Reconstructing NMR spectra of "invisible" excited protein states using HSQC and HMQC experiments

Carr-Purcell-Meiboom-Gill (CPMG) relaxation measurements employing trains of 180 degrees pulses with variable pulse spacing provide valuable information about systems undergoing millisecond-time-scale chemical exchange. Fits of the CPMG relaxation dispersion profiles yield rates of interconversion, relative populations, and absolute values of chemical shift differences between the exchanging states, |Deltaomega|. It is shown that the sign of Deltaomega that is lacking from CPMG dispersion experiments can be obtained from a comparison of chemical shifts in the indirect dimensions in either a pair of HSQC (heteronuclear single quantum coherence) spectra recorded at different magnetic fields or HSQC and HMQC (heteronuclear multiple quantum coherence) spectra obtained at a single field. The methodology is illustrated with an application to a cavity mutant of T4 lysozyme in which a leucine at position 99 has been replaced by an alanine, giving rise to exchange between ground state and excited state conformations with a rate on the order of 1450 s(-1) at 25 degrees C.