The conformation Conversion of Gramicidin A from Double Helix to Helical Dimer in Phospholipids bilayer by Fluorescence Spectroscopy

 

Ta-Hsien Lin1, Hsien-bin Huang2, H.-A. Wei3, S.-H. Shiao3, B. A. Wallace4, Y. C. Chen5 (陳怡成)

1Department of medical Research & Education, Veterans General Hospital-Taipei

2Institute of Biochemistry, National Yang-Ming University

3Institute of Molecular Biology, National Chung-Cheng University

4Department of Crystallography, Birkbeck College, University of London

5Department of Medical Technology, Tzu-Chi University

 

The conformation of gramicidin A is flexible and can form different motifs either helical dimer (channel) or double helix (nonchannel), depending on its environment. Although the helical dimer is the predominated form in synthetic saturated phospholipids bilayer, the conformation of gramicidin A in lipids vesicle strongly depends on those at initial manner before membrane-incorporated state. In staurated lipids membrane, the conversion of conformation between these two structures is irreversible which only the double helix can becomes into helical dimer. In the present result, the conversion of conformation from double helix to helical dimer in DMPC vesicles was mainly studied using fluorescence spectroscopy. It has been found that the whole process is quite extended and takes more than 24 hours at 60oC. This conformation conversion is also dependent on the temperature and lipids/peptide ratio. Unlike the previous study by size-exclusion HPLC, results show that the process for Trp residues should be dynamic, and the mechanism may be at least involve two steps. The whole process shows a three-state model with a fast conversion rate (with rate constant k1) between the first and second states and a slow rate (with rate constant k2) between the second and third states. The k1 is sensitive to temperature and almost independent of lipids/peptide molar ratio at constant temperature. On the other hand, the k2 becomes fast with the increasing lipids/peptide molar ratio but less sensitive to temperature as compared with the k1. This whole process clearly demonstrates that the complicated interaction between the 4 Trp side chains and lipid vesicles plays an important rule. Evidences seem to support the untwist and reorientation mechanism which the four Trp side chain interacts with lipids strongly and provides the essential element for twisting the gramicidin A to change its conformation from double helix to helical dimer.