Transverse relaxation of 31P in 5′-AMP

Abstract

The influence of proton decoupling on the transverse relaxation time ( T 2) of 31P in 5′-AMP was examined. The 31P transverse relaxation time is an order of magnitude smaller when the protons are not decoupled. An expression was derived for scalar relaxation of the second kind for nucleus A in an AX 2 system in which relaxation of X is in the slow relaxation limit. Values calculated using this expression were in good agreement with experimental T 2 values indicating that transverse relaxation of 31P in 5′-AMP in the absence of proton decoupling is dominated by scalar relaxation of the second kind via scalar coupling to the two 5′ protons. With broadband proton decoupling, 31P spin-echo attenuation attributable to magnetization transfer is a potential problem. The 31P T 2 measured in the presence of proton decoupling was compared with results obtained using the off-resonance rotating frame spin-lattice relaxation ( T 1 ϱ off) experiment in the absence of proton decoupling, with broadband decoupling, and with coherent decoupling of the 5′ protons. The T 1 T 2 ratio derived from the T 1 ϱ off experiments was in agreement with that obtained by measurement of T 1 and of T 2 at two different levels of broadband (both noise and square-wave modulated) proton-decoupling power and with coherent proton decoupling, revealing no complications in 31P T 2 measurements in the presence of broadband proton decoupling for 5′-AMP (or probably other phosphates).