Determination of secondary structure of normal fibrin from human peripheral blood.

The secondary structure of human fibrin from normal donors and from bovine and suilline plasma was studied by FT-IR spectroscopy and a quantitative analysis of its secondary structure was suggested. For this purpose, a previously experimented spectrum deconvolution procedure was applied to the analysis of conformation-sensitive Amide bands od infrared spectrum. This procedure was applied to Amide I and III analysis of bovine and suilline fibrin, obtained industrially, and to Amide III analysis of human fibrin clots. The analysis of both Amide I and III in the first case was useful in order to test the reliability of the method. We found bovine, suilline and human fibrin to contain about 30% α-helix (Amide I and III components at 1653 cm−1 and 1312 and 1284 cm−1, respectively), 40% β-sheets (Amide I and III components at 1625 cm−1 and 1231 cm−1, respectively) and 30% turns (Amide I and III components at 1696, 1680, 1675 cm−1 and 1249 cm−1, respectively). The good agreement of our quantitative data obtained separately by Amide I and Amide III analysis and consistent with a previous fibrinogen (from commercial sources) study, leads us to believe that the amounts of secondary structures found are accurate. Only one information about β sheet content of fibrin was already reported in literature, but it does not exist any complete analysis of the other secondary structures. Fibrin, being an insoluble protein, has never been structurally analysed by CD or UV methods, which require soluble proteins; on the other hand, X-Ray analysis has never been performed owing to the non-crystalline nature of the fibre network. The increase of β sheets in fibrin with respect to the fibrinogen molecule could be due to the formation of inter-molecular β sheets during the formation of the staggered overlapping fibrillar array and their lateral association. As the increase in β sheets content occurs at the expense of α-helical structures, it could also be suggested that the portion which undergoes the conformational transition involves the solvent-exposed α-helix present in the central fragment E, which is sensitive to thrombin attack, and α-helical portions present in the D distal globular domain.