 action metal ion tobacco mosaic viru ribonucl acid transit metal ion stabil secondari structur tobacco mosaic viru ribonucl acid tmv-rna evidenc reduct absorb chang heat nucleic acid solut spite stabil secondari structur heat presenc metal ion loss biolog activ hydrolysi phosphodiest bond heavi metal ion iib first-transit element studi caus rapid loss biolog activ tmv-rna ph 6 5 65 calcium magnesium ion secondari structur 65 ph 8 5 caus rapid loss biolog activ ph 5 8 room temperatur lead ion catalyz hydrolysi rna i' 3' - mononucleotid long stand nucleosid form 