In order to examine the effectiveness of reduced glutathione (GSH) attack mechanism for cerebral retention of ^9^9^mTcO^3^+- or ^9^9^mTcN^2^+- labeled diaminodithiols (N~2S~2), an ab initio MO study was performed on a model system in which N,N'-di(mercaptoethyl) ethylendiamine (DTEN) was taken as a model compound for diaminodithiols, and CH~3S^- anion as the mimic of GSH. The calculation results show that CH~3S^- anion can coordinate to the ^9^9^mTc of ^9^9^mTcO-DTEN, converting the latter to an anionic species [^9^9^mTcO-DTEN-SCH~3]^- that can not penetrate the intact brain blood barrier (BBB). Similar reaction does not occur in the case of ^9^9^mTcN-DTEN. Thus the retention of ^9^9^mTcN-N~2S~2 complex by GSH attack can not be expected. Th effectiveness of GSH attack mechanism for the cerebral retention of ^9^9^mTcO-N~2S~2 is sensitive to the substitutions for protons on the N~2S~2 backbone. The rule governing the deprotonation from the amino groups in a N~2S~2 ligand when that ligand coordinates to TcO^2^+ core is also discussed based on ab initio MO calculations.

Key words: MERCAPTAN, AB INITIO CALCULATION, MOLECULAR ORBITAL THEORY, TECHNETIUM, RADIONUCLIDE RETENTION