Polyproline circular dichroism

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. 

Bour and Keiderling (1993) report ah initio simulation of the vibrational circular dichroism of coupled peptides in the amide I and II region. Using the MFP model and the 4-3IG basis set they were able to reproduce the VCD sign pattern and the relative intensities of spectra of proteins in the a-helical, /)-sheet 3io-helical and polyproline II conformations. 

Pioneering work by the Alix laboratory on the secondary structure of human elastin and the solubilized K-elastin, estimated the molecule to be composed of 10% a-helices, 35% P-strands and 55% undefined conformation. These estimations were based on Fourier transform infrared (FTIR), near infrared Fourier transform Raman spectroscopy and circular dichroism (CD) (15). To further investigate the nature of the elasticity, polypeptides of hydrophobic sequences containing exons 3, 7, and 30 of human elastin were analyzed by CD and Classic Raman spectroscopy, revealing polyproline II (PPII) helix secondary structures in both the aqueous and solid phase. Further analysis of exon 30 by FTIR spectroscopy determined that this sequence was characterized by both PPII as well as p-sheets structures (15). The presence of these structures were dependent on temperature, concentration and / or time, where lower temperatures and concentrations favored the PPII structure and higher temperatures and concentrations favored p-sheets (16). 

Figure 7 Circular dichroism spectra of compounds 36-41 in 0.1 mM ethanol. AH compoimds exhibit a negative CD signal at 210-215 nm typical for a polyproline 11 structure in solution. The signal intensity increases with the number of proline residues (36 weakest-41 strongest). (Taken with permisson from Ref. 45.)...

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