The stability of the two-bonded collagen triple helix.

In the triple helical structure of collagen proposed by RAMACHANDRAN and co-workers1, 2 (see ref. 2 for earlier references), there are two N H . . . 0 hydrogen bonds for every three residues in each chain. In the modification of the RAMACHANDRANKARTHA structure proposed by RICI~ A N D C R I C K a, the three chains are pushed slightly farther apart from their positions in this two-bonded structure and only one set of hydrogen bonds is formed. The reason adduced for this further separation of the chains was that some of the atoms in neighbouring chains are impossibly close to one another in the two-bonded structure. On the basis of the limiting contact distances 4 which have been found to be in good agreement with a large number of observed polypeptide conformationsS, s, the two-bonded structure has been found to be perfectly permissible 2. However, this is a purely qualitative statement. We have therefore calculated the Van der Waals interaction energy of the two structures. This calculation shows that the two-bonded structure has in fact a lower energy than the one-bonded structure, even with regard to the Van der Waals contribution to the energy, quite apart from the fact that it has one additional N H . . . 0 hydrogen bond for every three residues. The Van der Waals potential functions used for the calculation were those proposed by SCHERAGA and co-workers v and extended by BRANT AND FLORY 8. The numerical constants of the potential, of the form V(r) ---a exp ( b r ) c / r e were evaluated following the method described in ref. 8. The coordinates of the backbone atoms and the two atoms of the side group attached to the c¢ carbon atom (the two hydrogens for glycine and one hydrogen and the fl carbon a tom for the other residues) were taken from ref. 2 for the two-bonded structure (called Structure A in this paper). For the one-bonded structure (Structure B), the coordinates of such a structure worked out at Madras 9, corresponding to a value of 2.91 A for the unit height h (the same as for the two-bonded structure) were employed. This structure is essentially similar to the one due to RICH AND CRICK 3 (Structure B', which however has h ---2.86 A) and the earlier one-bonded structure published from Madras 1°, with h ---2.95 A. The Van der Waals interaction energy was calculated including all neighbours of an a tom which occur in residues separated by up to ± 9 A along the axis of the triple helix.