L-Arginine-15N4 ((S)-(+)-Arginine-15N4) hydrochloride, the 15N-labeled variant of L-Arginine hydrochloride ((S)-(+)-Arginine hydrochloride), serves as the nitrogen donor in the synthesis of nitric oxide. This potent vasodilator is crucial, especially during a sickle cell crisis where its levels are notably diminished.