The introdurtion of intramolecular amide bonds iiito synthetic polypeptides produces molecules with organized spatial structure which are good models for the tertiary structure of proteins. Polarization of fluorescence measurements were used to study the internal structure and the overall rigidity of the intramoleciilarly crosslinked polypeptides.
The graph of [ ( l l p ) + (1/3)] against T/v changes from a straight line to a continuous curve: the temperature a t which this change occurs and the slope of the straight line segment measure the stability of the internal structure of the molecule. The introdartion of one to six crosslinks produces an organized internal structure that becomes more stable as the number of crosslinks increases. In contrast to the fluorescence measurements, the intrinsic viscosities, reflecting the overall hydrodynamic domain of the molecules, change (decrease) to the same extent whether one, four, or six intramolecular crosslinks are present. The overall rigidity of the polymers can be assessed by the rotational relaxation time pl, and the polarization at 10ยฐC. pl0. Both of these criteria show that the presence of six crosslinks significantly increases the rigidity, but one or four does not. The various hydrodynamic measurements may be fitted into a hierarchy of discrimination: intrinsic viscosity, sedimentation, and diffusion for size and shape; rotational relaxation time and polarization of fluorescence at a given temperature for overall molecular rigidity; and transition temperature and rate a t which it is attained for internal molerular detail.