An Expedient Synthesis of Cationic Rhodamine Fluorescent Probes Suitable for Conjugation to Amino Acids and Peptides

Abstract

Rhodamine 19 benzyloxycarbonylmethyl ester bromide 8 and rhodamine 19 4-chloromethyl-1-phenylmethyl ester chloride 12 act as precursors to cationic fluorescent probes. Molecules containing free amine or carboxylate functional groups, respectively, can be attached in a one-step procedure, yielding the desired probes without the need for chromatographic purification. As a proof of concept the method was applied to the attachment of amino acid and dipeptide residues through either the N- or C-termini. The precursor molecules 8 and 12 are readily synthesized from the inexpensive, commercially available dye rhodamine 6G.

Key words

fluorescent probes - rhodamines - amino acid derivatives - peptides

References

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15

The relative quantum yields at 495 nm of derivatives 10, 13 and 15 were 1.26, 1.03 and 0.95 × that of rhodamine 6G respectively (see Ref. [18] for the method used).

16

The crystals of compound 14 were obtained by crystallization from Et₂O-EtOH: C₄₁H₄₆IN₃O₇, M = 819.71, triclinic, space group P-1, a = 12.6230(2), b = 13.4724(3), c = 13.5600(3) Å, α = 101.865(1), β = 107.003(1), γ = 113.602(1)°, V = 1878.77(7) ų, Z = 2, D_c = 1.449 gcm^-3, µ = 0.907 mm^-1, F(000) = 844, crystal dimensions 0.35 × 0.32 × 0.28 mm³. The intensities of 28224 reflections were measured on a Nonius Kappa-CCD diffractometer (MoKα radiation, T = 100.0(1) K, 4.19 < θ < 30.48, 10822 unique reflections). The structure was solved and refined using the SHELXTL package (see Ref. [17] ). Non-hydrogen atoms were assigned anisotropic thermal parameters. Hydrogen atoms were included in calculated positions and treated as riding atoms. The refinement which included 469 parameters, converged with R1[I>2σ(I)] = 0.0375 (for 8449 reflections with I>2σ(I)) and wR2 (all unique data) = 0.0970. Atomic coordinates and further crystallographic details have been deposited at the Cambridge Crystallographic Data Centre, deposition number CCDC-191818, and copies of this data can be obtained in application to CCDC, 12, Union Road, Cambridge CB2 1EZ, UK. [Fax: [phone]; E-mail: [e-mail]].

17

Sheldrick, G. M. SHELXTL/PC Version 5.1, Windows NT Version, Bruker AXS Inc., Madison, USA.