Differential scanning calorimetry and H-2 nuclear magnetic resonance and Fourier transform infrared spectroscopy studies of the effects of transmembrane ex-helical peptides on the organization of phosphatidylcholine bilayers
C. Pare et al., Differential scanning calorimetry and H-2 nuclear magnetic resonance and Fourier transform infrared spectroscopy studies of the effects of transmembrane ex-helical peptides on the organization of phosphatidylcholine bilayers, BBA-BIOMEMB, 1511(1), 2001, pp. 60-73
We have studied the effects of the incorporation of the a-helical transmemb
rane peptides Ac-K-2-L-24-K-2-amide (L-24) and Ac-K-2-(L-A)(12)-K-2-amide (
(LA)(12)) on the thermotropic phase behavior of 1,2-dipalmitoyl-d(62)-sn-gl
ycero-3-phosphocholine (DPPC-d(62)) and 1-palmitoyl-d(31)-2-oleoyl-sn-glyce
ro-3-phosphocholine (POPC-d(31)) lipid bilayer model membranes by different
ial scanning calorimetry (DSC) and the conformational and orientational ord
er of the phospholipid chains by Fourier transform infrared (FTIR) spectros
copy and H-2 nuclear magnetic resonance (H-2-NMR) spectroscopy, respectivel
y. Our DSC and FTIR spectroscopic studies indicate that the peptides L-24 a
nd (LA)(12) both decrease the temperature and enthalpy of the gel/liquid-cr
ystalline phase transition of DPPC-d(62) bilayers, with (LA)(12) having the
greater effect in this regard. An examination of the frequencies of the CH
2 and CD2 symmetric stretching bands of the infrared spectra of liquid-crys
talline states of the peptide-free and peptide-containing DPPC-d(62) and PO
PC-d(31) samples, and a comparison with the orientational order as measured
by H-2-NMR spectroscopy as well as with the chain order as measured by ele
ctron spin resonance spectroscopy, lead us to conclude that the CH2 (or CD2
) stretching frequencies of lipid hydrocarbon chains are not a reliable mea
sure of chain conformational order in lipid bilayers containing significant
amounts of peptides or other lipophilic inclusions. In contrast, the resul
ts of our H-2-NMR spectroscopic studies present a consistent picture in whi
ch both L-24 and (LA)(12) increased in a similar way the time-averaged orie
ntational order of the lipid chains of their liquid-crystalline lipid bilay
er hosts. The comparison of the effects L-24 and (LA)(12) On phosphatidylch
oline bilayers indicates that the gel-to-liquid-crystalline phase transitio
n appears to be more sensitive to small changes in transmembrane peptide su
rface topology than hydrocarbon carbon chain orientational order in the liq
uid-crystalline state. (C) 2001 Elsevier Science B.V. All rights reserved.