Interaction between proteins and salt solutions. III. Effect of salt type and concentration on the shrinkage temperature

Abstract

SynopsisThe effect of salt type and concentration on the t,ransformation of an oriented crystalline collagen tendon into a crosslinked network under conditions of equilibrium swelling was investigated. Our main observations are the following. The degree of swelling of crystalline tendons increases a t low salt concentration C,, and decreases a t higher Cs for a wide variety of salts. The observation is not reconcilable with swelling taking place in interfibrillar spaces or structural voids. Within the tropocollagen units and at their ends, regions of reduced organization are postulated (as suggested by Bear and by Schmitt) which are able to interact with the diluent in the amorphous-like manner. At least four different factors should be considered in assessing the role of salt and salt concentration on the shrinkage temperature T, under isoelectric conditions. They are:( 1 ) specific effects, ( 2 ) diluent effects, ( 3 ) crosslinking effects, and ( 4 ) nonequilibrium effects. The diluent effects are correlated with the salting-in-salting-out power of the ions which was characterized in Part I of this series. Smaller amounts of diluents are generally available to the tendon when the salt has a higher salting-out power, and this corresponds to higher shrinkage temperatures, other conditions being the same. The crosslinking effect raises T, due to a reduction of the diluent content and, probably for p-benzoquinone and formaldehyde, also to a reduction of the conformational entropy in the molten state. Noneqiiilibrium effects arise from the fact that shrinkage and recrystallization are kinetically hindered when the tendon is highly deswollen in strong salting-out solutions, or when the salt has a crosslinking power. The specific effect is the only effect which is not related to the amount of dihient present in the tendon. Its origin is less clear. For anions such as C1-and SCN-, it is possibly related to an ability of the ion to prevent intersegmental hydrogen bonding and water carbonyl bridges. The competition of several of the above eflects for a given salt soliition makes possible various types of dependence of T, upon Cs: T8 may either continuously decrease or continuously increase with increasing C,, or it may go through a minimum. I n absence of salt,, the cooperative character of the transition at the pH a t which maximum swelling occurs appears extremely reduced. The large swelling maintains the tendon in the elongated state and this simulates a continiioiis decrease of T, on lowering pH. In presence of small quantities of salt, which reduce swelling, the transition is sharp and T, is decreased with pH up to pH 2, when maximum swelling occurs, and then reincreases on further lowering of the pH. The dependence of T, upon Cs is more complex than under isoelectric conditions. There is generally an increase of T, with C, which is equivalent to an increase of the denaturation temperature with C. for helical polyelectrolytes in solution. At higher salt concentrations, however, T. may decrease again, and possibly in...

Transport properties of charged membranes undergoing conformational transitions

“…the L s/L o vs. pH curve) indicates the occurrence of a complete melting transition at such higher temperature. This body of results is in line with results previously obtained [3,4,15] for cross-linked collagen tendons, indicating that at low temperatures the increase in degree of swelling due to Donnan effects may be accompanied by a large decrease of the degree of crystallinity without complete melting or shrinkage. Donnan effects and complete melting (i.e., shrinkage) occur, instead, simultaneously at relatively high temperatures, the exact value of the melting temperature depending upon the chemical composition of the collagen, the degree of cross-linking and the solvent type [15].…”

“…The large increase of swelling which occurs when the pH is lowered between 4 and 2 is primarily associated with the large osmotic forces (Donnan effect) due to the increase of the net cationic charge on the protein. The deswelling observed when pH <2 is attributed to the increasing concentration of the acid once the number of fixed charges no longer increases [4] with decreasing pH. The fact that the degree of swelling is larger at 52 than at 25 ~ particularly between pH 3 and 2, is attributed to the occurrence of a complete melting transition at the higher temperature.…”

“…Using the density of charge data of Table 1, taking [4] X l = 0.5, and fitting the network structure parameter re/Vo to the experimentally observed value of V~IVo at pH =3.5, the dotted line in Fig. 8 was calculated.…”

Electrolytes control flows of water and sucrose through collagen membranes

“…The first difference in behavior can be ascribed to residual amorphous regions, which can be highly swollen at low concentrations. The second difference has already been observed in collagen tendons (Ciferri et aL, 1965). A possible explanation might lie in the cross-link- …”

“…This cooperative phenomenon of melting in the membrane results in large changes in its water content, its physical structure and dimension, and in the frictional interactions between solute, solvent and membrane. In addition to those effects caused by melting, further changes in water content and physical dimension are created by salting in or salting out (depending upon the type of electrolyte used: see Ciferri, Rajagh & Puett, 1965;Veis, 1967) and, at higher electrolyte concentrations, by mechanical stresses developed in the membrane which is clamped at a prescribed area (Gliozzi, Penco, Battezzati & Ciferri, in press). These induced changes in the physical state of the membrane are reflected in more or less abrupt changes in the phenomenological coefficients: the solute permeability cos, the reflection coefficient a, and the filtration coefficient Lp.…”