Investigation of finite-pulse radiofrequency-driven recoupling methods for measurement of intercarbonyl distances in polycrystalline and membrane-associated HIV fusion peptide samples

Abstract

Two finite-pulse radiofrequency-driven recoupling (RFDR) methods were compared and applied to the measurement of 3–6 Å ¹³CO¹³CO distances in polycrystalline and membrane-associated HIV fusion peptide (HFP) samples. The RFDR methods were based on π pulses and were relatively straightforward to implement and insensitive to pulse imperfections. The two tested methods were: (i) constant-time double-quantum buildup with finite pulses (fpCTDQBU) for which the pulse sequence maintained a constant transverse relaxation period while allowing a variable period of dipolar dephasing; and (ii) constant-time finite-pulse rf-driven recoupling (fpRFDR-CT) for which the duration of transverse relaxation increased with increasing dephasing period. The fpRFDR-CT method yielded higher signal-to-noise and an accurate determination of a ∼5 Å intercarbonyl distance was made in a crystalline peptide which had T₂ ≈ 55 ms. In some contrast, the HFP samples had T₂ ≈ 15 ms and the fpRFDR-CT data were dominated by transverse relaxation. Examination of the fpCTDQBU sequence showed: (i) the most rapid signal buildup was obtained with application of one ¹³C π pulse per rotor period rather than one ¹³C π pulse per multiple rotor periods and (ii) the data were insensitive to ∼15 ppm transmitter offset and to ∼5° variation of π pulse nutation angle. For HFP samples which were ¹³CO labeled at a single residue, analyses of the fpCTDQBU data were interpreted with a model of mixed parallel and antiparallel β-strand arrangements in the N-terminal region of HFP and loss of parallel β-sheet structure in the C-terminal region of HFP. Copyright © 2007 John Wiley & Sons, Ltd.