Sphingomyelin consists generally of a sphingosine base with an 18-carbon chain and a double bond at position 4, attached to a phosphorylcholine fatty acid. The fatty acid chains are generally long, and, saturated or monounsaturated but can vary. There is no exact structure or molecular weight for this product. In egg SM, the dominant fatty acid is 16:0 (66%) followed by 18:0 (10%); see:
Ramstedt, B., P. Leppimaki, M. Axberg, and P. Slotte. 1999. Analysis of natural and synthetic sphingomyelins using high-performance thin-layer chromatography. Eur. J. Biochem. 266:997–1002.