Side-chains configurations in coiled coils revealed by the 5.15-angstrom meridional reflection on hard alpha-keratin X-ray diffraction patterns

The origin of the 5.15-Angstrom meridional reflection on hard alpha-keratin X-ray diffraction patterns is discussed in terms of side-chains conformati ons. We show it to reveal specific configurations of the side chains which are common to all two-stranded alpha-helical coiled coils. Combining litera ture data on crystallised coiled coil pieces and molecular dynamics results with our X-ray diffraction pattern simulations, we propose rules for the a ttribution of chi(1) torsion angles for coiled coils involved in fibres who se structure cannot be resolved at atomic resolution: in a (a b c d e f g) heptad repeat, a and d residues, respectively, adopt mean t and g(+) config urations, whereas statistical rules are given for the other residues. (C) 1 999 Academic Press.