Nucleated Condensation

Fig. 5. Protein folding. The unfolded polypeptide chain coUapses and assembles to form simple stmctural motifs such as -sheets and a-hehces by nucleation-condensation mechanisms involving the formation of hydrogen bonds and van der Waal s interactions. Small proteins (eg, chymotrypsin inhibitor 2) attain their final (tertiary) stmcture in this way. Larger proteins and multiple protein assembhes aggregate by recognition and docking of multiple domains (eg, -barrels, a-helix bundles), often displaying positive cooperativity. Many noncovalent interactions, including hydrogen bonding, van der Waal s and electrostatic interactions, and the hydrophobic effect are exploited to create the final, compact protein assembly. Further stmctural...

This nucleation-condensation mechanism has certain characteristics. First, the nucleation site does not need to be extensively preformed in the denatured state. It may or may not be detectable in isolated peptides, but it is essential only in that it is extensively formed in the transition state. Further, it need not be formed completely in the transition state because of the onrush of cooperative stabilizing interactions, however, it may be in the process of being formed in the transition state. Second, although much of the structure is from local, contiguous residues, there are important stabilizing contributions from long-range interactions i.e., from contacts with residues that are far removed in sequence. 

The nucleation-condensation mechanism can be accommodated in modified framework and hydrophobic-collapse models the framework model must be modified so that formation of secondary structure is linked to the formation of tertiary interactions and the hydrophobic collapse model must have the formation of tertiary interactions linked to the formation of secondary structure. Another variation of concerted structure formation is the hydrophobic zipper. 68 Whatever the distinctions of names, stable tertiary and secondary structural interactions must form concurrently. 

Nucleation-collapse—see nucleation-condensation mechanism Nucleation-condensation mechani sm 583,585-588... 

A mechanism called nucleation-condensation describes the folding.30,37,38 (The term nucleation-collapse is also used.39,40) This mechanism involves a nucleus that consists primarily of adjacent residues (Table 19.1). The nucleus does not form stable structure without assistance from interactions made with residues that are distant in sequence. Formation of the small nucleus cannot be solely rate-determining because a significant fraction of the overall structure... 

Colloidal dispersions can be formed either by nucleation with subsequent growth or by subdivision processes [12,13,16,25,152,426], The nucleation process requires a phase change, such as condensation of vapour to yield liquid or solid, or precipitation from solution. Tadros reviews nucleation/condensation processes and their control [236], Some mechanisms of such colloid formation are listed in Table 7.1. The subdivision process refers to the comminution of particles, droplets, or bubbles into smaller sizes. This process requires the application of shear. Some of the kinds of devices used are listed in Table 7.2 [228]. 

The current version of GEM-AQ has five size-resolved aerosols types, viz. sea salt, sulphate, black carbon, organic carbon, and dust. The microphysical processes which describe formation and transformation of aerosols are calculated by a sectional aerosol module (Gong et al. 2003). The particle mass is distributed into 12 logarithmically spaced bins from 0.005 to 10.24 pm radius. This size distribution leads to an additional 60 advected tracers. The following aerosol processes are accounted for in the aerosol module nucleation, condensation, coagulation, sedimentation and dry deposition, in-cloud oxidation of SO2, in-cloud scavenging, and below-cloud scavenging by rain and snow. 

Gas-to-particle conversion processes in the atmosphere consist of formation of low-vapor-pressure gases formed by atmospheric chemical reactions followed by new particle formation (nucleation), condensation of the low-vapor-pressure material on existing particles, or both. Chemical reaction producing the low-vapor-pressure product also occurs in cloud droplets, with the product remaining in the condensed phase of clear-air aerosol particles following cloud evaporation. Substances of intermediate vapor pressure may reversibly... 

Depending on the protein, the free energy landscape differs as is illustrated in Fig. 6. For some two-state proteins, with an independently stable secondary structure, the diffusion-collision mechanism is preferred. Other proteins,for which the secondary structure is less stable on its own, fold cooperatively using the nucleation-condensation pathway. In all cases there is still two state behavior, because there is only one rate limiting barrier. 

N. Ferguson, R. Day, C. M. Johnson, M. D. Allen, V. Dagget, A. Fersht (2005) Simulation and experiment at high temperatures Ultrafast folding of a thermophilic protein by nucleation-condensation. J. Mol. Biol. 347, pp. 855-870... 

The nucleation condensation model of protein folding (Chapter 2)... 

Kalani, a. Christohdes, P. D. 2002 Simulation, estimation and control of size distribution in aerosol processes with simultaneous reaction, nucleation, condensation and... 

The Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic hosts the leading Aerosol Laboratory in the country. The main effort is aimed at experimental study of individual parts of the condensation process (nucleation, condensation and evaporation, heat and mass transfer) as well as at more complex phenomena such as gas-phase synthesis of nano-particles, and combustion or atmospheric aerosols. 

---