| Fluorescent carbon dots(CDs) as a new type of carbon nanomaterials that have developed rapidly in recent years, have attracted tremendous attention of the researchers for their stable chemical properties, excellent optical performance, good biocompatibility and low toxicity of biological toxicity and easy for surface functionalization, etc. But because of their weak fluorescence intensity and the low fluorescence quantum yield, the carbon dots which were unmodified cannot be directly used in fluorescent tags and biological imaging applications of biomedical research. In order to make the fluorescent performance of CDs more advanced and make CDs more suitable for biological analysis and the other biomedical applications,the surface of the carbon dots could be chemically modified which greatly increase the properties of carbon dots and become more suitable for biological applications. At the moment, due to the wide application of carbon dots in the cell markers or biological imaging, the CDs are likely to interact with biological protein or nucleic acid in the body, and lead to changes in their conformations, such as, the loss of physiological activity. In this work, we investigate the interaction of CDs with protein in this research under simulated physiological conditions, these results have an important and guiding significance to the development of functional, safety CDs and the applications of CDs in biological field.Firstly, the fluorescence CDs without modification were synthesized.Subsequently nitrogen, sulfur or boron elements were connected to CDs. All CDs were characterized with the methods of fluorescence spectrum, ultraviolet spectrum,infrared spectrum, element analysis and transmission electron microscopy(TEM).The quasi-spherical CDs with sizes below 20 nm were dispersed perfectly. The results of elemental analysis and infrared spectrum show that hetero-atoms N, S and B were connected to CDs successfully. At the same time, there was an obvious improvement of the fluorescence quantum yield of CDs which were connected with hetero-atom.Secondly, the interactions of CDs and HSA under simulated physiologicalconditions were investigated with fluorescence spectroscopy and MST. The results of fluorescence spectroscopy and MST showed that complexes were formed between the four CDs and HSA. The binding constants Ka, the number of binding site and the type of interaction force were acquired. The binding of CDs to HSA mainly located within site ?. The results acquired from traditional fluorescence spectroscopy were coin-cident with those obtained from MST. The data from synchronous fluorescence spectroscopy, three-dimensional fluorescence spectra and circular dichroism spectra showed that the addition of CDs had no impact on the structure of HSA.At last, the interaction of CDs with CAT under mimic physiological conditions was conducted with the traditional fluorescence spectrometry and MST. The results showed that the CDs could quench the intrinsic fluorescence of CAT and CAT could quench the fluorescence of CDs with the formation of complexes by static quenching.The binding constants Ka and thermodynamics parameters ΔH, ΔG and ΔS at different temperatures were calculated. The results from synchronous fluorescence spectro-scopy, three-dimensional fluorescence spectra and circular dichroism spectra showed that there were no effects on the structure of CAT with the addition of CDs. Furthermore, we investigated the effect of CDs on the activity of catalase by evaluating the catalytic action of catalase to hydrogen peroxide through ultraviolet spectrophotometry method. The studies showed that the inhibition rates on the catalase increased with the increasing binding affinity between CDs and catalase(compared to the results under 25℃ from MST). There is a certain positive correlation between the affinity of CDs with enzyme and the inhibition of enzyme activity. Therefore, there may be a certain influence on the activity of enzymes when the CDs interact with them. |
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