Luminescent Bimetallic IrIII /AuI Peptide Bioconjugates as Potential Theranostic Agents

Abstract

Diverse iridium peptide bioconjugates and the corresponding iridium/gold bimetallic complexes have been synthesized starting from a cyclometallated carboxylic acid substituted Ir^III complex [Ir(ppy)₂ (Phen-5-COO)] by solid phase peptide synthesis (SPPS). The selected peptide sequences were an enkephalin derivative Tyr-Gly-Gly-Phe-Leu together with the propargyl-substituted species Tyr-Gly-Pgl-Phe-Leu to allow gold coordination (Pgl: propyrgyl-glycine, HC≡C-Gly), and a specific short peptide, Ala-Cys-Ala-Phen, containing a cysteine residue. Introduction of the gold center has been achieved via a click reaction with the alkynyl group leading to an organometallic Au-C(triazole) species, or by direct coordination to the sulfur atom of the cysteine. The photophysical properties of these species revealed predominantly an emission originating from the Ir complex, using mixed metal-to-ligand and ligand-to-ligand charge transfer excited states of triplet multiplicity. The formation of the peptide bioconjugates caused a systematic redshift of the emission profiles. Lysosomal accumulation was observed for all the complexes, in contrast to the expected mitochondrial accumulation triggered by the gold complexes. Only the cysteine-containing Ir/Au bioconjugate displayed cytotoxic activity. The absence of activity may be related to the lack of endosomal/lysosomal escape for the cationic peptide conjugates. Interestingly, the different coordination sphere of the gold atom may play a crucial role, as the Au-S(cysteine) bond may be more readily cleaved in a biological environment than the Au-C(triazole) bond, and thus the Au fragment could be released from or trapped in the lysosomes, respectively. This work represents a starting point in the development of bimetallic peptide bioconjugates as theranostics and in the knowledge of factors that contribute to anti-proliferative activity.

Keywords: bimetallic compounds; cell imaging; gold; iridium; peptide bioconjugates; theranostic agents.

Figure 1

Gold(I) peptide derivatives A, B, C and D.12

Scheme 1

Synthetic procedure for the preparation of complex 1.

Figure 2

POV‐ray view of one of the molecules present in the asymmetric unit of complex 1. Most relevant bond distances (Å) and angles (°): Ir(1)−C(11) 2.008(14), Ir(1)−C(22) 2.008(12), Ir(1)−N(1) 2.048(11), Ir(1)−N(2) 2.033(11), Ir(1)−N(3) 2.143(11), Ir(1)−N(4) 2.155(11); N(1)‐Ir(1)‐N(2) 173.5(4), N(4)‐Ir(1)‐N(3) 78.1(5).

Scheme 2

Representation of solid phase peptide synthesis (SPPS) of compound 2.

Figure 3

Chemical structure of compound 3 obtained by SPPS.

Scheme 3

Synthetic procedure of complex 4.

Scheme 4

Depiction of the synthetic pathways to obtain compound 5. Reaction conditions: i) DCM/TFA/TIS (90/5/5); ii) TFA/TIS/H2O (95/2.5/2.5); iii) [AuClPPh₃], DIPEA, DCM followed by TFA/DCM/Phenol (50/40/10); iv) [AuClPPh₃], DIPEA, DCM.

Figure 4

Emission spectra of complexes 1–5 recorded in DMSO and RT.

Figure 5

Flow cytometry analysis of ROS production induced by 5.

Figure 6

Fluorescence microscopy images from colocalization experiments of complexes 2 and 4 (green) with MitoTracker incubated with A549 cells. (A and A’) superimposition images. (B and B’) superimposition images including the phase contrast.

Figure 7

Fluorescence microscopy images from colocalization experiments of complexes 4 and 5 (green) with LysoTracker incubated with A549 cells. (A and A’) superimposition images. (B and B’) superimposition images including the phase contrast.