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Over the past year, we have made significant research advances in two areas related to the goals of our proposal, which was focused on the development of bio-inspired nanoparticle catalysts using peptide-mediated processes. Two specific peptides, one isolated from biocombinatorial approaches and one extracted from diatoms, were employed to generate highly intricate nanoscale systems. These materials possessed the ability to drive enhanced catalytic processes for the Stille reaction, which is able to form C-C bonds between aryl halides and organotin complexes. In the first avenue, we have employed catalysis as a method to elucidate the effects of the biological surface layer on peptide-capped Pd nanoparticles. These results suggest that the peptide sequence and surface arrangement play a significant role in mediating the reactivity and could serve as modification points to further enhance catalytic abilities. In the second area, a peptide template was used to generate non-spherical Pd nanostructures that demonstrated catalytic activity as a function of the surface area and penetration depth. These results indicate that the template is intimately involved in the reaction process and could be exploited for selective catalysis. A further discussion of the advances in each area is presented below.

In the first area, expansion of the analysis concerning the generation of Pd nanoparticle catalysts employing the Pd-specific Pd4 peptide (TSNAVHPTLRHL) was studied. The research has been focused on understanding the effects of the peptide sequence and its binding to the nanoparticle surface. Previous computational studies have demonstrated that the peptide binds to Pd through the histidines at positions 6 and 11 of the sequence. In this event, a kinked loop structure is envisioned at the particle surface, thus contributing to the exposure of the Pd atoms for catalytic reactivity. To more fully determine such effects, three analogues peptides were prepared that employed a histidine to alanine substitution at the 6, 11, and 6 and 11 positions, termed the A6, A11, and A6,11 peptides, respectively. Each sequence was demonstrated to have the ability to fabricate Pd nanoparticles of similar sizes (<4 nm in diameter) and each nanoparticle was able to drive Stille coupling; however, significant differences were observed in their catalytic reactivity. For instance, using the native Pd4-based materials, a turnover frequency (TOF) of 2234 ± 99 mol product (mol Pd × h)^-1 was observed, but using the Pd materials prepared with the A6 peptide, a greater than two-fold enhancement in the TOF was determined to 5224 ± 381 mol product (mol Pd × h)^-1. Interestingly, any alanine substitutions at the 11 position resulted in decreased catalytic activities, with TOF values of 1298 ± 107 mol product (mol Pd × h)^-1 and 361 ± 21 mol product (mol Pd × h)^-1 for the Pd nanoparticles capped with the A11 and A6,11 peptides, respectively. Furthermore, CD spectroscopic evidence confirmed that all four sequences presented significantly different surface structures, thus indicating that a different interface was presented to solution for the catalysis. This suggests that the peptide sequence is intimately involved in the catalytic process and could be employed to modify the reactivity. Future research is focused on elucidating the peptide binding motifs to allow for de novo design of sequences for enhancement of catalytic reactivity.

In the second avenue of research supported by funds from the PRF, a unique peptide template was used to generate non-spherical Pd nanostructures. Here, the R5 peptide of diatoms (SSKKSGSYSGSKGSKRRIL), which self-assembles to form aggregates of ~800 nm, is able to coordinate Pd²⁺ ions via the primary amines, that upon reduction, generates linearized Pd nanomaterials. By judiciously increasing the Pd concentration in the reaction, spheres, nanoribbons, or nanoparticle networks can readily be prepared. As a result the catalyst is generated within a peptide framework, which exposes the Pd surface for optimized reactivity. Furthermore, by changing the composite structure and morphology, changes to the reactive surface area and catalyst penetration depth within the scaffold occur. For instance, the spherical nanoparticles have the highest surface area, which results in a TOF value of 452 ± 16 mol product (mol Pd × h)^-1 for Stille coupling, while the nanoparticle network structures possess the smallest penetration depth within the peptide template, thus resulting in a TOF of 437 ± 14 mol product (mol Pd × h)^-1.  Interestingly, for the linear Pd nanoribbons, the surface area is lower as compared to the spherical nanoparticles and the penetration depth is larger as compared to the nanoparticle networks, which results in a minimized TOF value of 334 ± 38 mol product (mol Pd × h)^-1. These structural changes can be directly correlated to the nanomaterial-based system such that the materials with the highest surface area or the lowest penetration depth demonstrated the highest TOF value. This suggests that these two factors are critical to the overall reactivity of the system and could also be used to tune the functionality. At present, we are developing design criteria to elucidate their individual effects on reactivity for a variety of catalyses including C-coupling and nitrate reduction.

From both research directions supported by funds from the PRF, four papers have been published, with two additional manuscripts presently in review. Furthermore, these funds have supported the educational development of three graduate students, who have been highly productive in generating an understanding the interactions between biological and inorganic materials. These results lay the foundation for the development of multiple research pathways.