SECONDARY STRUCTURE AND TEMPERATURE BEHAVIOR OF THE ACETYLCHOLINE-RECEPTOR BY FOURIER-TRANSFORM INFRARED-SPECTROSCOPY

Fourier transform infrared spectroscopy (FT-IR) was used to test the secondary structure of purified acetylcholine receptor membranes from Torpedo californica. The secondary structure was estimated using the spectral features observed in the structure sensitive region of amide I and amide I' (between 1600 and 1700 cm-1), taking advantage of Fourier self-deconvolution and second-derivative techniques along with least-squares band fitting procedures. At least six different amide I' band components could be resolved in D2O and were tentatively assigned to beta-structures (1680 and 1636 cm-1), alpha-helices (1657 cm-1), aperiodic structures and/or distorted helices (1646-1648 cm-1), and turns (1690 and 1668 cm-1), respectively. The beta-band around 1637 cm-1, in particular, turned out to be complex since it reproducibly exhibited weak features near 1630 and 1627 cm-1, thereby suggesting the presence of different chain interacting beta-structures. The band near 1657 cm-1 was assigned to alpha-helices which traverse the membrane bilayers, while 1646-1648-cm-1 component was tentatively attributed to aperiodic structures and alpha-helices localized within the ''globular head'' of the receptor protein protruding from the membrane surface into the surrounding water. Least-squares band fitting procedures were applied in order to estimate relative amounts of secondary structures. The results suggest 36-43%, 32-33%, 14-24%, and 18-19% for beta-, alpha-helical, turn, and ''rest'' structures, respectively. Additionally, the temperature- and time-dependent variations of the secondary structure was tested by evaluating the changes of amide I and amide II band components of receptor membranes dispersed in H2O and D2O. The results are indicative of differing stability of the various protein secondary structures. The known activity loss of AChR membranes at temperatures above 20-degrees-C correlates well with considerable isothermic changes of beta-, aperiodic. and small loss of alpha-helical structures.