Response of GWALP Transmembrane Peptides to Titration of a Buried Lysine

Designed α-helical peptides such as GWALP23 serve as useful models for probing the influence of polar amino acids within a core transmembrane helical sequence. We incorporated lysine as a guest residue into the membrane-spanning host peptides GWALP23 and closely related Y5GWALP23 (acetyl-GGAL(W5/Y5)LALALAL12AL14ALALW19LAGA-amide). Lysine was introduced at either position 12 or 14 of the host sequences, for which position 12 corresponds to the center. Solid-state NMR spectra of 2H-Ala residues in peptides incorporated into oriented lipid bilayer samples reveal that L14K mutant peptides adopt well-defined orientations in DOPC, DMPC and DLPC. In each lipid membrane, the L14K substitution increases the helix tilt at neutral pH. The L12K substitution, on the other hand, reduces the 2H NMR spectral quality at neutral pH, particularly in the thicker DOPC, suggesting a lack of distinct orientation, as the system struggles to insert a charged lysine into the thicker bilayers. As the positively charged K12 amino group is titrated to higher pH values, nevertheless, the 2H NMR spectral quality improves in DOPC, and the K12 peptides adopt an average orientation nearly matching the one found for both host peptides GWALP23 and Y5GWALP23 (with L12). In similar fashion, titration of K14, in any of the tested lipid bilayer membranes, results in a smaller helix tilt, again much closer to that observed for the L14 peptides without a polar guest residue. Steady-state fluorescence measurements using the Y5GWALP23 series of peptides reveal spectral narrowing and modest blue shifts in Δmax from the W19 reporter, when either K12 or K14 is rendered non-ionized, suggesting a somewhat more hydrophobic environment for the Trp indole ring when the guest Lys side chain is neutral.