Article Information
PubMed
Related Articles
- Thermal Stability of Collagen Fibers in Ethylene GlycolThermal Stability of Collagen Fibers in Ethylene Glycol
Biophysical Journal, Volume 80, Issue 3, 1 March 2001, Pages 1480-1486
C.A. Miles and T.V. Burjanadze
AbstractThe mechanism that renders collagen molecules more stable when precipitated as fibers than the same molecules in solution is controversial. According to the polymer-melting mechanism the presence of a solvent depresses the melting point of the polymer due to a thermodynamic mechanism resembling the depression of the freezing point of a solvent due to the presence of a solute. On the other hand, according to the polymer-in-a-box mechanism, the change in configurational entropy of the collagen molecule on denaturation is reduced by its confinement by surrounding molecules in the fiber. Both mechanisms predict an approximately linear increase in the reciprocal of the denaturation temperature with the volume fraction () of solvent, but the polymer-melting mechanism predicts that the slope is inversely proportional to the molecular mass of the solvent (), whereas the polymer-in-a-box mechanism predicts a slope that is independent of . Differential scanning calorimetry was used to measure the denaturation temperature of collagen in different concentrations of ethylene glycol (=62) and the slope found to be (7.29±0.37)×10K, compared with (7.31±0.42)×10K for water (=18). This behavior was consistent with the polymer-in-a-box mechanism but conflicts with the polymer-melting mechanism. Calorimetry showed that the enthalpy of denaturation of collagen fibers in ethylene glycol was high, varied only slowly within the glycol volume fraction range 0.2 to 1, and fell rapidly at low . That this was caused by the disruption of a network of hydrogen-bonded glycol molecules surrounding the collagen is the most likely explanation.
Abstract | Full Text | PDF (168 kb) - Equilibrium Structure and Folding of a Helix-Forming Peptide...Equilibrium Structure and Folding of a Helix-Forming Peptide: Circular Dichroism Measurements and Replica-Exchange Molecular Dynamics Simulations
Biophysical Journal, Volume 87, Issue 6, 1 December 2004, Pages 3786-3798
Gouri S. Jas and Krzysztof Kuczera
AbstractWe have performed experimental measurements and computer simulations of the equilibrium structure and folding of a 21-residue -helical heteropeptide. Far ultraviolet circular dichroism spectroscopy is used to identify the presence of helical structure and to measure the thermal unfolding curve. The observed melting temperature is 296K, with a folding enthalpy of −11.6kcal/mol and entropy of −39.6cal/(mol K). Our simulations involve 45ns of replica-exchange molecular dynamics of the peptide, using eight replicas at temperatures between 280 and 450K, and the program CHARMM with a continuum solvent model. In a 30-ns simulation started from a helical structure, conformational equilibrium at all temperatures was reached after 15ns. This simulation was used to calculate the peptide melting curve, predicting a folding transition with a melting temperature in the 330–350K range, enthalpy change of −10kcal/mol, and entropy change of −30cal/(mol K). The simulation results were also used to analyze the peptide structural fluctuations and the free-energy surface of helix unfolding. In a separate 15-ns replica-exchange molecular dynamics simulation started from the extended structure, the helical conformation was first attained after ∼2.8ns, and equilibrium was reached after 10ns of simulation. These results showed a sequential folding process with a systematic increase in the number of hydrogen bonds until the helical state is reached, and confirmed that the -helical state is the global free-energy minimum for the peptide at low temperatures.
Abstract | Full Text | PDF (534 kb) - Conformational Changes in Single-Strand DNA as a Function of...Conformational Changes in Single-Strand DNA as a Function of Temperature by SANS
Biophysical Journal, Volume 90, Issue 2, 15 January 2006, Pages 544-551
J. Zhou, S.K. Gregurick, S. Krueger and F.P. Schwarz
AbstractSmall-angle neutron scattering (SANS) measurements were performed on a solution of single-strand DNA, 5′-ATGCTGATGC-3′, in sodium phosphate buffer solution at 10°C temperature increments from 25°C to 80°C. Cylindrical, helical, and random coil shape models were fitted to the SANS measurements at each temperature. All the shapes exhibited an expansion in the diameter direction causing a slightly shortened pitch from 25°C to 43°C, an expansion in the pitch direction with a slight decrease in the diameter from 43°C to 53°C, and finally a dramatic increase in the pitch and diameter from 53°C to 80°C. Differential scanning calorimeter scans of the sequence in solution exhibited a reversible two-state transition profile with a transition temperature of 47.5±0.5°C, the midpoint of the conformational changes observed in the SANS measurements, and a calorimetric transition enthalpy of 60±3kJ mol that indicates a broad transition as is observed in the SANS measurements. A transition temperature of 47±1°C was also obtained from ultraviolet optical density measurements of strand melting scans of the single-strand DNA. This transition corresponds to unstacking of the bases of the sequence and is responsible for the thermodynamic discrepancy between its binding stability to its complementary sequence determined directly at ambient temperatures and determined from extrapolated values of the melting of the duplex at high temperature.
Abstract | Full Text | PDF (211 kb) - …more
Copyright © 1996 The Biophysical Society. All rights reserved.
Biophysical Journal, Volume 71, Issue 4, 1994-2001, 1 October 1996
doi:10.1016/S0006-3495(96)79397-9
Research Article
Thermodynamic effects of mutations on the denaturation of T4 lysozyme
J.H. Carra, E.C. Murphy and P.L. Privalov
Abstract
We investigated the folding of substantially destabilized mutant forms of T4 lysozyme using differential scanning calorimetry and circular dichroism measurements. Three mutations in an alpha-helix in the protein's N-terminal region, the alanine insertion mutations S44[A] and K48[A], and the substitution A42K had previously been observed to result in unexpectedly low apparent enthalpy changes of melting, compared to a pseudo-wild-type reference protein. The pseudo-wild-type reference protein thermally unfolds in an essentially two-state manner. However, we found that the unfolding of the three mutant proteins has reduced cooperativity, which partially explains their lower apparent enthalpy changes. A three-state unfolding model including a discrete intermediate is necessary to describe the melting of the mutant proteins. The reduction in cooperativity must be considered for accurate calculation of the energy changes of folding. Unfolding in two stages reflects the underlying two-subdomain structure of the lysozyme protein family.
