Conformational Studies on Modified Proteins and Peptides

Cleavage of sperm whale myoglobin at arginine peptide bonds has been accomplished by tryptic hydrolysis of the protein after blocking the amino groups reversibly with citraconic anhydride. Peptides comprising the sequences 1 to 31, 46 to 118, and 119 to 139 were isolated and studied by optical rotatory dispersion and circular dichroism. In the free state, each peptide was much less helical than expected from its conformation in the intact protein. Thus Peptide 1–31 which, in the native protein, has 30 out of 31 residues in a helical configuration (i.e. 97% helical) was only 33% helical (from b0 values) in the free state. Helicity increased appreciably (to 54%) in 62% methanol. The longer Peptide 46–118, which is 62% helical in the intact protein, was 38% helical in the free state and helicity of this peptide was increased to about 50% in 62% methanol. This is a long peptide with an elaborate mode of folding which makes the contribution of long range interactions quite feasible. Peptide 119–139 occupies a region in the native protein which is 71% helical. However, its helical content in the free state (15%) was quite low, mainly due to minimal contribution of long range interactions and to some loss of contribution of short range interactions since it carries only a portion of a helix (helix H). Although helicity of this peptide increased (to 31%) in 62% methanol, it was still much lower than the corresponding value for the other two peptides in the same solvent. These studies show that the conformation of each of these peptides is less helical in the free form than expected from its location and configuration within the intact protein. Helicity increases as the probability of stabilization of folding by long range interactions improves. Also, the results indicate that two separated fragments of a helix will not be able to achieve a high degree of helicity. Furthermore, proteins may not start to assume native conformation during biosynthesis until virtually or entirely completed, depending on the extent of contribution to long range interactions (in the complete protein) of the yet unsynthesized portion.