Equilibrium unfolding of the poly(glutamic acid)20 helix.
The equilibrium structural ensemble of a 20-residue polyglutamic acid peptide (E(20)) was studied with FRET, circular dichroism, and molecular dynamics (MD) simulations. A FRET donor, o-aminobenzamide, and acceptor, 3-nitrotyrosine, were introduced at the N- and C-termini, respectively. Circular dichroism, steady state FRET, and time-resolved FRET measurements were employed to characterize the fraction helix and end-to-end distance under different pH conditions: pH 4 (60% alpha-helix), pH 6 (0% alpha-helix), and pH 9 (0% alpha-helix). At pH 4, the end-to-end distance was measured at 24 A and determined to be considerably less than the 31 A predicted for an alpha-helix of the same length. At pH 6 and 9, the end-to-end distance was measured at > 31 and 39 A respectively, both which are determined to be considerably greater than the 27 A predicted for a freely jointed random coil of the same length. To better understand the physical forces underlying the unusual helix-coil transition in this peptide, three theoretical MD models of E(20) were constructed: (1) a pure alpha-helix, (2) an alpha-helix with equivalent attractive intramolecular contacts, and (3) a weak alpha-helix with termini-weighted intramolecular contacts ("sticky ends"). Using MD simulations, the bent helix structure calculated from Model 3 was found to be the closest in agreement with the experimental data.
Finke
John M
JM
Department of Chemistry, Oakland University, Rochester, MI 48309-4477, USA. finke@oakland.edu
Jennings
Patricia A
PA
Lee
Jennifer C
JC
Onuchic
José N
JN
Winkler
Jay R
JR
D002942:Circular Dichroism; D003198:Computer Simulation; D019277:Entropy; D005454:Fluorescence Polarization; D031541:Fluorescence Resonance Energy Transfer; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008956:Models, Chemical; D008962:Models, Theoretical; D010455:Peptides / Q000737:chemistry*; D011099:Polyglutamic Acid / Q000737:chemistry*; D011108:Polymers / Q000737:chemistry; D017510:Protein Folding; D017433:Protein Structure, Secondary; D013696:Temperature
D010455:Peptides; D011108:Polymers; D011099:Polyglutamic Acid