**2. Materials and Methods**

#### *2.1. Materials*

Hydrogenous DPPC (h-DPPC) and chain-deuterated DPPC (d62-DPPC) were purchased as powder from Avanti Polar Lipids (>99%, Alabaster, AL, USA). Solutions of 1 mg·mL−<sup>1</sup> of h-DPPC, d62-DPPC, and h-DPPC/d62-DPPC (95:5 mol %) mixture, from now on contrast matched DPPC (cm-DPPC), were prepared in chloroform stabilized with ethanol (99.8%; Sigma Aldrich, St. Louis, MO, USA). Ultra-pure water was generated by passing deionized water through a Milli-Q unit (total organic content = 4 ppb; resistivity = 18 mΩ·cm, Millipore, Burlington, MA, USA). D2O (99.9%) was purchased from Sigma Aldrich and used as received.

The experiments were performed in HKM buffer pH = 7.2, whose composition is the following: 25 mM HEPES (4-(2-hydroxyethyl)piperazine-1-ethanesulfonic acid and N-(2-hydroxyethyl)piperazine-N'-(2-ethanesulfonic acid)), 125 mM potassium acetate, 5 mM magnesium acetate, and 1 mM DTT (threo-1,4-dimercapto-2,3-butanediol, DL-dithiothreitol, Cleland's reagent, and DTT). HEPES (in solution, 1 M in H2O, and powder 99.5%), potassium acetate (≥99.0%), magnesium acetate (≥99.0%), and DTT (≥99.0%) were purchased from Sigma Aldrich.
