Silver cluster interactions with Pterin: Complex structure, binding energies and spectroscopy

Metal nanoclusters (NCs) are widely present today in biosensing, bioimaging, and diagnostics due to their small size, great biocompatibility, and sensitivity to the biomolecular environment. Silver (Ag) NCs often possess intense fluorescence, photostability, and low photobleaching, which is in high demand during the detection of organic molecules. Pterins are small compounds, which are used in medicine as biomarkers of oxidative stress, cardiovascular diseases, neurotransmitter synthesis, inflammation and immune system activation. It is experimentally possible to detect pterin (Ptr) through the adsorption on Ag colloid. We optimized geometries and evaluated the binding energy in Ptr-Ag_n^q complexes (n = 1-6; q = 0, +1, +2) using quantum chemistry methods. Different Ptr atoms were preferential for silver attachment depending on NC charge and size. The highest E_b was obtained for the complexes between the Ptr⁰ and Ag₃²⁺ (-50.8 kcal mol^-1), between Ptr^-1 and Ag₃²⁺ (-64.8 kcal mol^-1), which means that these complexes should be formed preferably in aqueous solutions in acidic and alkaline media, respectively. The colorimetric detection of pterin with silver clusters does not seem to be promising. However, intense S_0→S₁ transitions of Ag₅⁺ complexes look promising for luminescent Ptr detection. SERS detection of pterin is better to be done at pH > 8 since deprotonated pterin Raman undergo more dramatic changes upon addition of Ag than the neutral pterin. The characteristics of absorption and vibrational spectra of silver-pterin should be exploited during biosensor development.