ROPS

The rate at which the drill bit penetrates the formation gives qualitative information about the lithology being drilled. For example, in a hard shale the rate of penetration ROP) will be slower than in a porous sandstone. 

Write a series of equations describing a synthesis of 1 bromo 2 Tietny rop ol from tert butyl bromide J... 

Higher polymeric forms, eg, (ROP ), also exist (see Phosphoric ACIDS AND PHOSPHATBs). 

FIG, 2-13 Temperahire-entropy diagram for nitrogen. Seotion of T-S diagram for nitrogen by E. S. Burnett, 1950. (Repainted from U.S. Bur Minos Rop. Invest. 4729.)... 

Between about Rop = 350,000 and 1 X 10 , the drag coefficient drops dramatically in a drag crisis owing to the transition to turbulent flow in the boundary layer around the particle, which delays aft separation, resulting in a smaller wake and less drag. Beyond Re = 1 X 10 , the drag coefficient may be estimated from (Clift, Grace, and Weber) ... 

Maximum normol occelerotion of collecting electrode plote produced by normol rop ( q")... 

In most four-helix bundle structures, including those shown in Figure 3.7, the a helices are packed against each other according to the "ridges in grooves" model discussed later in this chapter. However, there are also examples where coiled-coil dimers packed by the "knobs in holes" model participate in four-helix bundle structures. A particularly simple illustrative example is the Rop protein, a small RNA-binding protein that is encoded by certain plasmids and is involved in plasmid replication. The monomeric sub unit of Rop is a polypeptide chain of 63 amino acids built up from two... 

Figure 3.8 Schematic diagram of the dimeric Rop molecule. Each subunit comprises two a helices arranged in a coiled-coil structure with side chains packed into the hydrophobic core according to the "knobs in holes" model. The two subunits are arranged in such a way that a bundle of four a helices is formed.

The coiled-coil structure of the leucine zipper motif is not the only way that homodimers and heterodimers of transcription factors are formed. As we saw in Chapter 3 when discussing the RNA-binding protein ROP, the formation of a four-helix bundle structure is also a way to achieve dimerization, and the helix-loop-helix (HLH) family of transcription factors dimerize in this manner. In these proteins, the helix-loop-helix region is preceded by a sequence of basic amino acids that provide the DNA-binding site (Figure 10.23), and... 

Figure 17.16 Ribbon diagram representations of the structures of domain B1 from protein G (blue) and the dimer of Rop (red). The fold of B1 has been converted to an a-helical protein like Rop by changing 50% of its amino acids sequence. (Adapted from S. Dalai et al.,...

They started from the sequence of a domain, Bl, from an IgG-binding protein called Protein G. This domain of 56 amino acid residues folds into a four-stranded p sheet and one a helix (Figure 17.16). Their aim was to convert this structure into an all a-helical structure similar to that of Rop (see Chapter 3). Each subunit of Rop is 63 amino acids long and folds into two a helices connected by a short loop. The last seven residues are unstructured and were not considered in the design procedure. Two subunits of Rop form a four-helix bundle (Figure 17.16). 

Table 17.3 Amino acid sequences of domain Bl, the designed protein Janus, and Rop...

Red residues have been changed to the same type as present in Rop, blue residues have been changed to a different type, and black are unchanged. 

During this process of designing sequence changes, models were built and assessed to ensure that there were no obvious steric clashes and that the hydrophobic core was well packed. Furthermore, secondary structure prediction was also used to monitor the progress of change and to choose among different possible substitutions. The final sequence (see Table 17.3) contains 28 changes it had 50% identity to B1 and the similarity to Rop had increased from 5.4% identity to 41%. 

A gene encoding this sequence was synthesized and the corresponding protein, called Janus, was expressed, purified, and characterized. The atomic structure of this protein has not been determined at the time of writing but circular dichroic and NMR spectra show very clear differences from B1 and equally clear similarities to Rop. The protein is a dimer in solution like Rop and thermodynamic data indicate that it is a stably folded protein and not a molten globule fold like several other designed proteins. 

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