Biomolecular Studies Using Bromine 77 and Indium 111 Perturbed Angular Correlations.

Time - differential - perturbed - angular - correlation (TD PAC) spectroscopy has been applied to probe biological molecules in aqueous solution, using the 172 KeV - 245 KeV gamma-gamma cascade in the decary of ('111)In and the 755 KeV - 250 KeV cascade in the decay of ('77)Br. Carrier -free ('111)In is found to bind to the bioluminescent protein, aequorin, with a dissociation constant of about 10('-7) M. The quadrupole coupling constant for the daughter ('111)Cd -aequorin species is about 2 x 10('8) rad/sec. This estimate was obtained from the first few nanoseconds of the TD PAC spectrum after correcting for the effects of finite resolving time. Conjectures about the shape and internal flexibility of the aequorin molecule are presented to explain the relaxation effects observed in the TD PAC spectrum. To substantiate these, preliminary calculations of the effects of rotation of aequorin about a single fixed axis are presented. The work on ('77)Br is the first substantial investigation of ('77)Br as a rotational probe. Unperturbed values are determined for the angular correlation coefficients for the gamma-gamma cascade in ('77)Se from ('77)Br decay. These are A(,2) = -0.479 (+OR-) 0.012 and A(,4) = -0.046 (+OR -) 0.005. The PAC spectrum with ('77)Br('-) ion in aqueous solution at 25(DEGREES)C is a single exponential with the relaxation constant of 1.8 x 10('7) l/sec. Such strong relaxation is apparently due to a covalent ('77)Se daughter species with a quadrupole coupling constant of about 1.5 x 10('9) rad/sec and a rotational correlation time of about 3 x 10('-12) sec. The quadrupole moment of the ('77)Se 5/2- intermediate state is estimated to be about 1 barn. The PAC spectra of ('77)Br-tyrosine, ('77)Br-human serum albumin (HSA), and ('77)Br-deoxyuridine are also presented. Very strong perturbations are observed in the spectrum of the radio-brominated protein.