Which molecule has the lowest transition temperature (Tm)?
Transition temperature dependence reflects chain packing efficiency influenced by length unsaturation. DOPC dioleoyl phosphatidylcholine contains two eighteen carbon chains each with cis delta nine double bond bending chain thirty degrees creating double kinks and large cross sectional volume reducing van der Waals contacts. Its Tm about minus seventeen to minus twenty Celsius lowest among common PCs. POPC one palmitoyl saturated sixteen plus one oleoyl unsaturated Tm minus two, DPPC dipalmitoyl two saturated sixteen packs tightly all trans Tm forty one due to high cooperative enthalpy, DMPC dimyristoyl two saturated fourteen shorter fewer contacts Tm twenty four despite saturation. Therefore degree of unsaturation dominates over length lowering Tm within this series. Psychrophilic bacteria enrich DOPC like lipids maintaining fluidity at zero degrees. Differential scanning calorimetry endotherm width also broadens with unsaturation. This ranking underpins liposome formulation for temperature sensitive drug delivery and calibration fluorescence anisotropy probes DPH TMA DPH used measuring fluidity in laboratory assessments and examination questions about membrane biophysics quantitative aspects and thermodynamic principles.
Ref: Lewis et al., Biochemistry 1987, Phosphatidylcholine Tm table DOPC lowest.