Incorporation of Molecular Nanoparticles Inside Proteins: The Trojan Horse Approach in Theranostics

Abstract

Metrics & MoreArticle Recommendations CONSPECTUS: Molecular nanoparticles, MNPs, characterized by well-defined chemical formulas, structures, and sizes can interact with a variety of proteins. Fullerenes, carboranes, and gold nanoclusters well represent the diversity of MNPs properties available in nanoscience. They can have diameters smaller than 1.5 nm, be hydrophilic or hydrophobic, and can use a paraphernalia of means to establish local and global interactions with the amino acidic residues of proteins. Proteins, endowed as they are with an assortment of pockets, crevices, and gaps are natural supramolecular hosts to incorporate/hide/transport MNPs directly in water with a facile and "green" approach. This Account identifies and discusses the rules that govern the interactions and binding between MNPs and proteins. Fullerenes are composed solely by carbon atoms arranged to form hollow polyhedra. Hydrophobic interactions occur between aliphatic residues and the fullerene surface. The amino acids most effectively interacting with fullerenes are aromatic residues that establish π−π stacking interactions with the cage. Amphiphilic and charged residues produce also cation−π, anion−π, and surfactant-like interactions with the cages.Carboranes are composed of boron, carbon, and hydrogen atoms, also arranged to form cages. They are hydrophobic with unusual properties originating from the presence of boron atoms. Hydride-like hydrogens bound to the boron atoms govern carborane chemistry. These negatively charged hydrogens do not participate in classic hydrogen bonding with water and promote hydrophobic interactions with proteins. On the contrary, the electronegativity of these hydrogens drives the formation of unconventional dihydrogen bonds with the acidic hydrogen atoms of positively charged amino acid. Carboranes also establish C−H•••π and B−H•••π interactions with aromatic residues. Gold nanoclusters, AuNCs, are synthesizable with atomically precise stoichiometry. Amino acid residues with sulfur atoms or with nitrogen-containing heterocycles are the strongest Au binders. The proteins can act as supramolecular hosts but also as templates for the synthesis of AuNCs directly inside the protein core. Of the pristine amino acids, tryptophan, tyrosine, phenylalanine, and aspartic acid are the most efficient reducing groups. In a peptide sequence, the best Au-reducing moieties are obtained by nitrogencontaining residue such as glutamine, asparagine, arginine, and lysine. The investigation of the interactions between AuNCs and proteins therefore adds further complexity with respect to that of fullerenes and carboranes. The selection of the host proteins should consider that they will have to contain active sites for metal ion accumulation and ion reduction where AuNC can form and stabilize. This Account further discusses the hybridization of MNPs with proteins in view of creating innovative multifunctional theranostic platforms where the role of proteins is akin to that of "Trojan Horses" since they can (i) hide the MNPs,...

Dissecting the Supramolecular Dispersion of Fullerenes by Proteins/Peptides: Amino Acid Ranking and Driving Forces for Binding to C60

“…Concurrently, the larger the contact area between the amino acid side chain and the C 60 , the larger the stabilizing effect due to vdW interactions (dispersion forces). Shape complementarity represents a quick way to estimate both the stabilizing van der Waals and hydrophobic interactions between proteins and carbon nanomaterials [ 21 , 45 , 62 , 63 ].…”

“…The potential of fullerenes in bio-medical applications is significant and can only partly be exploited because their extremely hydrophobic nature makes them rare in physiological environments. Of all the strategies that have been developed to make fullerenes bio-available, the “Trojan horse” approach, in which they are stashed away in water-soluble proteins, appears to be the greenest and most environmentally friendly [ 21 ]. Propensity rules able to identify where a fullerene can be accommodated in a protein were developed.…”

“…Supramolecular approaches can preserve these properties [ 19 ]. Recently, it was demonstrated that it is possible to use peptides [ 20 ] and proteins [ 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 ] as supramolecular hosts for the non-covalent dispersion of fullerenes [ 21 ]. Different proteins, such as albumins [ 22 , 23 ], lysozyme [ 23 , 24 , 25 , 26 ], pepsin [ 27 ], trypsin [ 27 ], or natural protein surfactants [ 28 ] were used for the mono-molecular dispersion of pristine fullerenes in a physiological environment.…”

EGFR-Targeted Photodynamic Therapy

“…The bioconjugation of a photosensitizer to a protein is able to bypass many of the restrictions typical of photosensitizers, such as (i) the poor water solubility or low biocompatibility, (ii) the dependency of their ability to generate ROS on the features of the physiological environment, (iii) aggregation phenomena, (iv) the non-specificity of the cellular uptake, and (v) the poor pharmacokinetic or pharmacodynamic properties [42,[88][89][90][91][92]. Bioconjugates of EGF with disulfochloride aluminum phthalocyanine [Pc(Al)] and disulfochloride cobalt phthalocyanine [Pc(Co)] were prepared, using EGF as a vector for targeted delivery of the phthalocyanines to cancer cells [93].…”

Photoluminescent nanocluster-based probes for bioimaging applications

“…The integration of the advantageous features of metal NCs and the active functional groups of proteins within a single system looks extremely useful for the development of tracking agents. A wide range of proteins have been employed in the synthesis of NC-based bioimaging probes, including hemoglobin, human serum albumin, lactoferrin, and resilin-mimetic proteins [67][68][69][70].…”