The human immunodeficiency virus (HIV) is "enveloped" by a membrane, and infection of a host cell begins with fusion between viral and target cell membranes. Fusion is catalyzed by the HIV gp41 protein which contains a functionally critical ~20-residue apolar "fusion peptide" (HFP) that associates with target cell membranes. In this study, chemically synthesized HFPs were associated with host-cell-like membranes and had "scatter-uniform" labeling (SUL), that is, only one residue of each amino acid type was U-13 C, 15 N labeled. For the first sixteen HFP residues, an unambiguous 13 C chemical shift assignment was derived from 2D 13 C/ 13 C correlation spectra with short mixing times, and the shifts were consistent with continuous β-strand conformation. 13 C-13 C contacts between residues on adjacent strands were derived from correlation spectra with long mixing times and suggested close proximity of the following residues: Ala-6/Gly-10, Ala-6/Phe-11, and Ile-4/Gly-13. Specific antiparallel β-strand registries were further tested using a set of HFPs that were 13 CO-labeled at Ala-14 and 15 N-labeled at either Val-2, Gly-3, Ile-4, or Gly-5. The solid-state NMR data were fit with 50-60% population of antiparallel HFP with either Ala-14/Gly-3 or Ala-14/Ile-4 registries and 40-50% population of structures not specified by the NMR experiments. The first two registries correlated with intermolecular hydrogen bonding of 15-16 apolar N-terminal residues and this hydrogen-bonding pattern would be consistent with a predominant location of these residues in the hydrophobic membrane interior. To our knowledge, these results provide the first residue-specific structural models for membrane-associated HFP in its β-strand conformation.Correspondence to: David P. Weliky, weliky@chemistry.msu.edu. Supporting Information Available: 2D PDSD spectra of HFP-F at 10 and 500 ms exchange times; 1D slices for the 2D PDSD spectra of HFP-C and HFP-D; REDOR spectra at 24 and 32 ms dephasing times for HFP-I and HFP-K samples prepared with 25 µM initial HFP concentration; derivation of (ΔS/S 0 ) cor from (ΔS/S 0 ) exp and σ cor from σ exp ; comparison of (ΔS/S 0 ) exp with (ΔS/S 0 ) cor for the HFP-H, I, J, and K samples; model geometries for the HFP-H, HFP-J, and HFP-K samples; and (ΔS/S 0 ) sim for all models for the HFP-H, HFP-I, HFP-J, and HFP-K samples. This material is available free of charge via the Internet at