The helix–coil transformation for tropocollagen solutions and its relationship to transformations involving the crystalline form of the protein

✍ Scribed by Estella Bianchi; Giuseppina Conio; Alberto Ciferri

• Publisher: Wiley (John Wiley & Sons)
• Year: 1966
• Tongue: English
• Weight: 836 KB
• Volume: 4
• Category: Article
• ISSN: 0006-3525
• DOI: 10.1002/bip.1966.360040902

✦ Synopsis

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,, on the transformations: C -RC (I), H + RC (11), and H -* C (111) under near-isoelectric conditions. Processes I and I1 occur in the direction from ordered to disordered form on increasing temperature, while process 111 occurs in the direction crystal -f 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 11, but the order of the anions for increasing the stability of the dissolved helices, according to process 111, is reversed with respect to the Hofmeister series. TI and TI1 are greatly and continuously depressed by some salts on increasing C. while, for other salts, they are a t first slightly depressed and then raised. TIII, instead, is at first raised and then lowered on increasing C,. For a given salt the field of stability of each form is determined, on a pseudo-phase diagram, by the relative positions of the TI versus C,, the 2' 11 versus C,, and the 2' 111 versus C, curves. The similarity of the effect of temperature, salt type, and concentration on processes I and I1 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 TI11 versus C, curve is justified on the hasis that it reflects the usual change from salting in to salting out on increasing C,, allowing for the fact that the dissolved phase is stable in the low-temperature range. The reversal of the or- der of anions for process I11 can be most simply justified in terms of the slight cationic 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 KCI concentrations are able to cause isothermal precipitation of fibers and probably constitute a controlling factor for the fibrogenesis in uivo.