Site-specific dynamic nuclear polarization NMR agents

1. A dynamic nuclear polarization (DNP) agent for DNP nuclear magnetic resonance of an analyte, the DNP agent having the structure A-X-L-R, wherein A is an amphiphilic group, wherein X is a coupling group, is selected from the group consisting of an amine-reactive coupling group, an aldehyde-reactive coupling group, a sulfhydryl-reactive coupling group, and a combination thereof, wherein x is capable of site-specific binding with the analyte or with A, wherein L is a bond or a linker group, and wherein R is a poly-radical group, comprising two or more radicals connected through a flexible or rigid linker, where each radical is independently selected from the group consisting of a nitroxide radical, a triarylmethyl radical, and a combination thereof.

2. The DNP agent of claim 1 , wherein R is a poly-radical selected from the group consisting of a di-radical, a tri-radical, a tetra-radical, and a combination thereof.

3. The DNP agent of claim 1 , wherein each radical is independently selected for the group

wherein each occurrence of X is independently CH 2 , NH, O, or S;

wherein each occurrence of Z is independently H or a substituted or unsubstituted alkyl, heteroalkyl, alkenyl, or alkynyl group having from 1 to 30 carbon atoms.

4. The DNP agent of claim 1 , wherein R has the formula

wherein each occurrence of R 1 is independently an —X 1− L 1 -M 1− ;

wherein each occurrence of X 1 is selected from the group consisting of —O—, —S—, or —N(R 2 )—, wherein R 2 selected from the group consisting of H, hydroxyl, halide, and substituted and unsubstituted alkoxy, heteroalkoxy, alkyl, heteroalkyl, aryl, aryloxy, aralkyl, aralkyloxy, alkenyl, and alkynyl groups having from 1 to 30 carbon atoms

wherein each occurrence of L 1 is independently a bond or selected from the group consisting of substituted and unsubstituted alkoxy, heteroalkoxy, alkyl, heteroalkyl, aryl, aryloxy, aralkyl, aralkyloxy, alkenyl, and alkynyl groups having from 1 to 12 carbon atoms;

wherein each occurrence of M 1 is independently a nitroxide radical or a triarylmethyl radical.

5. The DNP agent of claim 4 , wherein each M 1 is independently selected for the group

wherein each occurrence of X is independently CH 2 , NH, O, or S;

wherein each occurrence of Z is independently H or a substituted or unsubstituted alkyl, heteroalkyl, alkenyl, or alkynyl group having from 1 to 30 carbon atoms.

6. The DNP agent of claim 1 , wherein the linker group is selected from the group consisting of substituted and unsubstituted alkoxy, heteroalkoxy, alkyl, heteroalkyl, aryl, aryloxy, aralkyl, aralkyloxy, alkenyl, and alkynyl groups having from 1 to 12 carbon atoms.

7. The DNP agent of claim 1 , wherein the coupling group forms a covalent bond to the analyte.

8. The DNP agent of claim 1 , wherein the coupling group is a sulfhydryl-reactive coupling group selected from the group consisting of a maleimide group, a methanethiosulfonate group, a haloacetyl group, a pyridyl disulfide group, and a combination thereof.

9. The DNP agent of claim 8 , wherein the sulfhydryl-reactive coupling group is a maleimide group having the structure

wherein R 5 is selected from the group consisting of substituted and unsubstituted alkoxy, heteroalkoxy, alkyl, heteroalkyl, aryl, aryloxy, aralkyl, aralkyloxy, alkenyl, and alkynyl groups having from 1 to 30 carbon atoms.

10. The DNP agent of claim 8 , wherein the sulfhydryl-reactive coupling group is a methanethiosulfonate group having the structure

wherein R 4 is selected from the group consisting of substituted and unsubstituted alkoxy, heteroalkoxy, alkyl, heteroalkyl, aryl, aryloxy, aralkyl, aralkyloxy, alkenyl, and alkynyl groups having from 1 to 30 carbon atoms.

11. The DNP agent of claim 8 , wherein the sulfhydryl-reactive coupling group is a haloacetyl group having the structure

wherein R 3 is selected from the group consisting of substituted and unsubstituted alkoxy, heteroalkoxy, alkyl, heteroalkyl, aryl, aryloxy, aralkyl, aralkyloxy, alkenyl, and alkynyl groups having from 1 to 30 carbon atoms; and

wherein Hal is a halogen.

12. The DNP agent of claim 8 , wherein the sulfhydryl-reactive coupling group is a pyridyl disulfide group having the structure

wherein each occurrence of R 2 is independently selected from the group consisting of hydrogen, hydroxyl, halide, and substituted and unsubstituted alkoxy, heteroalkoxy, alkyl, heteroalkyl, aryl, aryloxy, aralkyl, aralkyloxy, alkenyl, and alkynyl groups having from 1 to 30 carbon atoms or, when taken together with the atoms to which they are attached form a heterocycle having from 2 to 30 carbon atoms.

13. The DNP agent of claim 1 , wherein the coupling group is a amine-reactive coupling group selected from the group consisting of an isothiocyanate, an isocyanate, an acyl azide, an NHS ester, a sulfonyl chloride, an aldehyde, an epoxide, an oxirane, a carbonate, an aryl halide, an imidoester, a carbodiimide, an anhydride, a fluorophenyl ester, and a combination thereof.

14. The DNP agent of claim 1 , wherein the coupling group is an aldehyde-reactive coupling group selected from the group consisting of a hydrazide, an alkoxyamine, a primary amine, and a combination thereof.

15. The DNP agent of claim 1 , wherein the coupling is achieved by click chemistry, including but not limited to coupling between alkyne and azide and by a combination with other coupling schemes.

16. The DNP agent of claim 1 , wherein the coupling group is a non-covalent coupling group that binds non-covalently to the analyte with a K d of 10 −13 M to 10 −16 M.

17. The DNP agent of claim 16 , wherein the non-covalent coupling group is selected from the group consisting of biotin, a biotin derivative, avidin, an avidin derivative, streptavidin, a streptavidin derivative, and a combination thereof.

18. The DNP agent of claim 1 , wherein the analyte is an antibody and the coupling group is an antigen that binds specifically with the antibody.

19. The DNP agent of claim 1 , wherein the analyte is a polynucleotide and the coupling group is an aptamer that binds specifically with an active site of the polynucleotide.

20. The DNP agent of claim 1 , wherein A is an amphiphilic group, and wherein the amphiphilic group is selected from the group consisting of an amphiphilic polymer, a lipid, and a conjugate thereof.

21. The DNP agent of claim 1 , wherein the analyte is selected from the group consisting of a protein and a nucleic acid.

22. A method of NMR measurement of an analyte comprising an NMR-detectable nucleus, the method comprising the steps of:

providing a frozen sample containing the analyte and a DNP agent according to claim 1 ;

applying radiation having a frequency that excites electron spin transitions in the DNP agent at an intensity to polarize the NMR-detectable nucleus; and

detecting a signal from nuclear spin transitions in the NMR-detectable nucleus.

23. The DNP agent of claim 1 , wherein L is a bond.