I fiddled with my keyboard to try and draw the structures,
(CS, did I get it right?) No, the computer has removed the space between 2 bonds, but you get the idea.
Frustrating, I spent an hour on it!!
I think it is the conversion of NAC, an amino acid into its amide
The carboxylic acid group -COOH in NAC is converted to CONH2 in the amide

CH3-C=O
l
NH-CH-C=O
l l
CH2 OH
l
SH

N-acetylcysteine


CH3-C=O
l
NH-CH-C=O
l l
CH2 NH2
l
SH

N-acetylcysteine amide
They are different molecules, and presumably the amide form makes it more fat soluble (lipophilic) so it can cross the BBB

See also

Biomed Chromatogr. 2006 May;20(5):415-22. Links
Separation and quantification of N-acetyl-l-cysteine and N-acetyl-cysteine-amide by HPLC with fluorescence detection.Wu W, Goldstein G, Adams C, Matthews RH, Ercal N.
Department of Chemistry, University of Missouri-Rolla, Rolla, MO 65409, USA.

N-acetyl-l-cysteine (NAC) is a well-known antioxidant that is capable of facilitating glutathione (GSH) biosynthesis and replenishing intracellular GSH under oxidatively challenging circumstances. N-acetyl-cysteine-amide (NACA), the amide form of NAC, is a newly designed and synthesized thiol-containing compound which is believed to be more lipophilic and permeable through cell membranes than NAC. The metabolic and antioxidant effects of these compounds in vitro and in vivo are under investigation. However, an analytical method that can separate and quantify both compounds simultaneously is not yet available, to the best of our knowledge. Because of their structural similarities, the two compounds are difficult to separate using earlier HPLC methods which were designed for NAC quantification. Therefore, the goal of this work was to develop an HPLC method with fluorescence detection for simultaneous quantification of NAC and NACA in biological blood and tissue samples. A gradient HPLC program with fluorescence detection (lambda(ex) = 330 nm, lambda(em) = 376 nm) using N-(1-pyrenyl)maleimide (NPM) as the derivatizing agent was developed. The calibration curves were linear over a concentration range of 25-5000 nm (r(2) > 0.997). The coefficients of variation for within-run precision and between-run precision ranged from 0.67 to 5.23% and for accuracy ranged from 0.98 to 10.54%; the percentage relative recovery ranged from 94.5 to 102.8%. This new method provides satisfactory separation of NAC and NACA, along with other biological thiols, in 20 min with a 5 nm limit of detection (LOD) per 5 microL injection volume.

PMID: 16167305 [PubMed - indexed for MEDLINE]




Ron