The Effects of Salts on the Stability of the Collagen Helix Under Acidic Conditions

Abstract

SummaryIn the acid pH region, the relative effects of various salts on the thermal stability of the collagen helix are quite different from their effects at neutral pH. The magnitude of the decrease in thermal stability brought about by the salts studied depends mainly on the nature and concentration of the anion and very little on the nature of the cation, whereas at neutral pH the nature of both anions and cations affects the collagen helix stability, the effects of the two ions being roughly additive. The magnitude of the effect of salts at acid pH is much greater than that at neutral pH whereas for a non-ionized denaturant, urea, the magnitudes at both neutral and acid pH are similar. The data are discussed in terms of possible interactions between salts and the positively charged protein with particular consideration of the effects of salts on the pKa of protein carboxyl groups.

The Introduction of Synthetic Cross‐links Into Tropocollagen and Their Effects on the Thermal Stability of the Molecule

“…It has already been shown (Dick & Nordwig 1966, Bianchi et al 1967, Woodlock & Harrap 1968) that further protonation of carboxylate groups brought about by lowering the pH leads to a decrease in the thermal stability of the tropocollagen molecule as compared to that at pH 3.0. Thus reaction between the cationic chrome complex and the carboxylate groups of collagen, whether resulting in a cross-link or not, would lead to an increase in the net positive charge on the collagen molecule.…”

“…Thus reaction between the cationic chrome complex and the carboxylate groups of collagen, whether resulting in a cross-link or not, would lead to an increase in the net positive charge on the collagen molecule. It has already been shown (Dick & Nordwig 1966, Bianchi et al 1967, Woodlock & Harrap 1968) that further protonation of carboxylate groups brought about by lowering the pH leads to a decrease in the thermal stability of the tropocollagen molecule as compared to that at pH 3.0. One might expect that binding of the cationic chrome complex would produce a sirnilar effect and therefore that the depression in thermal stability of the molecule should correlate with its uptake in chrome.…”

“…However, the reaction was carried out in the absence of added salt whereas the T, measurements were made in the presence of 0.1 M KCI. which, at pH 3.0, depresses Tm by about 5" C (Woodlock & Harrap 1968). these cases is reduced because free carboxyl groups which would normally have been available to complete a cross-link are more likely to be masked by their own monofunctional reaction with the cationic chrome complex at such high concentrations of the latter.…”

“…The transition temperature (Tm) was arbitrarily taken as the average value of the temperatures at which the helix to random coil transition was one quarter and threequarters completed (Woodlock & Harrap 1968). The temperatures of solutions were controlled by means of jacketed tubes, one decimeter in length, connected to a circulating water bath.…”

“…Heating curves were plotted by raising the temperature of the solution at a rate of 1" C/5 min. The transition temperature (Tm) was arbitrarily taken as the average value of the temperatures at which the helix to random coil transition was one quarter and threequarters completed (Woodlock & Harrap 1968). Values of T, were reproducible to -: 0.01" C. The solvent used was dilute HCl, pH 3.0,-0.1 M KCI.…”