I need to run noesy for a monomeric peptide of about 1.5kDa at 800 MHz 1H frequency. NOE peaks are weak at 100 ms mixing but are a lot stronger at 300 ms.
Is it acceptable to use NOESY data at that mixing time - 300ms without running into correlations due to spin-diffusion?
If ALL of your NOe peaks are indeed very weak at 100 ms then it is safe to use longer mixing times (provided that the low intensity is NOT due to lack of adequate sample; I am assuming that sample conc. is > 1 mM and that the viscosity is equal to or less than than of an aqueous solution).
Instead of increasing the mixing time to 300 ms you may increase the sensitivity of the experiment by increasing the sample concentration by a factor of 3 or use a more sensitive probe, e.g. a cryogenic probe. These two alternatives would allow you to obtain substantial increases in sensitivity with lower mixing times.
It would be safer to use acquire a series of NOESY spectra with different mixing times and use a relaxation matrix analysis. (for your case 50ms?, 100ms, 150ms, 200ms and 300ms)
It would be better to use ROESY/CAMELSPIN spectra as the effects of spin-diffusion are less in these spectra. See work of Bothnerby et al.
Note: Computational studies show that NO mixing time (no matter how short) will guarantee the complete absence of spin diffusion and that to be completely safe you would have to obtain a series of NOESY spectra with different mixing times (time series) and use relaxation matrix analysis. However, the standard practice is to use the observed NOe's to obtain upper bounds (and NOT to obtain precise quantitative values of distances) and for such a purpose your mixing time seems reasonable.