Lipids Are Covalently Attached to Rigid Corneocyte Protein Envelopes Existing Predominantly as β-Sheets: A Solid-State Nuclear Magnetic Resonance Study

13C solid-state nuclear magnetic resonance at natural abundance was used to study isolated corneocyte envelopes from porcine stratum corneum. The presence of lipids covalently attached to the protein envelopes was detected by chemical shifts of methylene and methyl groups of the bound lipids. The corneocyte protein envelopes are rigid, as suggested by efficient 1H to13C cross polarization and 13C spin-lattice relaxation studies. The chemical shift of the carbonyl carbons of the protein envelopes supports the prediction that the chemically bound lipid envelope is attached to proteins arranged predominantly in the β-sheet conformation, allowing a dense palisade of ceramide molecules to form a water- impermeable external sheath.