By a simple click CuAAC (copper(I)‐catalysed azide alkyne cycloaddition) procedure several cyanine dye analogues have been attached to the side‐chain of an amino acid to yield chromophore amino acid conjugates with the potential to fluoresce upon binding to a target. Due to the availabiltiy of the amino acid C and N termini for peptide coupling, these conjugates are suitable for easy incorporation into the backbone of peptides. The novel amino acid dyes prepared in this work, although intrinsically non‐fluorescent, gave rise to strong fluorimetric responses upon binding to double‐stranded (ds) DNA or RNA, the emission response to various polynucleotide secondary structures being controlled either by linker length or a halogen atom located on the cyanine part of the molecule. Molecular modelling confirmed the mode of binding to different polynucleotides, which was responsible for the recognition. Interestingly, cell localisation experiments showed that the dyes were specifically localised in mitochondria at variance with the localisation of the parent dyes, which accumulate in cell nuclei, which suggests that the amino acid tail (containing a triazole ring) might function as a novel mitochondria‐directing appendage.
X-band electron paramagnetic resonance spectroscopy has been used to investigate the rotational diffusion of a stable, positively charged nitroxide 4-trimethylammonium-2,2,6,6-tetramethylpiperidine-1-oxyl iodide (Cat-1) in a series of 1-alkyl-3methylimidazolium tetrafluoroborate room-temperature ionic liquids (RTILs) having alkyl chain lengths from two to eight carbons. The rotation of Cat-1 is anisotropic with the preferential axis of rotation along the NO • moiety. The Stokes−Einstein−Debye law describes the mean rotational correlation time of Cat-1, assuming that the hydrodynamic radius is smaller than the van der Waals radius of the probe. This implies that the probe rotates freely, experiencing slip boundary condition, which is solvent-dependent. The rotational correlation time of Cat-1 in RTILs can very well be fitted to a power-law functionality with a singular temperature, which suggests that the apparent activation energy of rotation exhibits non-Arrhenius behavior. Compared to the rotation of perdeuterated 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl (pDTO), which is neutral, the rotation of Cat-1 is several times slower. The rotational anisotropy, the ratio of the rotational times of pDTO and Cat-1, and the apparent activation energy indicate the transition from a homogeneously globular structure to a spongelike structure when the alkyl chain has four carbons, which is also observed in molecular dynamics computational studies. For the first time, we have been able to show that the rotational correlation time of a solute molecule can be analyzed in terms of the Cohen−Turnbull free volume theory. The Cohen−Turnbull theory fully describes the rotation of Cat-1 in all ionic liquids in the measured temperature range.
We studied the diffusivities of a nitroxide radical at various temperatures in six glass-forming molecular liquids by electron spin resonance. By comparing the radical diffusivities and solvent self-diffusivities, we found that the radical diffusivities are lower than the self-diffusivities at high temperatures and approach them at low temperatures in all liquids. This crossover behavior was considered as evidence that a single-molecule diffusion process transforms into a collective process with temperature lowering. The crossover phenomenon was analyzed by a novel, simple diffusion model, combining collective and single-molecule diffusion processes, and it was compared to the Arrhenius crossover phenomenon. The obtained results suggest that future studies of tracer diffusion could contribute to a better understanding of diffusion mechanisms in glass-forming liquids. The proposed diffusion model could be used to study the crossover phenomena of tracer diffusion measured by other techniques, and it could serve as a base for developing more advanced models.
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