Kinetic and mechanistic studies on the interaction of glycyl-L-alanine,glycyl-L-asparagine,and glycyl-L-tyrosine with hydroxopentaaquarhodium(III) ion

The kinetics of the interaction of three glycine-containing dipeptides,namely,glycyl-l-alanine (l1-l′h),glycyl-l-asparagine (l 2-l′h),and glycyl-l-tyrosine (l3-l′h) with [rh (h2o)5 oh ]2+ has been studied spectrophotometrically in aqueous medium as a function of the molar concentration of [ rh (h2o)5oh]2+,[dipeptide],ph,and temperature at constant ionic strength. reactions were studied at ph 4.3,where the substrate complex exists predominantly as the hydroxopentaaqua species and dipeptides as the zwitterion. the reaction has been found to proceed via two parallel paths: both processes are ligand dependent. the rate constant for the processes are k1 ∼ 10-3 s-1 and k2 10-5s-1. the activation parameters for both the steps were evaluated using eyring's equation. the low δ h 1 ≠ and large negative value of δ s1 ≠ as well as δ h2 ≠ and δ s2 ≠ indicate an associative mode of activation for both the aqua ligand substitution processes for both the parallel paths. the product of the reaction has been characterized by ir and esi-mass spectroscopic analyses. © 2013 biplab k. bera et al.