The Helix-Coil Transition in Solutions of Polyglutamic Acid

“…The peptide could form monomeric ␣-helices, which subsequently aggregate to structures with a lower helix content. Poly(Glu) is known to aggregate slowly below pH 4.6, and our data were collected at pH 3.3 (21,22). Aggregation is unlikely, however, for the following reasons.…”

Section: Resultsmentioning

confidence: 98%

The α-helix folds on the millisecond time scale

Clarke

Doig²,

Stapley³

et al. 1999

Proc. Natl. Acad. Sci. U.S.A.

115

122

View full text Add to dashboard Cite

It has long been believed that nucleation of the ␣-helix is a very fast reaction, occurring in around 10 ؊7 s. We show here that helix nucleation, in fact, takes place on the millisecond time scale. The rate of ␣-helix nucleation in two polyalanine-based peptides and in lysine and glutamic acid homopolymers was measured directly by stopped-f low deep UV CD with synchrotron radiation as the light source. Synchrotron radiation CD gives far superior signal to noise than a conventional instrument. The 16-aa AK peptide folds with first-order kinetics and a rate constant of 15 s ؊1 at 0°C. The rate-determining step is presumably the initiation of a new helix, which occurs at least 10 5 times slower than expected. Helix folding occurs in at least two steps on the millisecond time scale for the longer peptides, with a transient overshoot of helix content significantly greater than at equilibrium, similar to that seen in the folding of several proteins. We suggest that the overshoot is caused by the formation of a single long helix followed by its breakage into the two or more helices present at equilibrium.If we are to clearly understand protein folding, it is essential to understand the folding of the major substructures, the ␣-helix and ␤-sheet. Relaxation times for the helix͞coil transition of Glu and Lys homopolymers previously have been measured by electric field jump (1, 2), temperature jump (3-5), and resonant ultrasound methods (6-8). Temperature jump, IR spectroscopy (9), and N-terminal reporter group fluorescence (10) also have been applied to measure the kinetics of unfolding of a 21-residue poly(Ala)-based helical peptide.There are two microscopic rates in helix folding (11). First, there is the fast propagation of an existing helix by the addition of a single residue to the end of a helix. The rate of initiation of a new helix, presumably by the formation of a single turn, stabilized by one i, iϩ4 hydrogen bond, will be slow because it requires the entropically unlikely event of the simultaneous restriction of three successive residues. Previous results have been used to derive a rate for extension of helices by a single turn of 1 ϫ 10 7 to 7 ϫ 10 10 ⅐s Ϫ1and to infer very fast initiation rates for the coil-to-helix transition.Here we directly measure the rate of nucleation of ␣-helices from the denatured state. Helix formation in AK16 (sequence Ac-YGAAKAAAAKAAAAKA-NH 2 ) was initiated by a 10-fold dilution from 5 M GuHCl and in AQ28 (sequence Ac-A(QAAAA) 5 QGY-NH 2 ) by a 20-fold dilution from 6 M GuHCl. We also studied poly(L-lysine) and poly(L-glutamic acid). The 3-kDa poly(Lys) sample consisted of polymers in the molecular mass range of 1.5 to 4.5 kDa; 5-kDa poly(Lys) ranged from 1.6 to 10 kDa; 4.4-kDa poly(Glu) ranged from 3 to 14 kDa; 7-kDa poly(Glu) ranged from 3 to 22 kDa; and 20-kDa poly(Glu) ranged from 6 to 40 kDa. These polypeptides form ␣-helices when neutral and random coil when charged. Initiation of helix folding therefore was performed by a pH jump, from 8.0 to 11.5 for poly(Lys) an...

show abstract

“…Thus, we have Gei()/N = 2.3 RT {pH + log [(1 -c)/a] -pK0} dcx (10) where = Z/N, N being the total number of ionizable groups in the molecule, or the degree of ionization. Equation 10 means that the electrostatic free energy of a polyacid having the degree of ionization Z G1(x)/N, is given by the area under the graph of pH + [log (1 -c)/c] -pK0 versus Alternatively, AGe1 can be expressed by L\Gei NAe/Ib (11) in terms of the electrostatic potential at the place where H originally existed, Therefore, Gei(X)/N can also be calculated from Gei()IN RT 42dOC (12) where (13) Equation 12 means that the electrostatic free energy of a macro-ion can be calculated by charging up the molecule from the uncharged state to the …”

Section: K0 = [-Coo -](H)/[-cooh]mentioning

confidence: 99%

Potentiometric titration and conformation of synthetic and natural polyelectrolytes

Nagasawa¹

1971

Pure and Applied Chemistry

View full text Add to dashboard Cite

From the analysis of the potentiometric titration data of polyelectrolytes, we can obtain the electrostatic free energy of the macromolecules. Therefore, potentiometric titration is a useful experimental procedure to study the conformation and conformational change of macromolecules. A few examples of the application of potentiometric titration to the studies on polypeptides, protein and DNA are discussed. When the potentiometric titration method is not applicable, the numerical solution of the Poisson-Boltzmann equation without use of the Debye-Hflckel approximation may be employed to estimate the electrostatic potential at the surface of macromolecules. To confirm the reliability in the values thus estimated without experiments and in the values of the electrostatic free energy calculated from the surface potential, the calculated potentiometric titration data are compared with experimental data of various polyelectrolytes and proteins. The relationship between potentiometric titration data and conformations of stereoregular polyelectrolytes is also discussed.

show abstract

The Helix-Coil Transition in Solutions of Polyglutamic Acid

Cited by 299 publications

References 0 publications

Applicability of broken-rodlike chain model to conformational analysis of polypeptide chain

Applicability of broken-rodlike chain model to conformational analysis of polypeptide chain

The α-helix folds on the millisecond time scale

Potentiometric titration and conformation of synthetic and natural polyelectrolytes

Contact Info

Product

Resources

About