Influence of Molecular Dipole Moment on the Redox-Induced Reorganization of α-Helical Peptide Self-Assembled Monolayers: An Electrochemical SPR Investigation

Abstract

Self-assembled monolayers (SAMs) of ferrocene-labeled α-helical peptides were prepared on gold surfaces and studied using electrochemical surface plasmon resonance (EC-SPR). The leucine-rich peptides were synthesized with a cysteine sulfhydryl group either at the C-or N-terminus, enabling their immobilization onto gold surfaces with control of the direction of the molecular dipole moment. Two electroactive SAMs were studied, one in which all of the peptide dipole moments are oriented in the same direction (SAM1), and the other in which the peptide dipole moment of one peptide is aligned in the opposite direction to that of its surrounding peptide molecules (SAM2). Cyclic voltammetry combined with SPR measurements revealed that SAM reorientations concomitant with the oxidation of the ferrocene label were more significant in SAM2 than in SAM1. The substantially greater change in the peptide film thickness in the case of SAM2 is attributed to the electrostatic repulsion between the electrogenerated ferrocinium moiety and the positively charged gold surface. The greater permeability of SAM1 to electrolyte anions, on the other hand, appears to effectively neutralize this electrostatic repulsion. The film thickness change in SAM2 was estimated to be 0.25 ± 0.05 nm using numerical simulation. The timescale of the redox-induced SPR changes was established by chronoamperometry and time-resolved SPR measurements, followed by fitting of the SPR response to a stretched exponential function. The time constants measured for the anodic process were 16 and 6 ms for SAM1 and SAM2 respectively, indicating that the SAM thickness changes are notably fast.

Self-assembled monolayers formed by helical peptide building blocks: a new tool for bioinspired nanotechnology

“…This effect was ascribed to the attractive interchain interactions taking place between opposing helical macrodipoles in the bicomponent SAM. Kraatz and co-workers 70,71 also demonstrated that, even if peptide SAMs are dynamical systems, in the antiparallel dipole arrangement, the peptide chains experience a more restricted motion due to stronger intermolecular interactions. The assembly of supramolecular structures on the basis of the intermolecular interaction of molecular electric dipoles (molecular dipole engineering) has been therefore proposed as a promising tool for the design of nanomaterials and nanodevices.…”

The self‐assembly of redox active peptides: Synthesis and electrochemical capacitive behavior

“…This free carboxyl group is important for further coupling of an antibody, enabling the use of the peptide in molecular recognition applications such as by forming functionalized surface for biosensor applications when coupled to a metallic plate. The amino acid Isoleucine was chosen due to the hydrophobicity and volume of side chain, resulting in the presence of hydrophobic and intramolecular hydrogen bonding in the peptide molecules . The volume of the branched increased in the order alkanethiol < Ala < Ile.…”

A controllable mechanistic transition of charge transfer in helical peptides: from hopping to superexchange

“…[53][54][55] This is in accordance with experimental observations where the dipole moment can facilitate charge transfer if the charge propagation (in the form of hole transfer) follows the same direction as the dipole moment in helical peptides. 12,56 Since each of the forward superexchange charge transfer steps has a negative energy difference (DE Aib1,Aib6 ), it is therefore energetically more favorable for the transference of cationic (+1) charge from the states Aib1 to Aib6, than the backward superexchange charge transfer step (Aib6 / Aib1), which is energetically unfavorable according to the Arrhenius' equation. Notably, there is a positive energy gap (DE Aib1,Aib4 ) in 3 (approx.…”