The synthesis of chemically clean and environmentally friendly nanoparticles through pulsed laser ablation in liquids has shown a number of advantages over conventional chemical synthesis methods and has evolved into a thriving research field attracting laboratory and industrial applications. The fundamental understanding of processes leading to the nanoparticle generation, however, still remains elusive. In particular, the origin of bimodal nanoparticle size distributions in femto- and picosecond laser ablation in liquids, where small nanoparticles (several nanometers) with narrow size distribution are commonly observed to coexist with larger (tens to hundreds of nanometers) ones, has not been explained so far. In this paper, joint computational and experimental efforts are applied to understand the mechanisms of nanoparticle formation in picosecond laser ablation in liquids and to explain the bimodal nanoparticle size distributions. The results of a large-scale atomistic simulation reveal the critical role of the dynamic interaction between the ablation plume and the liquid environment, leading to the generation of large nanoparticles through a sequence of hydrodynamic instabilities at the plume-liquid interface and a concurrent nucleation and growth of small nanoparticles in an expanding metal-liquid mixing region. The computational predictions are supported by a series of stroboscopic videography experiments showing the emergence of small satellite bubbles surrounding the main cavitation bubble generated in single pulse experiments. Carefully timed double pulse irradiation triggers expansion of secondary cavitation bubbles indicating, in accord with the simulation results, the presence of localized sites of laser energy deposition (possibly large nanoparticles) injected into the liquid at the early stage of the bubble formation.

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A minimalist Cys2His2 zinc finger peptide, Lys-Tyr-Ala-Cys-Ala-Ala-Cys-Ala-Ala-Ala-Phe-Ala-Ala-LysAla-Ala-Leu-Ala-Ala-His-Ala-Ala-Ala-His-Ala-Lys, has been synthesized. Metal binding studies using Co2+ as a probe indicated that this peptide forms a 1:1 peptide/metal complex with a dissociation constant comparable to that observed for other zinc finger peptides. At high peptide concentrations, a 2:1 peptide/metal complex also forms, with four cysteinates coordinated to Co2+. Additional studies with sequence variants in which the canonical hydrophobic residues were changed to alanine, or in which one of the residues between the cysteines and the histidines was deleted, revealed an even more pronounced formation of the 2:1 complex over the 1:1 complex. In addition, the absorption spectra of the 1:1 peptide/Co2+ complexes of the variant peptides are significantly different from those seen for complexes of the parent peptide or those of more typical zinc finger peptides. NMR studies revealed that the parent peptide folds in the presence of Zn2+ to a structure very similar to that observed for other zinc finger peptides of this class. Taken together, these results suggest that the metalbinding and canonical hydrophobic residues are necessary and sufficient to determine the structure of this class of zinc finger peptides.The amino acid sequence of a protein contains the information sufficient to determine its three-dimensional structure (1). A variety of experiments have demonstrated that this information is not distributed uniformly but that a smaller set of residues are crucial for correct structure formation. For example, the elegant work of Kaiser and Kezdy (2) showed that simplified (or minimalist) versions of naturally occurring peptides could be designed and synthesized, retaining many of the structural (and, in many cases, functional) properties of the native materials. This approach has been extended to more complex oligomeric structures by DeGrado et al. (3). The uneven distribution of folding information has also been demonstrated by Sauer and coworkers (4) using combinatorial mutagenesis. Here, a protein sequence is randomized in a number of positions, functional molecules are selected, and their amino acid sequences are determined. It has been found that some positions are quite tolerant ofa wide range ofamino acids whereas others have very stringent restrictions. These observations suggest that a large number of sequences that have only a small subset of residues fixed may fold in functionally equivalent manners. The Cys2His2 zinc finger sequences typified by transcription factor TFIIIA may represent a naturally occurring set of sequences of this type. After the determination of the sequence of a cDNA clone of TFIIIA (5), it was noted that the deduced amino acid sequence contained nine tandem sequences that shared the consensus (Phe,Tyr)-Xaa-Cys-Xaa2,4-Cys-Xaa3-Phe-Xaa5-Leu-Xaa2-His-Xaa3,4-His-Xaa2-6 where Xaa represents relatively variable amino acids (6,7). Subsequently, a large number of other ...