Conformational calculations on the Ala14-Pro27 LC1 segment of rabbit skeletal myosin.

In order to define the conformational characteristics of a singular Ala14-Pro27 segment in myosin LC1, conformational calculations were performed using the Simplex algorithm of Nelder and Mead (Computer J. 7 (1965) 308-313) in the ACME program proposed by Tournarie (J. Appl. Cryst. 6 (1973) 309-346). The (Ala-Pro) n = 1 unit was assigned a given conformation x; the conformation energy was then minimized for n = 1 to n = 7 by adjusting structural parameters (angle values). Similarly, 13 different possible conformations were optimized and compared, showing that a (beta 2R)7 conformation is favored by about 20 kcal per mol over the next most probable conformation (C7R)7. In the beta 2R conformation, the (Ala-Pro)7 segment is a wide helix, 15 A in length and 8.65 A in diameter, while the C7R conformation results in a semi-extended structure of 25 A long, with an approximate diameter of 6 A. These characteristics are in agreement with available experimental data and putative functions of the LC1 N-terminus.

[Conformation of angiotensin II]. / Conformation de l'angiotensine II.

A molecular model is proposed for Angiotensin II which could be close to an active form. This model corresponds to a complexed conformation and agrees with many experimental results. Its conformational energy ranges around the conformations already proposed, so that complexation must highly favour it.

Geometry of the five-membered ring mathematical demonstration of the pseudorotation formulae.

The geometrical relations between the 15 typical parameters (bond lengths and angles, torsion angles) of a five-membered ring are derived for any ring then for a regular one. It is demonstrated that for the case of the 20 symmetrical C2 and CS conformations, only geometrical considerations are needed to obtain the pseudorotation formulae for the torsion angles. However, the puckering intensity as well as the bond angle values cannot be expressed from geometrical constraints alone but would require energetical considerations.

Conformational calculations on gastrin C-terminal tetrapeptide.

Energy optimizations were performed on some typical conformations of the gastrin C-terminal peptide amide NAc-Trp-Met-Asp-Phe-NH2. Two families of lowest energy conformations were found corresponding to: (a) alpha-helical structures; (b) conformations having beta-structure at the level of Trp residue, and C7-structure at the level of Asp residue. The two aromatic rings were folded on the peptide backbone and ca. 5 A distant from each other (centre to centre). The last family, favoured by energy and population probability, can better account for conformational experimental results and biological activity observations.