Abstract:
Therllloelynalllics of lllodel 11lel1ll)rane systeills containing 1110nollnsaturatecl I)lloSI)holil)
ids is strongly infllienced l)y the I)osition of the C==C dOlll)le })ond in tIle acyl chain.
The telllI)eratllres of both chain-nlelting (TM) and La -+ HI! (TH) I)hase traIlsitions
are lowered by IIp to 20°C when C==C is Inoved froln positions 6 or 11 to I)osition 9
in an 18-carl)on chain. This work is an attellll)t to ellicidate the uIlderlying Illoleclilar
Illechanisllls reSI)Onsi])le for tllese draillatic tllerillodynaillic changes.
Mixtllres of di-18: 1 l)hoSI)hatidylethanolanline with C==C at l)ositioIlS 6, 9, 11 were
llsed, witll a sI1lall aI1lOlint of I)erdellterated tetradecanol, known to })e a gooel rel)Orter
of the cllain Illoleclilar order. SI)ectral second 11I0I1lents were llsed to Illonitor tIle La -+
HII I)hase transition, which was fOllnd to ])e ])road (2-6°C), with a slight llysteresis
on heatiIlg/cooling. The orientational order I)rofiles were nleasllred 1lSiIlg 2H Illiclear
Illagnetic resonance and changes in these order I)rofiles between La aIld HII I)hases silow
l)oth a local increase in order in the vicinity of the C==C bonds and an o\Terall decrease ill
the average orientational order of the chain as a whole. These Sll])tle changes recluire })oth
high-fidelity SI)ectrosCol)y and a careflll data analysis that takes into aCCOllnt the effects
due to l)artiall1lagnetically-indllced orientational ordering of the l)ilayers. In tIle COIltext
of SOllle recently rel)Orted cross-relaxation 11leaSlirenlents in Silllilar l)llOSI)llolil)iels, 0111'
reslilts sllggest that large-anll)litllde conforlllational changes in the interior of tIle I110del
111eI11])ranes I)lay a 1110re significant role than I)reviollsly thOllght.
