ACCESSIBLE SURFACE AND INTRAMOLECULAR MOBILITY OF PROTEINS - STUDIES USING TRITIUM PLANIGRAPHY

The accessible surface of the lysozyme globule and the accessibility o f different types of amino acid residues therein were determined as a function of temperature over the range 77-293 degrees K by tritium pla nigraphy. The protein was used as a powder of 10+/-1% water content. T he accessible surface area was found to increase with increasing tempe rature from 160 to 293 degrees K. According to the temperature depende nce of their accessibility, all amino acid residues could be arbitrari ly classified into three groups: with a significant dependence in the range 160-293 degrees K (K, R, H, P, L), with weak or no dependence (C , V, A, I, Y, F), and with a sharp dependence in the range 260-293 deg rees K (S, T, G, D+N, E+Q). The increase in accessibility is explained by ''thawing'' of intramolecular mobility. Enhanced label incorporati on was observed for residues of all types at 77 vs 160 degrees K, sugg esting a significant change in protein tertiary structure. This effect cannot be explained in terms of intramolecular mobility changes.