Moffitt parameter

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 secondary and tertiary structure of a partially purified 7S globulin was examined by Fukushima (7) based on optical rotatory dispersion, infrared and ultraviolet difference spectra. Antiparallel (5 -structure (352) and random coil (60%) predominated with only 5% helical structure present. The contribution of the three structures was calculated from molecular ellipticity values obtained by circular dichroism (11) and from the Moffitt parameters in ORD (11, 12). Between 210 and 250 nm, the experimental CD curve for the 7S protein was similar to the CD curve computed from ORD Moffitt parameters with the major dissimilarity occurring at 208-213 nm. 

Another important factor for the polymer conformation is the solvent effect. As the usual solvents for the copolymers in question are DMSO and DMF, which have absorption in the UV region, CD spectral measurements are impossible. However, the optical rotation measurements and analyses using the Moffitt-Yang equation give the Moffitt parameter bo for the copolymers (Table 21). Hie parameter is known to be related to the helix content of poly-(a-amino acids). The b0 value of polycarboben-... 

The Relation of the Moffitt Parameters to Partial Helical Content... 

The computations of the Moffitt parameters (see Equation 2), utilizing the statistical procedures with error analyses developed by Sogami, Leonard, and Foster (57) to determine the best Xo values, and of the Drude parameters see Equation 1 a modified Drude plot of [mf] vs. [ra ]X2 (67) was used with error analyses were performed with the IBM 7094 computer. Correction for the dispersion of refractive index of the solvent was made as outlined (46). 

Figure 5. Reduced mean rotation and a0 as a function of the Moffitt parameter, b0, for the entire helix-coil transition of paramyosin in guanidine-HCl at 25°C. 46)...

Solid symbols and right-hand ordinate. Moffitt parameter a0 Moffitt parameters calculated from optical rotatory dispersion data from 315 to 240 my. using X0 = 220 my... 

For paramyosin [ra ]232 and the Moffitt parameters a0 and b0 are colinear for the entire helix-coil transition see Figure 5), but since b0 represents an averaging of rotatory contributions at many wavelengths and is relatively insensitive to the environment, it is the preferable parameter for the estimation of helix content. 

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). 

The Moffitt parameter b was evaluated from optical rotatory dispersion (ORD) da a measured by a Yanagimoto OR-100 Type spectropolarimeter using a tungsten lamp as the light source in a temperature range of 10 to 40 C. The reduced mean residue rotation, [m ], at a wave length X is obtained from the observed rotation a by... 

---