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. 2010 May 20;114(19):6636-41.
doi: 10.1021/jp100082n.

Raman Studies of Solution Polyglycine Conformations

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Free PMC article

Raman Studies of Solution Polyglycine Conformations

Sergei Bykov et al. J Phys Chem B. .
Free PMC article

Abstract

Polyglycine (polygly) is an important model system for understanding the structural preferences of unfolded polypeptides in solution. We utilized UV resonance and visible Raman spectroscopy to investigate the conformational preferences of polygly peptides of different lengths in water containing LiCl and LiClO(4). Lithium salts increase the solubility of polygly. Our study indicates that in solution the conformational ensemble of polygly, as well as central peptide bonds of gly(5) and gly(6), are dominated by the 3(1) extended helix, also known as the polyglycine II conformation (PGII). This preference of the polygly backbone for the PGII conformation in solution is likely a result of favorable interactions between carbonyl dipoles in these extended helices. We found that high concentrations of Li(+) stabilize the PGII conformation in solution, most likely by polarizing the peptide bond carbonyls that makes PGII-stabilizing carbonyl-carbonyl electrostatic interactions more favorable. This ability of Li(+) to stabilize 3(1)-helix conformations in solution gives use to the denaturing ability of lithium salts.

Figures

Figure 1
Figure 1
Two conformations of polygly occur in the solid state: β-sheet-like PGI and the extended 31-helix PGII.
Figure 2
Figure 2
UVRR spectra of polygly. A. Polygly (~ 1 mg/ml) in 1.5 M LiClO4 aqueous solution. B. Gly5 – Gly3 difference spectrum which approximates the spectra of the middle residues of gly5 in solution. C. Solid polygly powder precipitated as mostly PGII. D. Solid polygly precipitated as mostly PGI conformation. E. Gly5 PGI crystalline form. Bands from PGII conformation are marked in green, while PGI conformation bands are in blue.
Figure 3
Figure 3
488 excited Raman spectra of polygly in 9 M LiCl solution, solid polygly in the PGII form, solid polygly in the PGI form, gly5 in the PGI form. The spectra of polygly in solution and solid polygly in the PGII form are essentially identical. The homogeneous FWHM of the CH2 stretch is ~ 12 cm−1 as determined from spectra of gly5 crystals. The bandwidth of the νsCH2 of the solution sample is more then twice that of crystalline gly, indicating solution conformational inhomogeneity. The structure on the top shows the CH2 group in a peptide segment in the polygly chain.
Figure 4
Figure 4
UVRR spectra of gly3 in H2O (left) and D2O (right) at 0.3 M (top) and 9 M (bottom) of Li+. Blue 10°C, red 60°C. H2O and D2O contributions are numerically subtracted. The temperature induced frequency shifts between 10 °C and 60 °C are shown by black arrows. Green arrows indicate Li+ induced band shifts and band narrowing at 10 °C. The spectra are arbitrarily scaled. The molecular structures above schematically show the effects of dehydration and Li+ binding to the carbonyl oxygens on the peptide bond lengths.
Figure 5
Figure 5
Comparison of the Ramachandran plot and calculated energy map [kcal/mol] of the carbonyl-carbonyl dipole interactions for polygly (light areas correspond to the energy minima). βL and βP regions on the Ramachandran plot correspond to 31-helix (left and right-handed) energy map minima. This figure was kindly provided by Dr. Bosco Ho.

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