Polypeptides Moffitt parameters

Nuclease behaves like a typical globular protein in aqueous solution when examined by classic hydrodynamic methods (40) or by measurements of rotational relaxation times for the dimethylaminonaphth-alene sulfonyl derivative (48)- Its intrinsic viscosity, approximately 0.025 dl/g is also consistent with such a conformation. Measurements of its optical rotatory properties, either by estimation of the Moffitt parameter b , or the mean residue rotation at 233 nin, indicate that approximately 15-18% of the polypeptide backbone is in the -helical conformation (47, 48). A similar value is calculated from circular dichroism measurements (48). These estimations agree very closely with the amount of helix actually observed in the electron density map of nuclease, which is discussed in Chapter 7 by Cotton and Hazen, this volume, and Arnone et al. (49). One can state with some assurance, therefore, that the structure of the average molecule of nuclease in neutral, aqueous solution is at least grossly similar to that in the crystalline state. As will be discussed below, this similarity extends to the unique sensitivity to tryptic digestion of a region of the sequence in the presence of ligands (47, 48), which can easily be seen in the solid state as a rather anomalous protrusion from the body of the molecule (19, 49). 

The transition can perhaps most conveniently be followed polarimet-rically. Figure 1 shows the change in specific optical rotation of a 3% PBG solution (solvent, 70 volume % DCA-30 volume % DCE) as the temperature is varied through the transition range. From optical rotatory dispersion measurements one may obtain the Moffitt parameter, bQ, and from this it has been shown that for PBG the high temperature form (with positive [o ]d) corresponds to the helical conformation of the polypeptide (12). 

Helical fraction is the only quantity that can be estimated experimentally among the various quantities characterizing the conformation of polypeptides. There are several means of estimating helical fraction (15,16). The most commonly used is based on the assumption that the Moffitt-Yang parameter b0 derived from optical rotatory dispersion measurement is a linear function of fN. Thus... 

The proposal to assimilate the rotatory dispersions of proteins to those of synthetic polypeptides is, in barest form, the recommendation to treat these same data by the Moffitt expression despite the fact that they can be described by the simple Drude equation. Thus one proceeds to plot [m ] -(X — Xo)/Xo against Xo/(X2 — Xo) with Xo set to 212 m/j, and thereby obtains an intercept, a , and a slope, which can be given conformational interpretations through Eqs. (35) and (36). These parameters, which are in general cited simply as Oo and foo when their experimental origin is... 

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