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Solution Structures of the Prototypical 18 kDa Translocator Protein Ligand, PK 11195, Elucidated with 1H/13C NMR Spectroscopy and Quantum Chemistry
journal contribution
posted on 2012-04-18, 00:00 authored by Yong-Sok Lee, Fabrice
G. Siméon, Emmanuelle Briard, Victor W. PikeEighteen kilodalton translocator protein (TSPO) is an
important
target for drug discovery and for clinical molecular imaging of brain
and peripheral inflammatory processes. PK 11195 [1a;
1-(2-chlorophenyl)-N-methyl-(1-methylpropyl)-3-isoquinoline
carboxamide] is the major prototypical high-affinity ligand for TSPO.
Elucidation of the solution structure of 1a is of interest
for understanding small-molecule ligand interactions with the lipophilic
binding site of TSPO. Dynamic 1H/13C NMR spectroscopy
of 1a revealed four quite stable but interconverting
rotamers, due to amide bond and 2-chlorophenyl group rotation. These
rotamers have been neglected in previous descriptions of the structure
of 1a and of the binding of 1a to TSPO.
Here, we used quantum chemistry at the level of B3LYP/6-311+G(2d,p)
to calculate 13C and 1H chemical shifts for
the rotamers of 1a and for the very weak TSPO ligand, N-desmethyl-PK 11195 (1b).
These data, plus experimental NMR data, were then used to characterize
the structures of rotamers of 1a and 1b in
organic solution. Energy barriers for both the amide bond and 2′-chlorophenyl
group rotation of 1a were determined from dynamic 1H NMR to be similar (ca.17 to 18 kcal/mol), and they compared
well with those calculated at the level of B3LYP/6-31G*. Furthermore,
the computed barrier for Z to E rotation
is considerably lower in 1a (18.7 kcal/mol) than in 1b (25.4 kcal/mol). NMR (NOE) unequivocally demonstrated that
the E rotamer of 1a is the more stable
in solution by about 0.4 kcal/mol. These detailed structural findings
will aid future TSPO ligand design and support the notion that TSPO
prefers to bind ligands as amide E-rotamers.