Dihydrolipoic Acid Conjugated Carbon Dots Accelerate Human Insulin Fibrillation

Protein fibrillation is believed to play an important role in the pathology and development of several human diseases, such as Alzheimer’s disease, Parkinson’s disease and type 2 diabetes. Carbon dots (CDs), as a new type of nanoparticle have recently been extensively studied for potential biological applications, but their effects on protein fibrillation remain unexplored. In reality, any application in biological systems will inevitably have “contact” between proteins and CDs. In this study, human insulin was selected as a model protein to study the effects of CDs on protein fibrillation, as proteins may share a common mechanism to form fibrils. Hydrophobic CDs were conjugated with dihydrolipoic acid (DHLA-CDs) to facilitate their water solubility. Characterizations from thioflavin T fluorescence, circular dichroism spectroscopy and atomic force microscopy demonstrate that the presence of DHLA-CDs results in a higher rate of human insulin fibrillation, accelerating the conformational changes of human insulin from α-helix to β-sheet. This promoting effect is likely associated with the locally increased concentration of human insulin adsorbed on the surface of DHLA-CDs. other research suggested that hydrophilic coated QDs accelerate the fibrillation of proteins (i.e., human β2-microglobulin, α-synuclein and human serum albumin) by enhancing the formation of the critical nucleus [12,13]. Our current knowledge of nanoparticles influencing protein fibrillation still relies on a case-by-case approach, depending on physical/chemical properties of proteins and nanoparticles, such as components, charge, coating and size [12,14,15]. Another novel type of nanoparticle-carbon dots (CDs) has recently arisen to replace QDs for biological applications. Compared with QDs, CDs are currently attracting more and more research attention because of their unique properties, such as excitation wavelength dependent photoluminescence, excellent biocompatibility, low cytotoxicity, and optical stability [16-18]. These properties make CDs increasingly attractive for applications in biological systems. Several studies using CDs in biological systems have shown that CDs are nontoxic and can be used for imaging in cellular environment [16,19,20]. For any application of CDs in a biological system, they are inevitably interacting with proteins which may undergo conformational changes to form fibrils due to the presence of CDs. However, the effect of CDs on protein fibrillation has not been reported until most recently (March 2015) from our group [21]. In that study, water soluble CDs with size less than 6 nm are prepared by oxidizing carbon nano powder and these CDs are demonstrated to inhibit human insulin fibrillation in vitro in a concentration-dependent manner. Insulin fibrils or plaques have been observed in vivo in patients suffering from diabetes after insulin infusion or repeated injection [22,23]. Insulin fibrils can be easily formed in vitro under certain condition, such as high concentrations, low pH and high temperatures [3,24]. In this study, human insulin was chosen to study the effects of CDs on its fibrillation. It is worth noting that the CDs used in this study have completely different nature from those we reported most recently [21] in terms of size, surface properties and components. The CDs in the present study have a hydrophobic core capped by dihydrolipoic acid (DHLA-CDs) to increase their water Sheba J. Kuruvilla1, Shanghao Li1, Lorenzo Sansalone1, Blake Fortes1, Ian Zheng1, Patrica L. Blackwelder2,3, Cyrus Pumilia1, Miodrag Micic4,5, Jhony Orbulescu5 and Roger M. Leblanc1* 1Department of Chemistry, University of Miami, USA 2University of Miami Center for Advanced Microscopy and Marine Geosciences, University of Miami, USA 3Nova Southeastern University Oceanographic Center, USA 4Department of Engineering Design Technology, Cerritos College, USA 5MP Biomedicals LLC, USA *Address for Correspondence Roger M. Leblanc, Professor, Department of Chemistry, University of Miami, 1301 Memorial Drive, Coral Gables, Florida, 33146, USA, Tel: +1305-284-2194; Fax: +1-305-284-6367; E-mail: rml@miami.edu Submission: 23 January, 2015 Accepted: 16 March, 2015 Published: 20 March, 2015 Review Article Open Access Journal of Parkinson’s disease & Alzheimer’s disease Avens Publishing Group Inviting Innovations Avens Publishing Group Inviting Innovations