Low-Level Long-Range 1H−15N Heteronuclear Shift Correlation at Natural Abundance Using Submicro NMR Techniques

Gradient inverse-detected NMR methods have provided experimental access to long-range 1H-15N heteronuclear shift correlation data at natural abundance. When used in conjunction with newly developed submicro-inverse-detection gradient (SMIDG) NMR probes, sample size requirements are dramatically reduced. Long-range 1H-15N GHMBC data are shown for a 1 mg sample ( approximately 3 mol, 0.11 M) of the alkaloid strychnine (1) dissolved in 30 L of CDCl3. All of the correlations observed in previous studies using samples in the range of 20-40 mg in 5 mm NMR probes were observed in data acquired overnight on a 1 mg sample in a 1.7 mm SMIDG NMR probe. A previously unreported long-range correlation, 3JH-15a-N(19), was observed in data recorded over a weekend for the 1 mg sample. Data recorded over a weekend using a 1 mol (334 g) sample of strychnine (1) in 30 L of CDCl3 contained the majority of the responses from the 1 mg sample. The practical limit of detection for long-range 1H-15N heteronuclear shift correlation data at natural abundance would appear to be approximately 1 mol with data acquisition over a weekend or longer using present submicro-NMR probe technology.