Field Desorption Mass Spectra of Gastrine Peptides and Glutathione Derivatives

738 M. Przybylski et al. • Field Desorption Mass Spectra of Oligopeptides 738 Table I . Fragment ions formed b y direct cleavage o f peptide bonds in the f .d . mass spectra of Trp-Met-Asp-PheN H 2 (I), Met -Asp -Phe -NH 2 (II) , A s p P h e N H 2 ( I I I ) and M e t A s p O H (IV) . m/e Partial sequence N, C-terminal sequence ion a ( % relative intensity )b I I I I I I I V 433 Trp-Met -Asp N 3 (8) 411 Met A s p -Phe N H 2 C3 (6) MH+ (100) 318 Trp-Met N 2 (35) 280 A s p P h e N H 2 C2 (6) C2 (13) MH+ (100) 247 Met -Asp N 2 (11) MH+ (100) c 187 T r p N i ( ) 164 P h e N H 2 Ci (20) Ci (15) Ci (16) 132 Met Ni ( — ) Ni (8)d 117 A s p Ni (20) a Partial sequences starting f rom the N-terminus are assigned N i , N 2 , . . ., sequences starting f r o m the C-terminus Ci, C2 etc. b See Table I I for emitter heating currents at which spectra were obtained, c MH+-ion, m/e 265 of Met -Asp -OH. d Same m/e-value as Ci-ion of Met -Asp-OH. b y c o m p a r i s o n w i t h t h e f . d . m a s s s p e c t r a o f t h e m o d e l p e p t i d e s , M e t A s p P h e N H 2 , A s p P h e N H 2 ( F i g . 2), a n d M e t A s p O H ( s p e c t r u m n o t s h o w n ) . T h u s , a C 2 i o n is a l s o o b s e r v e d i n t h e s p e c t r u m o f M e t A s p P h e N H 2 ( m o l e c u l a r i o n o f A s p P h e N H 2 ) , a n d t h e C i i o n , P h e N H 2 a p p e a r s i n t h e s p e c t r a o f al l t h r e e h o m o l o g u e s p e p t i d e s . E x c e p t f o r t h e N i i o n t h a t is m i s s i n g i n t h e s p e c t r u m o f M e t A s p P h e N H 2 , N t e r m i n a l i o n s w e r e a l s o o b s e r v e d f o r t h e dia n d t r i p e p t i d e s ( T a b l e I ) . L e s s c o n s i s t e n t l y , o t h e r f r a g m e n t a t i o n p r o d u c t s w e r e o b s e r v e d . I n t h e s p e c t r a o f a l l p e p t i d e s , e l i m i n a t i o n o f w a t e r and/or N H 3 , a n d C 0 2 or C O O H l e a d s t o a b u n d a n t i o n s a t e m i t t e r t e m p e r a t u r e s a b o v e B A T , b u t c a n n o t be d i f f e r e n t i a t e d c l e a r l y w i t h i n t h e c o r r e s p o n d i n g c l u s t e r i o n s . T h e s e f r a g m e n t s s e e m t o o c c u r u n s p e c i f i c a l l y i n f . d . m a s s s p e c t r a o f p e p t i d e s [ 1 1 , 12], A m o n g o t h e r d i r e c t f r a g m e n t a t i o n r e a c t i o n s o f t h e p e p t i d e b a c k b o n e , s e v e r a l i o n s o r i g i n a t e b y N a l k y l c l e a v a g e , s u c h a s m/e 449 a n d 263 i n t h e s p e c t r u m o f T r p M e t A s p P h e N H 2 a n d m/e 262/263 a n d 1 4 9 i n t h e s p e c t r u m o f M e t A s p P h e N H 2 ( T a b l e I I ) . I n t h e f . d . s p e c t r a o f M e t A s p P h e N H 2 a n d A s p P h e N H 2 , s o m e f r a g m e n t s c o u l d a l s o b e ass i g n e d t o d i r e c t C C c l e a v a g e r e a c t i o n s , f o r e x a m p l e i o n s a t m/e 307/308 a n d m/e 1 9 1 / 1 9 2 (cf. F i g . 2). T h e f o r m a t i o n o f t h e s e i o n s w a s q u i t e i n c o n s i s t e n t a t