15– Solid State NMR · 15– Solid State NMR In solid state NMR, many of the anisotropic interactions are not averaged to zero, such that the Hamiltonian can usually be written

'

&

$

%

15– Solid State NMR

In solid state NMR, many of the anisotropic

interactions are not averaged to zero, such that the

Hamiltonian can usually be written as

H = Hext + Hint

= HZ + Hrf (t) + Hcs,aniso + HJ + HD + HQ,

(15.1)

where the last term is only present if we are dealing

with spins I ≥ 1.

Because the magnitude of the “internal” interactions

is large, in solid state NMR much effort is placed in

removing them, either via mechanical rotation

(magic angle sample spinning) or by rf pulses (in this

case ω1 = 50 − 150kHz).

15.1 Examples

In the following, we will briefly illustrate some

examples of experiments which are commonly used

in solid state and the type of information which can

be extracted from this data.

'

&

$

%

In general, solid state experiments are used for

1. extracting anisotropic parameters (e.g. CSA,

which in turn is used in relaxation for instance)

2. assignment (identification) of the resonance lines

3. extracting structural information

4. extracting dynamic information

5. extracting information on the interaction of one

molecule with another, e.g. lipid + protein

Anisotropic Parameters

In previous chapters (in particular the chemical shift

and dipolar chapters), we saw how we can measure

anisotropic parameters. To determine the CSA, for

example, a cross-polarization (CP) experiment is

applied to a static sample. Alternatively, a CP

experiment at slow MAS speeds can be recorded and

'

&

$

%

sideband intensities fitted to yield the CSA

parameters. In the next chapters, we will see in more

detail what a CP experiment is and what the result

of MAS is.

Assignments

Unlike in solution state, where the methods have

been established long ago (relatively speaking!),

assignments of high resolution spectra in (complex)

solids (e.g. proteins) have been relatively recent (last

ten years or so). So there is no “standard” set of

experiments used though typical ones incorporate

elements such as proton driven or rf driven spin

diffusion and 13C −15 N cross-polarization.

As before, the choice of experiments used for

assignment purposes depends on the labelling

pattern. For instance, for a protein, we can have a

fully 13C labelled sample only or a fully 13C/15N

sample. To date, 1H information is not used because

of the poor resolution, though many groups are

working on improving 1H −1 H decoupling

techniques (e.g. Shimon Vega), while others are

working methods which use 1H detection (e.g. Rob

'

&

$

%

Tycko).

Examples

Listing of experiments:

ref: S.K. Straus, Phil. Trans. B, 2004

13C experiments:

CP

CPt1/2 t1/2 t2/2 t2/2

COCα

t3τm τ'm

TPPM TPPMI

Sπ

π π ππ

π π π

TPPM

'

&

$

%

13C/15N experiments:

CP1

CP1 t1/2 t1/2

Cα

t2

TPPM TPPM1H

13C π πϕ2

CP2

LG

15N

a)

CP2 GARP-1

(π)ϕ1

ϕ3

ϕ4

b)CP1

CP1

t2

TPPM TPPM1H

13C CP2

LG

15N CP2

(π)ϕ1

(π)

t1

CO

ϕ2

ϕ3ϕ4

ϕ5

τm

π

c)CP1

CP1 t1/2 t1/2

Cα

t2

TPPM TPPM1H

13C π πϕ2

CP2

LG

15N

CP2 GARP-1

(π)ϕ1

ϕ3 π (π)

CO

ϕ4

ϕ6τm

y

Structure

Structural information can be obtained from the

chemical shift (though the trends aren’t as clear cut

as in solution state). More common is to use

experiments which correlate two orientationally

dependent parameters, such as dipolar interaction

with dipolar interaction or dipolar interaction with

'

&

$

%

CSA.

Examples:

ref: S.K. Straus, Phil. Trans. B, 2004

Dynamics

Dynamic information is obtained through the

relaxation parameters T1 and T2.

Interactions between molecules

If the systems are not too dynamic, it is possible to

measure distances between molecules (e.g. lipid and

a membrane-embedded peptide/protein) using, for

example, the REDOR pulse sequence.