen bonding, but rather by the steric mutual repulsion effects of the prolyl side chains. The polyproline helix is more extended than the a helix, with adjacent residues separated along the axis by 0.31 nm. 

The collagen triple helix has a unique CD spectmm. The spectrum shows a positive peak at 220-225 nm, and a negative peak at 195-200 nm. In contrast, the polyproline II helix has a positive peak at 228 nm, and a negative peak at 206 nm. The polyproline II like poly-4-hydroxyproIine helix has a positive peak at 219 nm and a negative peak at 205 nm (13). The thermal stability of the triple helix can be monitored easily by the CD... 

Additionally, a recent study on the gas-phase conformations of varying lengths of polyproline ions demonstrated that while PPI conformation is maintained in the gas phase, PPII conformation is not. The authors sustain that as the aqueous phase was removed from the PPII-structured polyproline during an electrospray process, the loss of water destabilized the PPII helix. Although it was not clear what conformations were formed from PPII polyproline in the gas phase, a mixture of cis- and frfliis-proline was evident (Coimterman and Clemmer, 2004). This study also clearly demonstrated the critical importance of water in stabilizing the PPII helix. 

Wennemers and coworkers reported that the azido group can be used as a conformation-directing element in 4-azidoproline containing polyprolines (Figure 11.9). In agreement with the above analysis, the polyproline PPII helix is stabilized by (4R)-azidoproline and destabilized by (4S)-azidoproline. Efficient functionalization of such polymers by click chemistry renders azido-containing polyprolines attractive molecular scaffolds. ... 

In contrast, polar media encourages the formation of the all-trans poly(L-pro-line)-n structure, which is an extended left-handed helix having a repeat length of three amino acids and a translation length of 3.12 A per residue. It should be noted that both polyproline structures exist as solvent-stabilized conformations which, unlike the a helix, do not rely on the formation of long-range, interresidue hydrogen bonds for their stability. Thus proline trimers [43] and tetramers [44] crystallized from aqueous solution have been shown to adopt the conformational... 

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