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The helix–coil transformation for tropocollagen solutions and its relationship to transformations involving the crystalline form of the protein
Author(s) -
Bianchi Estella,
Conio Giuseppina,
Ciferri Alberto
Publication year - 1966
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.1966.360040902
Subject(s) - chemistry , hofmeister series , salt (chemistry) , crystallography , phase diagram , random coil , phase (matter) , helix (gastropod) , organic chemistry , circular dichroism , ecology , snail , biology
Collagen molecules can exist, in the aggregate crystalline state (C) and in solutions, as tropocollagen helices (H) or random coils (RC). We have compared the role of temperature, salt type, and salt concentration, C 8 , on the transformations: C → RC (I), H → RC (II), and H → C (III) under near‐isoelectric conditions. Processes I and II occur in the direction from ordered to disordered form on increasing temperature, while process III occurs in the direction crystal → dissolved helices on lowering temperature. The order of both anions and cations for increasing the stability of the dissolved random coiled form is similar for processes I and II, but the order of the anions for increasing the stability of the dissolved helices, according to process III, is reversed with respect to the Hofmeister series. T I and T II are greatly and continuously depressed by some salts on increasing C 8 while, for other salts, they are at first slightly depressed and then raised. T III , instead, is at first raised and then lowered on increasing C 8 . For a given salt the field of stability of each form is determined, on a pseudo‐phase diagram, by the relative positions of the T I versus C 8 , the T II versus C 8 , and the T III versus C 8 , curves. The similarity of the effect of temperature, salt type, and concentration on processes I and II supports the contention that both processes are controlled by the interaction of the random coiled form with the salt solution. The phenomenon of crystallization by increasing temperature is complicated by large time effects and, often by lack of reversibility. Assuming, nevertheless, that there is an underlying equilibrium process, two possible interpretations are proposed. The peculiar trend of the T III versus C 8 curve is justified on the basis that it reflects the usual change from salting in to salting out on increasing C 8 , allowing for the fact that the dissolved phase is stable in the low‐temperature range. The reversal of the order of anions for process III can be most simply justified in terms of the slight cat ionic character of the tropocollagen units. Accordingly, the dissolved helices will be more; stable of the crystalline form depending upon the occurrence of a net charge different from zero on the protein, as well as upon low ionic strength and low temperature. Minute alterations of KCl concentrations are able to cause isothermal precipitation of fibers and probably constitute a controlling factor for the fibrogenesis in vivo .