Maximum entropy approach to the determination of solution conformation of flexible polypeptides by global conformational analysis and NMR spectroscopy - Application to DNS 1 -c-(D-A2bu 2 ,T rp 4 ,Leu 5 )- enkephalin and DNS 1 -c-(D-A 2 bu 2 ,T rp 4 , D-Leu 5 )enkephalin