Enantioselective fluorescence sensing of amino acids by modified cyclodextrins: role of the cavity and sensing mechanism.
Two selectors based on modified cyclodextrins containing a metal binding site and a dansyl fluorophore-6-deoxy-6-N-(N(alpha)-[(5-dimethylamino-1-naphthalenesulfonyl)aminoethyl]phenylalanylamino-beta-cyclodextrin-containing D-Phe (3) and L-Phe (4) moieties were synthesized. The conformations of the two selectors were studied by circular dichroism, two-dimensional NMR spectroscopy and time-resolved fluorescence spectroscopy. Cyclodextrin 4 was found to have a predominant conformation in which the dansyl group is self-included in the cyclodextrin cavity, while 3 showed a larger proportion of the conformation with the dansyl group outside the cavity. As a consequence, the two cyclodextrins were found to bind copper(II) with different affinities, as revealed by fluorescence quenching in competitive binding measurements. Addition of D- or L-amino acids induced increases in fluorescence intensity, which were dependent on the amino acid used and in some cases on its absolute configuration. The cyclodextrin 4 was found to be more enantioselective than 3, suggesting that the self-inclusion in the cyclodextrin cavity strongly increases the chiral discrimination ability of the copper(II) complex. Accordingly, a linear fluorescent ligand N(alpha)-[(5-dimethylamino-1-naphthalenesulfonyl)aminoethyl]-N(1)-propyl-phenylalaninamide, which has the same binding site and absolute configuration as 4, showed very low chiral discrimination ability. The enantioselectivity in fluorescence response was found to be due to the formation of diastereomeric ternary complexes, which were detected by ESI-MS and by circular dichroism. Time-resolved fluorescence studies showed that the fluorescence of the dansyl group was completely quenched in the ternary complexes formed, and that the residual fluorescence was due to uncomplexed ligand.
Pagliari
Sara
S
Dipartimento di Chimica Organica e Industriale, Università di Parma, Parco Area delle Scienze 17/A, 43100 Parma, Italy.
Corradini
Roberto
R
Galaverna
Gianni
G
Sforza
Stefano
S
Dossena
Arnaldo
A
Montalti
Marco
M
Prodi
Luca
L
Zaccheroni
Nelsi
N
Marchelli
Rosangela
R
D000596:Amino Acids / Q000737:chemistry*; D015374:Biosensing Techniques; D002240:Carbohydrate Sequence; D003300:Copper / Q000737:chemistry; D003505:Cyclodextrins / Q000138:chemical synthesis / Q000737:chemistry*; D005453:Fluorescence; D008024:Ligands; D008968:Molecular Conformation; D008969:Molecular Sequence Data; D009942:Organometallic Compounds / Q000138:chemical synthesis; D013237:Stereoisomerism
D000596:Amino Acids; D003505:Cyclodextrins; D008024:Ligands; D009942:Organometallic Compounds; D003300:Copper