31P nuclear relaxation studies of para- and diamagnetic cobalamins in their two isomeric forms

Abstract

A new, naturally occurring isomeric form of vitamin B-12 was reported by us earlier (Katada, M., Tyagi, S., Nath, A., Petersen, R.L. and Gupta, R.K. (1979) Biochim. Biophys. Acta 584, 149–163). The Mössbauer parameters of various derivatives of vitamin B-12, including the one-electron reduced species, cob(II)-alamin (vitamin B-12 r ), exhibit differences for the two isomeric forms. This and some other observations from our laboratory indicated that the new form (vitamin B-12′) presumably constitutes a conformational isomer possessing a different puckering of the corrin ring. The 31P nuclear relaxation studies of vitamins B-12 r and B-12′ r , B-12 and B-12′, and dicyanocobalamin as compared to dicyanocobalamin reported here are consistent with our earlier conjecture. The relaxation of the 31P nucleus in cob(II)alamins is primarily due to its dipolar interaction with the paramagnetic cobalt and therefore the measurement of the 31P nuclear relaxation times ( T 1 ) in the two isomeric forms permits us to calculate Co(II)- 31P distances. The Co(II)- 31P distance in vitamin B-12′ r is found to be larger than that in vitamin B-12 r . The 31P T 1 value in the diamagnetic cob(III)-alamins is determined predominantly by its dipolar interaction with the two closest protons flanking the phosphate group, one being situated on the ribose moiety and the other on the amino propanol group. The nuclear relaxation times T 1 of cyanocobalamin and dicyanocobalamin differ from those of their corresponding conformational isomers. The observed differences in 31P relaxation rates of the para- and diamagnetic cobalamins in their two isomeric forms can be understood on the basis of structural changes arising from variations in the nature of puckering of the corrin ring. The 31P chemical shifts in the two isomers remain essentially unchanged, indicating that the structural differences in the isomeric forms do not alter the electronic environment of the phosphorus nucleus.