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J. Phys. Chem. B 2009, 113, 9620–9627
Theoretical Insights into the Interaction Mechanism between Proteins and SWCNTs: Adsorptions of Tripeptides GXG on SWCNTs Yixuan Wang* and Hongqi Ai Department of Natural Science, Albany State UniVersity, Albany, Georgia 31705 ReceiVed: April 10, 2009; ReVised Manuscript ReceiVed: May 26, 2009
Adsorptions of nine tripeptides GXG, ranging from negatively (D) and positively (K) charged, to hydrophilic (N and S), and to hydrophobic (G, V, F, W, and Y) residues, on the two cluster models (C54H18 and C54) of (10,0) single-walled carbon nanotubes (SWCNTs) are systemically investigated with the MPWB1K and MP2 methods. The solvent effects are taken into account with the implicit CPCM model. The objective is to provide novel insights into the interaction mechanism between proteins and SWCNTs. Results reveal that the adsorption strength of two charged tripeptides is greatly affected by the solvent effect and the hydrogen saturability of the SWCNT models. In the gas phase, on the surface of C54H18, GKG has the strongest adsorption (adsorption energy (AE): -29.3 kcal/mol at the MP2 level), whereas the adsorption of the negatively charged GDG is the strongest on C54 (AE: -30.4 kcal/mol with MP2). However, because of strong solvation, the adsorptions of the charged residues (D and K) on both C54H18 and C54 surfaces in aqueous solution are either rather weak or even unbound. The two neutral hydrophilic residues (N and S) exhibit adsorptions on C54H18 in the gas phase (AE: -3.3 and -4.2 kcal/mol), yet are unable to adsorb on SWCNTs in aqueous solution (AE: +0.3 kcal/mol at MP2+CPCM). The five hydrophobic residues present relatively strong adsorption on SWCNTs, especially for the three aromatic residues (GFG, GYG, and GWG), regardless of the CNT model and whether they are in the gas phase or solution. These results indicate that in general the aromatic groups of proteins would play a very important role on functionalizing CNTs, which basically supports the relevant experimental observations. In addition, the electron correlation is essential for adsorptions of GXG on pristine SWCNTs, and the three aromatic residues have the highest electron correlation effects. The present investigation provides strong evidence that for the functionalization of CNTs via proteins it is most likely that hydrophobic interaction and van der Waals are the dominant driving forces. 1. Introduction Carbon nanotubes (CNTs) have a very low solubility in aqueous solution as well as in organic solvents, which has been a major barrier for a variety of potential biomedical applications. Strategic approaches toward solubilization of CNTs have been developed mainly through their surface functionalization of either covalent or noncovalent attachments to the sidewalls or tips of CNTs.1-7 The covalent modification involving chemical reactions between CNTs and molecules somewhat impairs the structural and electronic properties of CNTs. Therefore, noncovalent functionalization of CNTs has attracted increasing attention. Noncovalent functionalization could not only enhance the solubility of CNTs but also maintain their attractive geometric, electronic, and mechanical properties. Among numerous functional species for solublizing CNTs, biological and bioactive materials are of a special importance. The fundamental components in living systems, such as carbohydrates, proteins, and nucleic acids, as well as their precursors have been explored to noncovalently functionalize CNTs toward the delivery of therapeutic agents.7-10 Various natural proteins differing in sizes and structures are able to efficiently disperse single-walled carbon nanotubes (SWCNTs) in water due to spontaneous absorbing on sidewalls of CNTs.11 Characterizations by a variety of complementary techniques including UV-vis spectroscopy, IR and Raman * To whom correspondence should be addressed. E-mail: yixuan.wang@ asurams.edu.
spectroscopy, and atomic force microscopy (AFM) confirmed the dispersion at the individual nanotube level. In spite of extensive studies on the noncovalent functionalization of SWCNTs with proteins, as briefly discussed below, there still exist some controversies in terms of the types of CNTs, net charge of proteins, and in particular the nature of interaction between proteins and CNTs that leads to the protein-CNT conjugates.7 The noncovalent binding of proteins to pristine and acidoxidized SWCNTs, where sidewalls are partially terminated with carboxyl acidic groups (-COOH), was conventionally attributed to hydrophobic interactions.2,7,12-14 This attribution has been challenged by numerous other findings. For example, Dai et al. used the oxidized SWCNTs to bind three proteins such as streptavidin protein A (SpA), bovine serum albumin (BSA), and cytochrome c (cyt-c).15 AFM images showed that the density of proteins on the sidewalls of SWCNTs is the highest for cytc, followed by SpA and BSA, which follows the order of isoelectric points (pI): 9.2 for cyt-c (positively charged at a physiological pH of 7.4) and
