Effect of Orientation of the Peptide-Bridge Dipole Moment on the Properties of Fullerene–Peptide–Radical Systems

Abstract

We synthesized two series of compounds in which a nitroxide radical and a fullerene C 60 moiety were kept separated by a 3 10 -helical peptide bridge containing two intramolecular CO···H−N hydrogen bonds. The direction of the resulting molecular dipole moment could be reversed by switching the position of fullerene and nitroxide with respect to the peptide nitrogen and carbon termini. The resulting fullerene−peptide−radical systems were compared to the behaviors of otherwise identical peptides but lacking either C 60 or the free radical moiety. Electrochemical analysis and chemical nitroxide reduction experiments show that the dipole moment of the helix significantly affects the redox properties of both electroactive groups. Besides providing evidence of a folded helical conformation for the peptide bridge, IR and NMR results highlight a strong effect of peptide orientation on the spectral patterns, pointing to a specific interaction of one of the helical orientations with the C 60 moiety. Time-resolved EPR spectra show not only that for both systems triplet quenching by nitroxide induces spin polarization of the radical spin sublevels, but also that the coupling interaction can be either weak or strong depending on the orientation of the peptide dipole. As opposed to the concept of dyads, the molecules investigated are thus better described as fullerene−peptide−radical systems to stress the active role of the bridge as an important ingredient capable of tuning the system's physicochemical properties. ■ INTRODUCTIONSince its discovery in 1985, 1 the properties of the C 60 fullerene have been extensively investigated. Thorough understanding of its chemical reactivity has led to the synthesis of a large number of fullerene derivatives whose properties and applications have been studied from many viewpoints.2 Among them, interesting molecular systems are featured by fullerene−radical dyads where C 60 is covalently linked, through formation of fulleropyrrolidines, 3 either directly or via a molecular spacer to a free radical function. 4,5 Besides interest in synthesis and properties of C 60 and its derivatives, endohedral fullerenes 6 in which atom or small molecule is encapsulated in the C 60 nano cavity have become a field that attracts more attention due to their potential applications in medicine and biology. 7,8 In particular, the recently successful synthesis 9 of H 2 @C 60 by the molecular surgery method makes the material available in quantities from tens to hundreds of milligrams. Therefore, to probe the interactions of the motion of the endohedral molecule with the outside environment, 10 a series of H 2 @C 60 derivatives covalently linked to a nitroxide radical has been synthesized in which the nitroxide moiety acts as a relaxation agent, and the effect of the nitroxide radical on proton nuclear spin relaxation of the endohedral molecule has been investigated. 11In this Article, we describe properties and compare the behavior of compounds in which a free radical and a fullerene C 60 moiety are...

Dipole Moment Effect on the Electrochemical Desorption of Self‐Assembled Monolayers of 310‐Helicogenic Peptides on Gold

“…For example, different ET (or electron‐transport) rates have been found for opposite orientations of the peptides in SAMs; similar effects were observed for ET in freely diffusing donor–peptide–acceptor systems . Thermodynamics, however, could also be responsible for making ET rates different: as a matter of fact, we already showed that an oriented peptide dipole moment affects significantly the redox potentials of electroactive groups and gold clusters . For molecules in solution, as opposed to the concept of dyads, in which two moieties are separated by a spacer, we concluded that moieties separated by Aib peptides must be described also by stressing the orientation of the bridge, because its dipole moment is capable of tuning the system's physicochemical properties (not just electrochemistry) significantly .…”

“…Thermodynamics, however, could also be responsible for making ET rates different: as a matter of fact, we already showed that an oriented peptide dipole moment affects significantly the redox potentials of electroactive groups and gold clusters . For molecules in solution, as opposed to the concept of dyads, in which two moieties are separated by a spacer, we concluded that moieties separated by Aib peptides must be described also by stressing the orientation of the bridge, because its dipole moment is capable of tuning the system's physicochemical properties (not just electrochemistry) significantly . A final mention is about the important effect of oriented dipole moments on the metal work function and associated applications.…”

Bioinspired molecular electrets: bottom-up approach to energy materials and applications

“…[28][29][30][31] In synthetic polypeptide helices, the intrinsic dipoles rectify the directionality of CT. [32][33][34][35][36][37] The broadband gap [or highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap] of proteins, however, along with the inability to inject charges in them without chemically cleaving their backbones, [38][39][40] considerably limits their utility as electronic materials. Proteins and polypeptides (composed of native α-amino acids and their derivatives) mediate ET solely via tunneling, the efficiency of which is limited to about 2 nm.…”

Bioinspired approach toward molecular electrets: synthetic proteome for materials

“…When the structures with opposing dipole direction are not completely symmetrical, the differences in the electronic coupling can prevail over the dipole-induced effect for relatively large −ΔG CT (0) [125]. Since the beginning of the 21 st century, studies of dipole effects on CT have focused on systems comprising polypeptide helices [12,95,99,[126][127][128][129][130][131][132][133][134][135][136][137][138][139][140][141]. Donor-bridge-acceptor (DBA) constructs, where the bridge is a helix, allow for testing dipole-induced charge-transfer rectification, i.e.…”

“…the difference between the rates of ET along the dipole vs. ET against the dipole [137]. Self-assembled monolayers (SAMs) of polypeptide helices on gold surfaces show similar dipole-induced rectification of interfacial CT [95], i.e.…”