Azido alkanes as convenient reporters for mobility within lipid membranes

A test compound featuring azido and cyano end groups linked by an alkyl spacer, az-(CH₂)₁₁-CN (az11CN), was designed as a probe of mobility of synthetic lipid membrane interiors and mobility of solvents accessed via 2DIR spectral diffusion measurements. The results indicate that the az11CN compound is oriented in lipid membranes such that the azido group is well inside the membrane, while the cyano group resides in the region of moderate polarity around the carbonyl groups of the lipid. The lineshape of the azido moiety transition at 2100 cm⁻¹ shows significant broadening in nonpolar solvents such as hexane. In silico analysis demonstrated that the structural softness of the azido group, intrinsic Fermi resonance at the azido site, and broad wagging, twisting, and rocking vibrational CH₂ bands of the alkane chain result in a large number of Fermi resonances with the fundamental transition at 2100 cm⁻¹, thus providing a smooth and featureless linewidth.