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Solid-State NMR Investigation of the Depth of Insertion of Protegrin-1 in Lipid Bilayers Using Paramagnetic Mn²⁺

Jarrod J. Buffy ^*, Teresa Hong , Satoru Yamaguchi ^*, Alan J. Waring , Robert I. Lehrer  and Mei Hong ^*

^* Department of Chemistry, Iowa State University, Ames, Iowa 50011; and Department of Medicine, University of California at Los Angeles, School of Medicine, Los Angeles, California 90095

Correspondence: Address reprint requests to Mei Hong, Dept. of Chemistry, Iowa State University, Ames, IA 50011. Tel.: 515-294-3521; Fax: 515-294-0105; E-mail: mhong{at}iastate.edu.

The depth of insertion of an antimicrobial peptide, protegrin-1 (PG-1), in lipid bilayers is investigated using solid-state NMR. Paramagnetic Mn²⁺ ions bind to the surface of lipid bilayers and induce distance-dependent dipolar relaxation of nuclear spins. By comparing the signal dephasing of the peptide with that of the lipids, whose segmental depths of insertion are known, we determined the depths of several residues of PG-1 in 1,2 dilauryl-sn-glycero-3-phosphotidylcholine (DLPC) bilayers. We found that residues G2 at the N-terminus and F12 at the ß-turn of the peptide reside near the membrane surface, whereas L5 and V16 are embedded in the acyl chain region. The depths increase in the order of G2 < F12 < L5 < V16. These intensity-dephasing results are confirmed by direct measurement of the paramagnetically enhanced ¹³C transverse relaxation rates. The relative depths indicate that PG-1 is tilted from the bilayer normal, which is consistent with independent solid-state NMR measurements of PG-1 orientation in the same lipids (Yamaguchi et al., 2001). They also indicate that PG-1 is fully immersed in the lipid bilayer. However, a quantitative mismatch between the bilayer thickness and PG-1 length suggests a local thinning of the DLPC bilayer by 8–10 Å. The depth sensitivity of this Mn²⁺ dephasing technique is tunable with the Mn²⁺ concentration to focus on different regions of the lipid bilayer.

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