Effect of 4-Fluoroproline in the X-Position on the Stability of the Collagen Triple Helix

Introduction Collagen consists of three polypeptide chains that fold into a triple helix. Each natural chain contains many repeats of the sequence: XaaYaaGly, in which a third of the Xaa and Yaa residues are (2S)-proline (Pro). The pucker of a proline ring can be influenced by electronegative substituents, such as the hydroxyl in the naturally occurring residue, (2S,4R)-4-hydroxyproline (Hyp) [1,2]. This effect is stereoelectronic, as it depends on the configuration and electron-withdrawing ability of the substituent. In particular, the gauche effect exerted by an electron-withdrawing 4R substituent stabilizes the C -exo pucker, and that by a 4S substituent stabilizes the C -endo pucker. The degree of stabilization is likely to be greatest for fluorine, the most electronegative of atoms. The thermal stability of the triple helix is increased by replacement of proline in the Yaa position with Hyp [3] and, to a greater degree, (2S,4R)-4-fluoroproline (Flp) [4]. Molecular modeling of a triple helix of (ProProGly)10 strands has suggested that Pro in the Xaa position prefers to adopt a C -endo pucker, whereas Pro in the Yaa position prefers a C -exo pucker [5]. This pattern has been observed in a crystalline (ProProGly)10 triple helix [6]. The pyrrolidine ring pucker influences the range and distribution of the and main-chain dihedral angles of Pro, and can fix those dihedral angles for optimal packing of the triple helix. Increasing the preference for the desired C -exo conformation in the Yaa position by inclusion of either Hyp or Flp decreases the entropic penalty for triple-helix formation. Likewise, Hyp and Flp increase the preference of the main-chain dihedral angle for the trans ( = 180°) conformation [7]. Because all peptide bonds in collagen are trans, preorganization of by Hyp and Flp decreases the entropic penalty for triple-helix formation.