Table 2. Fluorescence Parameters Obtained for Arg Variants Investigated without and with POPC Liposomes at Different pH Valuesa.
| variant | noPOPC λmax (nm) | +POPC pH 8 λmax (nm) | +POPC pH 3 λmax (nm) | +POPC pH 3 helicity (mdeg) | fluor. pK/n | CD pK/n | τinsertion | τexit |
|---|---|---|---|---|---|---|---|---|
| R10 | 352.1 | 350.4 | 339.7 | –10.3 | 6.2/1.8 | 5.8/1.8 | 14 ms 0.2 s 5 s | 42 ms 194 ms |
| R14 | 352.1 | 350.7 | 340.3 | –11.9 | 6.0/1.1 | 5.8/2.6 | 9 ms 1.8 s 27 s | 499 ms 2.3 s 14.4 s |
| R15 | 352.4 | 353.4 | 340.3 | –7.5 | 5.9/2.8 | 5.6/2.6 | 60 ms 1.2 s 60 s | 42 ms 236 ms |
| R17 | 351.3 | 347.5 | 340.3 | –8.8 | 5.6/1.2 | 5.5/2.8 | 5 ms 0.06 s 4 s | 114 ms 876 ms 8.9 s |
a
Position of the maximum of fluorescence spectra, λmax; helicity at 222 nm; mid of transition (pK) and cooperativity of transition (n) for the peptides’ partitioning into the membrane as measured by fluorescence changes, and for the coil–helix transitions as measured by CD changes; and characteristic times of insertion into the membrane, τinsertion, and times of exit from the membrane, τexit, are shown. In solution, pHLIP forms an unstructured polymer at high pH (∼8), leading to the so-called state I. The interaction of pHLIP with the lipid bilayer of the POPC membrane at high pH (∼8) corresponds to state II. The transmembrane helical orientation of pHLIP triggered by low pH (3–5) is often called state III.