The strength of the 3 '-gauche effect dictates the structure of 3 '-O-anthraniloyladenosine and its 5 '-phosphate, two analogues of the 3 '-end of aminoacyl-tRNA

Anthranilic acid charged yeast tRNA(Phe) or E. Coli tRNA(Val) are able to f orm a stable complex with EF-Tu*GTP, hence the 2'- and 3'-O-anthraniloylade nosines and their 5'-phosphate counterparts have been conceived to be the s mallest units that are capable of mimicking aminoacyl-tRNA. Since the 3'-O- anthraniloyladenosine also binds more efficiently to the EF-Tu*GTP complex compared to its 2'-isomer, we have herein delineated the stereoelectronic f eatures that dictate the conformation of 3'-O-anthraniloyladenosine and its 5'-phosphate vis-a-vis their 2'-counterparts and we have also addressed ho w their structures and thermodynamic stabilizations are different from aden osine and 5'-AMP. It has been found that the electron-withdrawing anthranil oyl group exerts gauche effects of variable strengths depending upon whethe r it is at the 2'- or at 3'-position because of either the presence or abse nce of O2'-N9 gauche effect, [GE(O2'-C2'-C1'-N9)], thereby steering the pse udorotation of the constituent sugar moiety either to the North (N)-type (C 3'-endo) or South (S)-type (C2'-endo) conformation. The 3'-O-anthraniloylad enosine 5'-phosphate has a relatively more stabilized S-type conformation D elta G degrees = -4.6 kJ mol(-1)) than 3'-O-anthraniloyladenosine itself (D elta G degrees = -3.9 kJ mol(-1)), whereas the Delta G degrees for 2'-O-ant hraniloyladenosine and its 5'-monophosphate are respectively -0.9 and -1.8 kJ mol(-1), suggesting that the 3'-gauche effect of the 3'-O-anthraniloyl g roup is stronger than that of 2'-O-anthraniloyl in the drive of the sugar c onformation. Since the EF-Tu can specifically recognize the aminoacylated-t RNA from the non-charged tRNA, we have assessed the. free-energy (Delta G d egrees) for this recognition switch to be the least approximate to -2.9 kJ mol(-1) by comparison of Delta G degrees of the N=S pseudorotational equili brium for 3'-O-anthraniloyladenosine 5'-phosphate and 5'-AMP. The 3'-O-anth raniloyladenosine and its 5'-phosphate are much more flexible than the isom eric 2'-counterparts as is evident from the temperature dependent coupling constants analysis. The relative rate of the transacylation reaction of 2'( 3')-O-anthraniloyladenosine and its 5'-phosphate is cooperatively dictated by the two-state N=S pseudorotational equilibrium of the sugar, which in tu rn is controlled by a balance of the-stereoelectronic 3'- and 2'-gauche eff ects as well as by the pseudoaxial preference of the 3'-O- or 2'-O-anthrani loyl group. The reason for the larger stabilization of the 2'-endo conforme r for 3'-O-anthraniloyladenosine and its 5'-phosphate lies in the fact that the C3'-O3' bond. takes up an optimal gauche orientation with respect to t he C4'-O4' bond dictating the pseudoaxial orientation of the 3'-anthraniloy l residue, which can be achieved only in the S-type sugar conformation with adenin-9-yl and the 2'-OH groups in the pseudoequatorial geometry, compare d to the preferred C3'-endo sugar with a pseudoaxial aglycone and 2'-OH fou nd in the 3'-terminal adenosine moiety in the helical 3'-CCA end of uncharg ed tRNA.