Raju Nandhakumar

Ratiometric Fluorescent Chemosensor for Silver Ion at Physiological pH

Bis-pyrene derivative 1, bearing two pyrene and pyridine groups, was synthesized as a ratiometric fluorescent chemosensor for Ag(+) in aqueous solution. Fluorescent chemosensor 1 displayed a selective ratiometric change with Ag(+), which was attributed to the excimer-monomer emissions of pyrenes. A mechanism for the binding mode was proposed based on fluorescence changes, NMR experiments, and theoretical calculations.

BINOl-Based Chiral Receptors as Fluorescent and Colorimetric Chemosensors for Amino Acids

Three representative BINOL derivatives were examined for their chiral recognitions with D- and L-t-Boc-amino acid anions: an open system 1, which bears two urea groups and two pyrene groups; a closed ring system 2, which bears two urea groups with a closed ring system; and a dimeric system 3, which bears two benzylic amine groups and two pyrene groups. Dimeric system 3 displayed a ΔI(D)/ΔI(L) of 12.95 for t-Boc-alanine.

Reactive extraction of enantiomers of 1,2-amino alcohols via stereoselective thermodynamic and kinetic processes.

(R)-amino alcohol with an enantiomeric excess of >95% was resolved by reactive extraction processes from 2 equiv of racemic alcohol using a chiral receptor 2 as an enantioselective extractant. One resolution cycle is composed of three extractions: a stereoselective reversible imine formation, a stereoselective irreversible imine hydrolysis, and the recovery of 2 and enantiomeric amino alcohols.

Stereoconversion of Amino Acids and Peptides in Uryl‐Pendant Binol Schiff Bases

(S)-2-Hydroxy-2-(3-phenyluryl-benzyl)-1,1-binaphthyl-3-carboxaldehyde (1) forms Schiff bases with a wide range of nonderivatized amino acids, including unnatural ones. Multiple hydrogen bonds, including resonance-assisted ones, fix the whole orientation of the imine and provoke structural rigidity around the imine C==N bond. Due to the structural difference and the increase in acidity of the alpha proton of the amino acid, the imine formed with an L-amino acid (1-l-aa) is converted into the imine of the D-amino acid (1-D-aa), with a D/L ratio of more than 10 for most amino acids at equilibrium. N-terminal amino acids in dipeptides are also predominantly epimerized to the D form upon imine formation with 1. Density functional theory calculations show that 1-D-Ala is more stable than 1-L-Ala by 1.64 kcal mol(-1), a value that is in qualitative agreement with the experimental result. Deuterium exchange of the alpha proton of alanine in the imine form was studied by (1)H NMR spectroscopy and the results support a stepwise mechanism in the L-into-D conversion rather than a concerted one; that is, deprotonation and protonation take place in a sequential manner. The deprotonation rate of L-Ala is approximately 16 times faster than that of D-Ala. The protonation step, however, appears to favor L-amino acid production, which prevents a much higher predominance of the D form in the imine. Receptor 1 and the predominantly D-form amino acid can be recovered from the imine by simple extraction under acidic conditions. Hence, 1 is a useful auxiliary to produce D-amino acids of industrial interest by the conversion of naturally occurring L-amino acids or relatively easily obtainable racemic amino acids.

Highly Selective Fluorescent Recognition of Pyrophosphate in Water by a New Chemosensing Ensemble

AbstractA new chemosensing ensemble that displays sensitive and selective fluorescent recognition of pyrophosphate in water at pH 7.4 has been developed. The ensemble is constructed by a copper complex (receptor) and eosin Y (indicator), the constructed ensemble is capable of highly selectively discriminate pyrophosphate from other common existing anions such as CH3COO−, HSO4−, NO3−, H2PO4−, HPO42−, PO43−, NCS−, I−, Cl−, Br−, F−as well as some structurally similar carboxylates such as citrate, tartrate, oxalate, malonate, succinate and glutarate.n FigureA new chemosensing ensemble that can effectively differentiate pyrophosphate from common existing anions and structurally similar carboxylate anions in 100% aqueous solution at physiological pH has been developed.