Orthogonally protected derivative of homocysteic acid. As a Glu substitute, or for introducing a highly acidic moiety, it can be incorporated into various peptide effectors, such as receptor ligands and enzyme inhibitors. The neopentyl sulfonate can be cleaved with HF/m-cresol or by slow hydrolysis in 0.1% TFA in water/acetonitrile. This cysteic acid homolog occurs in nature; homocysteic acid, generated from plasma homocysteine, can damage endothelial tissue.