The conformational change and the identification
of the mechanical clamp for proteins can be derived by looking at the unfolding scenario. A
unfolding scenario shows distance du at which a given native contact is broken for the last
time. Contacts in the scenario diagrams are identified by the sequential distance |j - i|. A
contact is said to be broken if the distance between amino acids i and j exceeds 1.5σij. An
accumulation of many contacts unfolding events at a value of du indicates emergence of a
force peak. In such figure the y-axis shows the sequential separation between two amino
acids that make a native contact. The x-axis indicates at which displacement of the pulling
tip the contact is broken for the last time. Notice that thermal fluctuations may reinstate
a broken contact temporarily and that’s why we seek the last, and thus definitive, rupture
event.
As an example we show the unfolding diagram for the first chain of protein 1c4p. This protein contains an α helix (196-210), denoted as I, and eight β strands, denoted as A through H, as labelled consecutively from the N to C termini. These β strands form three β sheets: a, b, and c. The a sheet comprises four strands (A, B, G, E), b sheet two ( C, F), and c-sheet also two ( D, H). These letters are indicated on the unfolding scenario figure and show which secondary structures are involved in a contact that is broken at the distance du. The data symbols marked by asterisks correspond to contacts which do not involve any secondary structures. The remaining symbols are diversified and have a meaning identified by the letter symbols placed next to them. The force peak at about 140 Å involves a near simultaneous rupture of the contacts which relate to the terminal strands A and G: A+G (meaning between A and G), A+I, G+I (i.e. with the helix) as well as D+H, A+B, E+G, D+G, C+G, and A+C. Breaking these contacts results in destroying the a β-sheet. Later on, the remaining contacts, such as C+F in the b sheet are broken and the helix is the last to unravel.