Molecular Orbital Basis for Yellow Curry Spice Curcumin's Prevention of Alzheimer's Disease

It is demonstrated by using high-level ab initio computations that the yellow curcumin pigment, bis(4-hydroxy-3-methoxyphenyl)-1,6-diene-3,5-dione, in the east Indian root plant turmeric (Curcuma longa) exhibits unique charge and bonding characteristics that facilitate penetration into the blood−brain barrier and binding to amyloid β (Aβ). Alzheimer's disease is caused by Aβ accumulation in the brain cells combined with oxidative stress and inflammation. Consistent with the recent experimental work by Cole and co-workers (Yang, F., et al. J. Biol. Chem. 2004, 280, 5892−5901) that demonstrates curcumin pigment's binding ability to Aβ both in vivo and in vitro, it is shown here that curcumin possesses suitable charge and bonding features to facilitate the binding to Aβ. In addition, curcumin's anti-inflammatory and antioxidant properties are also attributed to electronic and structural features. It is shown that the presence of an enolic center and two phenolic polar groups separated by an essentially hydrophobic bridge of a conjugated network provides both hydrophobic and hydrophilic features to the curcumin pigment, thereby facilitating penetration into the blood−brain barrier through the former property and then binding to Aβ oligomer through the latter property. Both density functional and Møller−Plesset perturbation (MP2) computations have been carried out on the curcumin pigment to obtain fully optimized geometries in the gas phase and aqueous solution and also the atomic charges. Different isomers (keto and enol forms) have been considered to show that the enol form is the most favored and has all of the properties for an ideal antioxidant with also features to penetrate the blood−brain barrier and to bind to Aβ. This is demonstrated with natural bond charges, highest occupied and lowest unoccupied molecular orbitals, dipole moments, and Laplacian plots. The computed ionization potential and electron affinity show that curcumin has a low molecular hardness and thus has a propensity to dissociate its phenolic −OH, and the resulting charge undergoes delocalization throughout the structure, resulting in excitonic features. This feature seems to be also important for its binding capability to human proteins such as human serum albumin and Aβ. Keywords: Curcumin; molecular orbital studies; charges; polar and hydrophobic features