Data on transverse relaxation time T2 for backbone 15N nuclei are essential to study pico-nanosecond as well as micro-millisecond dynamics of proteins. The presence of chemical exchange produces an additional transverse relaxation term Rex to the experimental value RN(Nx,y), which is proportional on wN2 (Larmor frequency). Thus, the presence of chemical exchange for a residue may be noted by a transverse relaxation rate that is high compared to that of the neighbouring residues ( 99JB13-14 , 99JB369-14 , and 00JACS2867 ). A method for distinguishing between anisotropic tumbling and chemical exchange has been reported ( 01JB149-20 ). Otherwhise, slow dynamics ranging from microsecond to second time scale exchange processes can be studied from 15N relaxation dispersion experiments ( 01JACS11341 ).NMR experiments
For RS(Sx) measurements, after the initial refocused INEPT pulse train a continuous CW spin-lock is usually applied on the transverse S magnetization. Cross-correlation is removed by applying 180 1H pulse between spin-lock segments. Alternatively a Carr-Purcell-Meiboon-Gill (CPMG) pulse train can also be applied.
Several 15N T2 relaxation experiments have been proposed ( 89BIO8972 , 90BIO4394 , 92BIO5269 , 94JMRA121-111 , 94BIO5984 , 95BIO2408 , and 00JACS2867 , 01METH204 , 02BIO2655 , and 04JMR25-171 ).
The off-resonance effects of the CPMG sequence has also been study ( 97JMR355-124 , 00JB231 ). TROSY-based experiments have been proposed for this purpose ( 99JACS2331 , 99JB151-15 , 00JMR423-143 , and 01JB361-21 ).
Recently, relaxation dispersion CPMG-based experiments has been proposed to measure slow time-scale us-ms dynamic processes in 15N backbone amides, 13C methyl groups, 15N side-chain NH2 groups, amide protons, and 13CO backbone ( 01JACS967 , 01JACS4556 , 01JACS11341 , 01NAT912 , 01NAT932 , 02JACS1443 , 03JB243-25 , 04JB187-29 , 04NAT586 , 05JB337-31 ) in proteins. These sequences utilize a relaxation compensation element to average the relaxation of in phase and antiphase magnetization components, in conjunction with a Constant-time CPMG period, that enables efficient measurement of R2 over a wide range of effective RF fields. Alternative multiple-quantum relaxation dispersion CPMG experiments have been reported as a probe for millisecond time-scale dynamics in proteins ( 04JACS1886 , and 04JACS7320 ). Studies on error estimations and global fitting have been performed ( 05JB41-32 ).
Experimental: Two sets of T2 measurements can be carried out using different spin-lock fields. Usually about 10 spectra with relaxation delays ranging from 0.01 to 0.5 seconds are carried out using a CPMG duty cycle delay of 0.5 ms. Peaks heights or integrated peaks can be used to determine the corresponding relaxation rates as a function of spectral dispersion.
Mean T2 values of some proteins: