COLLAGEN STRUCTURES CONSIDERED AS STATES OF AGGREGATION OF A KINETIC UNIT. THE TROPOCOLLAGEN PARTICLE

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Abstract
Evidence is presented in support of the view that the various structural forms of collagen, including the fibrous and segmental long-spacing type, may be composed of very thin, possibly protofibrillar, particles having lengths in the long-spacing range (1500-3000 A, but primarily in the range of 1800-2300 A). The hypothesized particles are called "tropocollagen." Evidence for the existence of tropocollagen is as follows (1) From a common starting solution of ichthyocol dissolved in acid, collagen, fibrous long spacing, or segment long spacing may be precipitated in separate aliquots almost quantitatively. (2) Each of these forms may be redissolved and converted almost quantitatively into either of the forms. (3) Frayed segments of segmental long-spacing (SLS) reveal that they are composed of extremely thin filaments (possibly a few tropocollagen particles thick) having lengths equal to those of the SLS. (4) Time-sequence studies of the formation of fibrous long-spacing (FLS) and SLS reveal that very thin tactoidal particles of the order of 2500 A in length appear for a short time and then disappear as the long-spacing structure is formed. FLS may also be produced by adlineation of individual separate periods. Dense spheroids of the order of 500 A in diameter formed early in the precipitation of FLS and SLS and were considered to consist of clusters of tropocollagen particles co-ordinated around the negative groups of ATP or acid glycoprotein.

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