Synthesis of 2-guanidinyl pyridines and their trypsin inhibition and docking

A range of guanidine-based pyridines, and related compounds, have been prepared (19 examples). These compounds were evaluated in relation to their competitive inhibition of bovine pancreatic trypsin. Results demonstrate that compounds in which the guanidinyl substituent can form an intramolecular hydrogen bond (IMHB) with the pyridinyl nitrogen atom (6a-p) are better trypsin inhibitors than their counterparts (10-13) that are unable to form an IMHB. Among the compounds 6a-p, examples containing a 5-halo substituent were, generally, found to be better trypsin inhibitors. This trend was inversely related to electronegativity, thus, 1-(5-iodopyridin-2-yl)guanidinium ion 6e (K_i = 0.0151 mM) was the optimum inhibitor in the 5-halo series. Amongst the isomeric methyl substituted compounds, 1-(3-methylpyridin-2-yl)guanidinium ion 6h demonstrated optimum levels of trypsin inhibition (K_i = 0.0140 mM). In order to rationalise the measured enzyme inhibition, selected compounds were docked with bovine and human trypsin with a view to understanding active site occupancy and taken together with the K_i values the order of inhibitory ability suggests that the 5-halo 2-guanidinyl pyridine inhibitors form a halogen bond with the catalytically active serine hydroxy group.