The claim states that proteins generally contain sulfur but lack phosphorus. This is a fundamental biochemical fact based on the elemental composition of amino acids and standard protein structures; sulfur is present in cysteine and methionine, whereas phosphorus is typically a component of nucleic acids, lipids, and phosphorylated proteins rather than the canonical protein backbone. Because this is a well-established textbook fact of biochemistry, it falls under common knowledge and does not require citation to specific papers.