SYSTEMATIC-ERRORS ASSOCIATED WITH THE CPMG PULSE SEQUENCE AND THEIR EFFECT ON MOTIONAL ANALYSIS OF BIOMOLECULES

A theoretical approach to calculate the time evolution of magnetizatio n during a CPMG pulse sequence of arbitrary parameter settings is deve loped and verified by experiment. The analysis reveals that off-resona nce effects can cause systematic reductions in measured peak amplitude s that commonly lie in the range 5-25%, reaching 50% in unfavorable ci rcumstances. These errors which are finely dependent upon frequency of fset and CPMG parameter settings, are subsequently transferred into er roneous T-2 values obtained by curve fitting, where they are reduced o r amplified depending upon the magnitude of the relaxation time. Subse quent transfer to Lipari-Szabo model analysis can produce significant errors in derived motional parameters, with tau(e) internal correlatio n times being affected somewhat more than S-2 Order parameters. A haza rd of this off-resonance phenomenon is its oscillatory nature, so that strongly affected and unaffected signals can be found at various freq uencies within a CPMG spectrum. Methods for the reduction of the syste matic error are discussed. Relaxation studies on biomolecules, especia lly at high field strengths, should take account of potential off-reso nance contributions. (C) 1997 Academic Press.